Author Topic: How much noise floor and other things matter in oscilloscope usability  (Read 53703 times)

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Offline FiorenzoTopic starter

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Hello,
I am in the process to "buy" an oscilloscope and I really need some clarification because, even if I've read almost everything I have been able to find on the web and on this forum, there are some technical things I cannot sort out by my self.

For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?

I am going to use It for many different things, I am a "begginer" but I do digital stuff with embedded electronics and I also aim to learn more things as possible about analog electronics starting from working on an old valve radio that i would like to repair and experiment with.

Actually I bought few days ago a Rigol mso5000 and I did not be aware about its noisy front end.
After a lot of searching on the web and experimenting with such instrument It seem to me that It is at least 3 times more noisy than other comparabile oscilloscopes.

So how much this matter in electronics? How this could preclude its usability?

I have still about 30 days to give It back for free to Amazon but only one week to decide if I want to buy its direct competitor siglent "sds2000x plus" that now Is on offer with the LA probe discount bundle until 30dicember.

The Siglent appear to have a low noise front end but a different way to handle signal recording in its internal memory that i don't understand if it Is bad due to the fact that It cannot "zoom out"...

Also the Rigol has a very fast ADC when working in interleaved mode but at the same time I still don't understand if 8 Gsa/s are really needed with frequency up to 500Mhz and maybe the Siglent with Its 2Gsa/s is enough.... I don't find clear infos on the web apart the nyquist theorem.

I would be gratefull if someone with experience could help me because all this matter a lot for me, electronics has been in my heart from when I was young and I am keen to improve as best as possible day after day....

Thank you

« Last Edit: December 24, 2021, 12:39:18 pm by Fiorenzo »
 

Offline Kleinstein

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #1 on: December 23, 2021, 12:47:34 pm »
For most uses the noise of the scope is not really an issue. This is because a 1:10 probe is often the mayjor noise source.  Unless the scope is rather high in noise, the noise floor is mainly an issue with the few measurements done with an 1:1 probe or similar signal via coax.
For digital signals itself the scopes noise is usually not an issue, but it may be when looking at the supply ripple.

For comparing the noise, one should take the bandwidth into account. With a higher BW the RMS noise naturally goes up. This effect can make a new higher BW scope look higher noise. So  one usually should compare at the same BW, like the usual 20 MHz BW setting.

2 Gs/s are a bit on the low end for 500MHz BW. This may lead to some aliasing and a few more thoughts about what one is actually seeing at the short time scale end.
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #2 on: December 23, 2021, 01:28:17 pm »
For most uses the noise of the scope is not really an issue.
I strongly disagree with this. An oscilloscope with a lot of noise gives thick traces which make it hard to see the actual level of a signal. Ofcourse you can use high-res or bandwidth limiting but IMHO these should be targeted at cleaning up a signal and not masking a poorly designed analog front end. An oscilloscope with a low noise floor is easier to work with. I used to own an Agilent DSO7104A and it was just horrible to work with for analog stuff due to the massively thick traces it has due its own noise.

Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty.
« Last Edit: December 23, 2021, 01:36:25 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #3 on: December 23, 2021, 02:05:29 pm »
I find your replies very interesting.

So now I am thinking: for what type of signal and in what situation the noise floor of the oscilloscope is impacting the anologue measurements?
 

Offline Grandchuck

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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #5 on: December 23, 2021, 02:51:59 pm »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?

When you're measuring very small signals.

I am going to use It for many different things, I am a "begginer" but I do digital stuff with embedded electronics and I also aim to learn more things as possible about analog electronics starting from working on an old valve radio that i would like to repair and experiment with.

It won't make any difference at all on your digital stuff.

For the radio? If it turns out to be a problem you can easily add a preamplifier and make it even better than a lower-noise oscilloscope.




So how much this matter in electronics? How this could preclude its usability?

It's not a showstopper. You can still do everything, just maybe not as easily for a few specific things.

The real questions are: How often do you do those things? How much would you have to spend to get a lower-noise 'scope with the same abilities as your Rigol? Is the extra money well-spent?
« Last Edit: December 23, 2021, 02:53:36 pm by Fungus »
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #6 on: December 23, 2021, 02:53:04 pm »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?
When you're measuring very small signals.
Actually not because the relative noise floor remains more or less constant!
I just tried on an R&S RTM3004. At 1V/div I get 17mV stdev. At 10V/div I get 170mv stdev (Stdev= RMS with DC removed). The noise floor scales with the V/div setting.
« Last Edit: December 23, 2021, 02:57:19 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #7 on: December 23, 2021, 03:01:05 pm »
Grandchuck I read that article and many others but they doesn't give example of practical situation that are impatted from the noise floor of the oscilloscope. Which kind of signal need an oscilloscope with a low noise frontend?
I ask these because I do not have a clear comprehension of such subjects.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #8 on: December 23, 2021, 03:03:25 pm »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?
When you're measuring very small signals.
Actually not because the relative noise floor remains more or less constant!
I just tried on an R&S RTM3004. At 1V/div I get 17mV stdev. At 10V/div I get 170mv stdev (Stdev= RMS with DC removed). The noise floor scales with the V/div setting.

If the signal is large, you can see it, it's just that the traces on screen are thicker.

If the signal is small, it gets lost inside the trace.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #9 on: December 23, 2021, 03:06:33 pm »
Which kind of signal need an oscilloscope with a low noise frontend?

A 1mV signal.

(for example)
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #10 on: December 23, 2021, 03:15:51 pm »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?
When you're measuring very small signals.
Actually not because the relative noise floor remains more or less constant!
I just tried on an R&S RTM3004. At 1V/div I get 17mV stdev. At 10V/div I get 170mv stdev (Stdev= RMS with DC removed). The noise floor scales with the V/div setting.

If the signal is large, you can see it, it's just that the traces on screen are thicker.
Yes, and thick traces just suck. Try to make a cursor measurement on a trace that is 20% of a division even with V/div set to 1V/div. Needless to say that small variations of a signal are also lost in a noisy oscilloscope.

Don't confuse needing a pre-amplifier to look at signals that are outside the V/div range of an oscilloscope. That is a different subject!
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #11 on: December 23, 2021, 03:24:21 pm »
Ok, you all are giving me many usefull information, but now give a look to the attached photos.
The first image shows the apparent noise of CH1, It is set in AC mode, 1X attenuation, without bandwith limit and is also sampling in normal mode with the probe attached and grounded to its own tip.
The second image shows the same configuration but with the probe attached to the output of and old transformer just to check its ripple.

I see a lot of noise floor in this photos.
Do you agree? It seem too much to me, but as I said before I am not an expert in analoge stuff so maybe I am doing something wrong... Or maybe this oscilloscope is particularly noisy...
« Last Edit: December 24, 2021, 06:07:05 am by Fiorenzo »
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #12 on: December 23, 2021, 03:36:01 pm »
Which kind of signal need an oscilloscope with a low noise frontend?

A 1mV signal.

(for example)


What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

I don't want to buy an oscilloscope and find myself after a year that It is going to particularly limit my study in electronics, this are costly instruments, I can spend some more money than that spent for the Rigol but I must do the right choice and now I don't feel confident with my actual knowledge.
 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #13 on: December 23, 2021, 04:03:34 pm »
I haven't compared those two models directly, but comparisons with two lower models of Rigol and Siglent were enough to convince me that low noise was an important factor in some cases.  The Rigol was a lot noisier than typical analog scopes, whereas the Siglent is comparable with a good basic CRO.  The Siglent also has a for-real 500uV/div capability and the Rigol actually only went down to something like 2mV/div (or perhaps 5mV/div) and then expanded the signal and decreased the resolution for the lower ranges, a sort of digital zoom.  They had a 500uV/div range if you hacked them, but it was pretty useless.

As for some previous responses, I disagree that the 10X probe will be noisier than the scope, that might apply to a very quiet scope but with either of these the scope front-end noise will be the issue when you are looking at small signals with a 10X probe.  And you'll almost always be using a 10X probe because 1X is very limited bandwidth, so a 'low' signal will be anything below 50mV.  So when I've used a 10X probe with scopes like this on small signals (the lower versions of Siglent and Rigol) the Siglent is clearly superior (3-4X better at least) but sometimes I wish it were even better.  Always remember the bandwidth limiter will help if your signals are below 20MHz.

There was a mention that the noise appears on all scales, not just the lowest volts/div settings.  There's some of each, I suppose you could call them the analog-front-end noise and the ADC noise.  The Siglent clearly has more noise on the lowest volt/div settings and above 2mV/div, the 'ADC' noise is pretty minimal.  The Rigol will be worse in this regard because of the digital expansion for the lowest ranges.

You asked about sample rate and bandwidth.  2GSa/s is just enough for 500MHz, but the Siglent only has that with two channels active and so the BW is limited with 4 channels.  The Rigol has more than adequate samples for its 350MHz bandwidth under any conditions.  I'm not sure how much this matters for your uses.

The memory configuration and lack of zoom-out on the Siglent is a baked in trait that I don't think is going to ever change.  I find it to not be problem, but it has annoyed some people who are used to a different configuration.  I'm also annoyed because I think its usability could be improved with trivial effort by reducing the whole-record display to a bar on the top.  This whole thing is less of a problem on the SDS2000X series because in zoom mode you are wasting 1/4 of a fairly large screen.  Earlier models were wasting 1/2 of a much smaller screen.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #14 on: December 23, 2021, 04:11:46 pm »
What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

On an old valve radio you might want to use a 100X probe, in which case a 'small signal' that would merit using the lowest range of the scope might be hundreds of millivolts.  With a 10X probe, which is what you will almost always use, you might start caring about front-end noise at 50mV.  Low front end noise gives you better FFT performance as well.  There are all sorts of cases where noise is an issue and if you are just starting out,  I can't predict which ones you will run into.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #15 on: December 23, 2021, 06:31:40 pm »
Ok, you all are giving me many usefull information, but now give a look to the attached photos.
The first image shows the apparent noise of CH1, It is set in AC mode, 1X attenuation, without bandwith limit

Never use 1x mode without the bandwidth limit.



nb. 10x mode is what you should be using almost all the time. 1x mode is only for very special cases.
« Last Edit: December 23, 2021, 06:35:28 pm by Fungus »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #16 on: December 23, 2021, 06:34:32 pm »
What circuits works with such a low signal?  This is my question from the beginning.

The example that's usually given is "power supply ripple", but unless you're designing a switched mode power supply then you probably don't need that.

Yes, and thick traces just suck. Try to make a cursor measurement on a trace that is 20% of a division even with V/div set to 1V/div. Needless to say that small variations of a signal are also lost in a noisy oscilloscope.

Put it in the middle of the trace.

Oscilloscopes aren't that accurate anyway - only about 5% even on a low-noise oscilloscope.

Edit: You can also turn on color gradient mode and the true signal will be highlighted for you.  ::)



(I just learned that trick from the video below and you can be sure it will be repeated in all future "Rigol noise" threads...  >:D )
« Last Edit: December 23, 2021, 07:06:29 pm by Fungus »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #17 on: December 23, 2021, 06:50:07 pm »
If the signal you're looking at is periodic (repeatiing) then you can turn on hires mode and look at the average of multiple waveforms:


« Last Edit: December 23, 2021, 06:59:49 pm by Fungus »
 

Offline egonotto

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #18 on: December 23, 2021, 07:39:53 pm »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?
When you're measuring very small signals.
Actually not because the relative noise floor remains more or less constant!
I just tried on an R&S RTM3004. At 1V/div I get 17mV stdev. At 10V/div I get 170mv stdev (Stdev= RMS with DC removed). The noise floor scales with the V/div setting.

Hello,
this is much better than on RTA4004. R&S says: (50 Ohm 1GHz)
1 V/div  31.4 mV

Best regards
egonotto
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #19 on: December 23, 2021, 07:42:20 pm »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?
When you're measuring very small signals.
Actually not because the relative noise floor remains more or less constant!
I just tried on an R&S RTM3004. At 1V/div I get 17mV stdev. At 10V/div I get 170mv stdev (Stdev= RMS with DC removed). The noise floor scales with the V/div setting.

Hello,
this is much better than on RTA4004. R&S says: (50 Ohm 1GHz)
1 V/div  31.4 mV
I switched the 20MHz bandwidth limit on in order to allow making comparisons with other scopes. The example is just to show that the noise floor scales with the V/div setting.
« Last Edit: December 23, 2021, 07:44:02 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #20 on: December 24, 2021, 04:04:59 am »
Noise level almost never matters because oscilloscopes are time domain instruments with two more significant limitations:

1. Noise is typically lower than the aberrations in the pulse response, and this is also a limit for instruments with resolution higher than 8 bits, so removing the noise just gives a clearer view of an inaccurate result where it counts.

2. Input stage offset and drift is usually greater than the noise.

And unlike the above errors, a digital storage oscilloscope can reduce noise with averaging.

Where noise does matter is spectrum analysis, but again, other limits like distortion have a greater effect.  Optimizing oscilloscope input stages for pulse response requires compromise in noise and distortion.

Good low noise performance for a 100 MHz instrument should be about 40 microvolts RMS based on a input JFET source follower with 3.2 nV/Sqrt(Hz) noise, and many old analog oscilloscopes and DSOs achieve this.  This is also why sensitivity below a couple millivolts/division is questionable except if bandwidth is limited.

For instance my 2232 DSO has 25 points per division so at 2 millivolts/division each point is 80 microvolts, and peak-to-peak noise is 3 points or 240 microvolts which comes out to 40 microvolts RMS.  I measured slight lower than this on some of my analog oscilloscopes under similar conditions, although my favorite analog oscilloscope has a noise level about 3 times higher than this.
« Last Edit: December 24, 2021, 04:20:31 am by David Hess »
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #21 on: December 24, 2021, 06:15:24 am »
So, with these settings:
1x probe, no BW limit, normal acquisition mode

I did other photos:
1) probe grounded to its tip
2) probe connected to a Power supply to check the ripple
3) no probe attached to the oscilloscope

What do you think about the noise floor?
It seem high to me....
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #22 on: December 24, 2021, 06:18:01 am »
Here same settings but hi-res mode.

 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #23 on: December 24, 2021, 06:38:26 am »
Can you try again only set the vertical scale to 1mV/div and measure with no probe and with the shorted probe?  Then do the same at 100mV/div.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #24 on: December 24, 2021, 06:55:13 am »
So, with these settings:
1x probe, no BW limit

Again: You're not supposed to use 1x with no BW limit. Ever.

1x with no BW limit = noise.

2) probe connected to a Power supply to check the ripple

You can see the ripple, right? That's what counts.

Try the same thing with the bandwidth limiter on.
« Last Edit: December 24, 2021, 07:41:42 am by Fungus »
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #25 on: December 24, 2021, 06:57:43 am »
For very small signals, use the FFT mode on the scope to find the amplitude. FFT does help to bring out signals from the noise (for periodic signals). For e.g. you could observe a 2mVrms sine wave with even 10mVrms of noise, by using FFT. 
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #26 on: December 24, 2021, 07:52:00 am »
Ok I have taken other photos but now with BW limit set to 20Mhz and the probe connected but grounded to its tip.

Photos:
1) 1x probe
2) 10x probe

« Last Edit: December 24, 2021, 08:01:41 am by Fiorenzo »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #27 on: December 24, 2021, 07:57:37 am »
Ok I have taken other photos but now with BW limit set to 20Mhz and the probe connected but grounded to its tip.

Photos:
1) 1x probe
2) 10x probe

What does it look like at 1v/div? That's what you'll normally be seeing when you're working.


 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #28 on: December 24, 2021, 08:01:55 am »
Fungus you asked me to what I am comparing the noise:
I saw some photos on the web of the siglent sds2000x+ and the measured RMS and peak to peak value, what I see on the Rigol seem to me many times higher....
Because I am no expert I cannot say the Rigol have too much noise and this is a problem for me. I am trying to understand/learn from you if It could be acceptable or a problem in some circumstance because I hope to do not change the oscilloscope for at least 5/10 years.
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #29 on: December 24, 2021, 08:03:17 am »
Ok I have taken other photos but now with BW limit set to 20Mhz and the probe connected but grounded to its tip.

Photos:
1) 1x probe
2) 10x probe

What does it look like at 1v/div? That's what you'll normally be seeing when you're working.

I didn't change the volt scale only the probe attenuation.

In the afternoon I am going to take the photos you asked, I cannot now.
 

Online Andreas

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #30 on: December 24, 2021, 08:45:41 am »
In what situation really matter to have an oscilloscope with a "low" noise floor?

The most annoying situation is when you use a FFT to detect small signals within a larger complex signal.
E.g. 50 Hz mains hum within a pre-amplified signal. (AC transformer too near to the setup creating a large error nearly not visible in the time domain).

E.g. actual Chopper frequencies and overlay of different stages (dependant on temperature) at the output of a Chopper stabilized OP-Amp. (had the problem that the measured noise increased suddenly at special temperatures due to interferences)

with best regards

Andreas

 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #31 on: December 24, 2021, 09:38:12 am »
For most uses the noise of the scope is not really an issue. This is because a 1:10 probe is often the mayjor noise source.  Unless the scope is rather high in noise, the noise floor is mainly an issue with the few measurements done with an 1:1 probe or similar signal via coax.
For digital signals itself the scopes noise is usually not an issue, but it may be when looking at the supply ripple.

For comparing the noise, one should take the bandwidth into account. With a higher BW the RMS noise naturally goes up. This effect can make a new higher BW scope look higher noise. So  one usually should compare at the same BW, like the usual 20 MHz BW setting.

2 Gs/s are a bit on the low end for 500MHz BW. This may lead to some aliasing and a few more thoughts about what one is actually seeing at the short time scale end.

Thank you Kleinstein for your reply.

What typology of circuits/signals need measurements with a 1x probe?

Does a higher sample rate give a better visual resolution of square wave signals at high frequency  or its benefit is reduced by the amount of noise of the front end?
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #32 on: December 24, 2021, 09:40:17 am »
For most uses the noise of the scope is not really an issue.
I strongly disagree with this. An oscilloscope with a lot of noise gives thick traces which make it hard to see the actual level of a signal. Ofcourse you can use high-res or bandwidth limiting but IMHO these should be targeted at cleaning up a signal and not masking a poorly designed analog front end. An oscilloscope with a low noise floor is easier to work with. I used to own an Agilent DSO7104A and it was just horrible to work with for analog stuff due to the massively thick traces it has due its own noise.

Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty.

Thank you, interesting.
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #33 on: December 24, 2021, 10:03:06 am »
This was one fo ther main reasons for purchasing Lecroy scopes, (we also have Rigol 8000's) which are great general scopes and have some cracking features, but for low noise rail voltage measurments (and 2 channel FFT's) that are consistant, accurate (gain accuracy of 0.5%) and reliable coupled with the superb selection of probes then Wavepro HD is the R&D go to. We alosmuse powerananlysers (both DC and AC which are again very accurate) along with a 6705C.

nctnico is on the money, scope accuracy is very important imho
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #34 on: December 24, 2021, 10:10:59 am »
For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?

When you're measuring very small signals.

I am going to use It for many different things, I am a "begginer" but I do digital stuff with embedded electronics and I also aim to learn more things as possible about analog electronics starting from working on an old valve radio that i would like to repair and experiment with.

It won't make any difference at all on your digital stuff.

For the radio? If it turns out to be a problem you can easily add a preamplifier and make it even better than a lower-noise oscilloscope.




So how much this matter in electronics? How this could preclude its usability?

It's not a showstopper. You can still do everything, just maybe not as easily for a few specific things.

The real questions are: How often do you do those things? How much would you have to spend to get a lower-noise 'scope with the same abilities as your Rigol? Is the extra money well-spent?

Yes I understand what you mean, but I don't have the knowledge to understand ​all the "abilities" of my scope.
My budget is <2000€ but if it is possible to spend less of 500€ It would be (offcourse) better because I could buy other equipments.
It Is a difficult decision because there are so many models of oscilloscopes and no one is perfect.
I would like to have the right instrument that will let me expand my knowledge and build circuits without particular constrains, maybe I am exagerating because I don't have a complete understanding of all the fields of electronics, but at the same time I do not expect to work with Ghz exotic electronics.
 

Offline Kleinstein

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #35 on: December 24, 2021, 10:15:06 am »
For the DSO there are 2 types of noise: noise relative to the input (e.g. input amplifier noise)  and noise relative to the output (ADC noise). For the noise relative to the input the amplifier a 10:1 probe can be a major noise source. The 1 M resistor in the divider has a natural noise of some 130 nV/sqrt(Hz), which is higher than a reasonable JFET based input stage (more like 10 nV/sqrt(hz) range). A low noise for the input is nice, but not relevant when using a 10:1 probe. It can be when using a 1:1 probe.
An 1:1 probe is mainly used when one rellay needs low noise for very small signals (e.g. noise at some point on a circuit, supply ripple). It comes with the price of reduced bandwidth (except with a special, expensive active probe), but with significant lower input noise (e.g. factor 10 from the divider ratio and an additional factor of some 3-10 from not having the resistor noise).

The lower gain settings usually use an internal input divider and if not designed good this may add some noise to 1 or 2 of the higher ranges, which can be a bit annoying as it is avoidable. So ideally a full noise testing would test all ranges, at least with 1 BW setting.

The output related noise, e.g. from the limited resolution  ADC noise, of just ADC noise because of a limited effective ADC resolution that may be lower.
For the ADC noise the sampling rate can make a difference. So the same DSO may look noisy at the higherst sampling rate but looks much better at a lower sampling rate when more samples are averaged. The noise relevant bandwidth is different from the 3 dB bandwith and can be quite a bit higher if the sampling rate is high, or closer to the -3dB BW when the sampling rate is barely sufficient. So it needs some case for the comparison to get comparable condictions (e.g. same sampling rate, relatively close to the maximum, like some 1 Gs/s for the scopes in question).
With the high speed scopes the ADC noise can be a factor.

This is mainly relevant with non repetitve (single trigger) relatively fast signals (so one needs a high sampling rate).
Spectrumanalysis (FFT) is also an example where the noise can matter.  Here also the way the math is done (e.g. does it support averaging, if so the unused time for the math) can make a difference.

Besides the noise of the raw data the methods available for noise reduction can also make a difference. So how good is the averaging over multiple triggers or bandwidth limiting working.

A higher sampling rate can reduce the artifacts from signal parts beyound the Nyquist limit (f_s / 2). With a relatively low sampling rate for a given BW it would need either some extra anti aliasing filtering that can add phase shifts or one has to accept some artifacts if the signal actually contains very fast components. At the same BW, the higher sampling rate scope would usually give a slightly more accurate response.
Noise whise the highest sampling rate often comes with more noise, but after averaging of consecutive samples (the usual way to reduce the data rate - though it can be done worse) the noise improves and thus no panelty from a faster ADC.


edit:  I just remembered: the 10:1 probe is resistive only for low frequencies (e.g. < 10 kHz) but capacitive at higher frequenices. So there is not that much extra noise from the divider and input amplifier noise can still be a factor with the 10:1 probe (at least in the lower ranges).
« Last Edit: December 24, 2021, 11:19:10 am by Kleinstein »
 
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Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #36 on: December 24, 2021, 10:35:16 am »
Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty.

Eventually it depends on the individual attenuator and front end design. For instance, I'm aware of a low-cost scope model where the relative noise levels are best at 100mV/div, 1V/div and 10V/div; already a bit worse at 50mV/div, 500mV/div and 5V/div; even worse at 20mV/div, 200mV/div and 2V/div; and finally getting more and more worse at 10mV/div, 5mV/div and 2mV/div

 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #37 on: December 24, 2021, 12:12:21 pm »
Which kind of signal need an oscilloscope with a low noise frontend?

A 1mV signal.

(for example)
[/quote]

What kind of electronics works with such low intensity signals?
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #38 on: December 24, 2021, 12:29:55 pm »
What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

On an old valve radio you might want to use a 100X probe, in which case a 'small signal' that would merit using the lowest range of the scope might be hundreds of millivolts.  With a 10X probe, which is what you will almost always use, you might start caring about front-end noise at 50mV.  Low front end noise gives you better FFT performance as well.  There are all sorts of cases where noise is an issue and if you are just starting out,  I can't predict which ones you will run into.

Thank you for all your explanations. Very usefull. I am trying to understand: what kind of electronics work with such kind of low signals?
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #39 on: December 24, 2021, 01:42:30 pm »
What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

On an old valve radio you might want to use a 100X probe, in which case a 'small signal' that would merit using the lowest range of the scope might be hundreds of millivolts.  With a 10X probe, which is what you will almost always use, you might start caring about front-end noise at 50mV.  Low front end noise gives you better FFT performance as well.  There are all sorts of cases where noise is an issue and if you are just starting out,  I can't predict which ones you will run into.

Thank you for all your explanations. Very usefull. I am trying to understand: what kind of electronics work with such kind of low signals?
Few but if ever needing to measure low values indeed noise can get in the way.
When probing high impedance circuits and connection can effect the circuit operation and as 10x probes are most commonly used a 1mV/div scope setting need be used for a 10mV signal however it will only be displayed ~1div high.
To take this to extremes a popular tablet DSO has just 50mv/div max sensitivity which when coupled with a 10x probe permits only 500mV/div sensitivity which is useless for anything but the simplest of tasks.

When we need higher sensitivity 1x is used but at the expense of higher probe capacitance loading on the circuit so such use is often restricted to low impedance measurements like the ripple on a DC rail or that of a PSU.

When most move to a DSO forgetting to set the input attenuation to match the probe is a common newbie error instead of letting the scope display the correct measured value. This is where probe autosense like in the other DSO you are looking at is of substantial value.
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Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #40 on: December 24, 2021, 02:05:21 pm »
What typology of circuits/signals need measurements with a 1x probe?

20 MHz AC coupled power supply noise and ripple measurements work well with a 1x probe.  This is actually specified in the ATX power supply standard.  A 50 ohm cable can be used in place of a 1x probe but it will deliver worse performance.

Audio measurements are another area where 1x probes are useful.

Quote
Does a higher sample rate give a better visual resolution of square wave signals at high frequency  or its benefit is reduced by the amount of noise of the front end?

It does give better fidelity but how it affects noise depends on the implementation.  High end instruments now use an ADC which can trade sample rate with noise and resolution, but for most, sample rate has no effect.  The reason for this is that if the ADC is not doing noise shaping, then it operates at a constant (maximum) sample rate and different sample rates are produced by discarding samples during decimation, which has no effect on noise within the input bandwidth of the ADC.

Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty.

Most DSOs these days only have a single input divider.  Older DSOs have at least two which allows the input buffer to operate over 1/10th of input range so the input full power bandwidth does not limit performance.

Separately there is a low impedance output divider, usually now in the form of a PGA (programmable gain amplifier), with its own noise characteristics.  At high sensitivity noise is dominated by the input buffer, and at low sensitivity noise is dominated by the preamplifier and ADC.

For the noise relative to the input the amplifier a 10:1 probe can be a major noise source. The 1 M resistor in the divider has a natural noise of some 130 nV/sqrt(Hz), which is higher than a reasonable JFET based input stage (more like 10 nV/sqrt(hz) range). A low noise for the input is nice, but not relevant when using a 10:1 probe.

For a typical tip capacitance of a 10x probe, the 1 megohm shunt resistance is in parallel with about 100 picofarads of compensation capacitance producing a noise bandwidth of only 2.5 kHz, so its noise contribution is only about 6.5 microvolts RMS over a wide bandwidth which is close to insignificant.

For the same reason, the noise contribution from the roughly 500 kilohm resistance in series with the gate of the input transistor for protection adds basically no noise.  It is bypassed with about 1000 picofarads reducing its noise bandwidth to an insignificant level.

Quote
The lower gain settings usually use an internal input divider and if not designed good this may add some noise to 1 or 2 of the higher ranges, which can be a bit annoying as it is avoidable. So ideally a full noise testing would test all ranges, at least with 1 BW setting.

The internal high impedance input dividers are also capacitively compensated limiting their high frequency noise.  The output dividers are low impedance so require no compensation, but have low noise anyway.  In a modern DSO, these are part of the PGA.

Quote
For the ADC noise the sampling rate can make a difference. So the same DSO may look noisy at the higherst sampling rate but looks much better at a lower sampling rate when more samples are averaged. The noise relevant bandwidth is different from the 3 dB bandwith and can be quite a bit higher if the sampling rate is high, or closer to the -3dB BW when the sampling rate is barely sufficient. So it needs some case for the comparison to get comparable condictions (e.g. same sampling rate, relatively close to the maximum, like some 1 Gs/s for the scopes in question).

With the high speed scopes the ADC noise can be a factor.

I have only seen high end DSOs take advantage of noise shaping in the ADC.  There is probably some effect on noise for DSOs which use an interleaved ADC for multiple channels.

Some old DSOs with relatively low real time sample rates, like 100s of MSamples/second, have less ADC noise (and preamplifier noise) than the quantization noise of their ADC.  When I first saw this on my 2232, I thought something was broken or misconfigured.  This might actually be considered a disadvantage when averaging where added noise would produce a better result and I have actually seen this happen with the averaged signal producing a stair-step from the ADC's quantization noise.
« Last Edit: December 24, 2021, 02:09:10 pm by David Hess »
 

Offline tautech

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #41 on: December 24, 2021, 02:07:07 pm »
Here's a simple example where the noise increases at the higher input sensitivities required to properly scale the higher attenuation probes.
Done some years back and grabbed from an old post.
SDS1104X-E with 1x, 10x, 100x and 1000x probes all connected to the probe Cal output.

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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #42 on: December 24, 2021, 07:17:44 pm »
What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

On an old valve radio you might want to use a 100X probe, in which case a 'small signal' that would merit using the lowest range of the scope might be hundreds of millivolts.  With a 10X probe, which is what you will almost always use, you might start caring about front-end noise at 50mV.  Low front end noise gives you better FFT performance as well.  There are all sorts of cases where noise is an issue and if you are just starting out,  I can't predict which ones you will run into.

Thank you for all your explanations. Very usefull. I am trying to understand: what kind of electronics work with such kind of low signals?
Few but if ever needing to measure low values indeed noise can get in the way.
When probing high impedance circuits and connection can effect the circuit operation and as 10x probes are most commonly used a 1mV/div scope setting need be used for a 10mV signal however it will only be displayed ~1div high.
To take this to extremes a popular tablet DSO has just 50mv/div max sensitivity which when coupled with a 10x probe permits only 500mV/div sensitivity which is useless for anything but the simplest of tasks.

When we need higher sensitivity 1x is used but at the expense of higher probe capacitance loading on the circuit so such use is often restricted to low impedance measurements like the ripple on a DC rail or that of a PSU.

When most move to a DSO forgetting to set the input attenuation to match the probe is a common newbie error instead of letting the scope display the correct measured value. This is where probe autosense like in the other DSO you are looking at is of substantial value.

Thank you tautech and everyone again for the time spent replying. I find everything you wrote very usefull and interesting
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #43 on: December 24, 2021, 07:30:31 pm »
What typology of circuits/signals need measurements with a 1x probe?

20 MHz AC coupled power supply noise and ripple measurements work well with a 1x probe.  This is actually specified in the ATX power supply standard.  A 50 ohm cable can be used in place of a 1x probe but it will deliver worse performance.

Audio measurements are another area where 1x probes are useful.

Quote
Does a higher sample rate give a better visual resolution of square wave signals at high frequency  or its benefit is reduced by the amount of noise of the front end?

It does give better fidelity but how it affects noise depends on the implementation.  High end instruments now use an ADC which can trade sample rate with noise and resolution, but for most, sample rate has no effect.  The reason for this is that if the ADC is not doing noise shaping, then it operates at a constant (maximum) sample rate and different sample rates are produced by discarding samples during decimation, which has no effect on noise within the input bandwidth of the ADC.

Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty.

Most DSOs these days only have a single input divider.  Older DSOs have at least two which allows the input buffer to operate over 1/10th of input range so the input full power bandwidth does not limit performance.

Separately there is a low impedance output divider, usually now in the form of a PGA (programmable gain amplifier), with its own noise characteristics.  At high sensitivity noise is dominated by the input buffer, and at low sensitivity noise is dominated by the preamplifier and ADC.

For the noise relative to the input the amplifier a 10:1 probe can be a major noise source. The 1 M resistor in the divider has a natural noise of some 130 nV/sqrt(Hz), which is higher than a reasonable JFET based input stage (more like 10 nV/sqrt(hz) range). A low noise for the input is nice, but not relevant when using a 10:1 probe.

For a typical tip capacitance of a 10x probe, the 1 megohm shunt resistance is in parallel with about 100 picofarads of compensation capacitance producing a noise bandwidth of only 2.5 kHz, so its noise contribution is only about 6.5 microvolts RMS over a wide bandwidth which is close to insignificant.

For the same reason, the noise contribution from the roughly 500 kilohm resistance in series with the gate of the input transistor for protection adds basically no noise.  It is bypassed with about 1000 picofarads reducing its noise bandwidth to an insignificant level.

Quote
The lower gain settings usually use an internal input divider and if not designed good this may add some noise to 1 or 2 of the higher ranges, which can be a bit annoying as it is avoidable. So ideally a full noise testing would test all ranges, at least with 1 BW setting.

The internal high impedance input dividers are also capacitively compensated limiting their high frequency noise.  The output dividers are low impedance so require no compensation, but have low noise anyway.  In a modern DSO, these are part of the PGA.

Quote
For the ADC noise the sampling rate can make a difference. So the same DSO may look noisy at the higherst sampling rate but looks much better at a lower sampling rate when more samples are averaged. The noise relevant bandwidth is different from the 3 dB bandwith and can be quite a bit higher if the sampling rate is high, or closer to the -3dB BW when the sampling rate is barely sufficient. So it needs some case for the comparison to get comparable condictions (e.g. same sampling rate, relatively close to the maximum, like some 1 Gs/s for the scopes in question).

With the high speed scopes the ADC noise can be a factor.

I have only seen high end DSOs take advantage of noise shaping in the ADC.  There is probably some effect on noise for DSOs which use an interleaved ADC for multiple channels.

Some old DSOs with relatively low real time sample rates, like 100s of MSamples/second, have less ADC noise (and preamplifier noise) than the quantization noise of their ADC.  When I first saw this on my 2232, I thought something was broken or misconfigured.  This might actually be considered a disadvantage when averaging where added noise would produce a better result and I have actually seen this happen with the averaged signal producing a stair-step from the ADC's quantization noise.

Mr. David you gave a lot of explanations, this is very kind and usefull.

Trying to do a recap: at this point It seem to me that a "low noise" oscilloscope is important when working with FFT analysis, power supply ripple, audio signal, and high impedance circuits?

In regard of the mso5000 with its high sample rate of 8GSa/s I am not sure if in the balance It is an advantage due to its apparently noisy front end.
As an ignorant, at the beginning I thought: the Rigol is better because It has better specs, so I bought it. Could you suggest me a different model if you think It could be better?
« Last Edit: December 24, 2021, 07:50:23 pm by Fiorenzo »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #44 on: December 24, 2021, 07:40:03 pm »
In regard of the mso5000 with its high sample rate of 8GSa/s I am not sure if in the balance It is an advantage due to its apparently noisy front end.

It definitely is. It will help reduce the Gibbs Phenomenon on your digital circuits.

(Ever wonder how "ringing" can occur before a signal starts to rise? Undersampling combined with sin(x)/x reconstruction...)
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #45 on: December 24, 2021, 07:41:06 pm »
Could you suggest me a different model if you think It could be better?

At the same price? Not a chance.

You'll have to go up quite a lot in price to get something better. Think: The difference would buy a good multimeter, a soldering iron and a bench power supply.

(And the oscilloscope still wouldn't be perfect, you'd just have other things to worry about)

OTOH you could even go down to a 500 Euro oscilloscope like the Siglent SDS1204X-E. It's probably all you need and you'd have enough money left over to create an awesome lab.
« Last Edit: December 24, 2021, 08:05:16 pm by Fungus »
 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #46 on: December 24, 2021, 07:50:14 pm »
It definitely is. It will help reduce the Gibbs Phenomenon on your digital circuits.

(Ever wonder how "ringing" can occur before a signal starts to rise? Undersampling combined with sin(x)/x reconstruction...)

Meh, ringing on step responses isn't usually Gibbs unless something has been designed or set wrong.  You can have ringing (and even pre-ringing) on an analog scope. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #47 on: December 24, 2021, 08:03:33 pm »
Meh, ringing on step responses isn't usually Gibbs unless something has been designed or set wrong.

It can be a mixture of both.

You can have ringing (and even pre-ringing) on an analog scope.

But you can't have Gibbs.   ::)
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #48 on: December 24, 2021, 08:07:55 pm »
Could you suggest me a different model if you think It could be better?

At the same price? Not a chance.

You'll have to go up quite a lot in price to get something better. Think: The difference would buy a good multimeter, a soldering iron and a bench power supply.

(And the oscilloscope still wouldn't be perfect, you'd just have other things to worry about)

OTOH you could even go down to a 500 Euro oscilloscope like the Siglent SDS1204X-E. It's probably all you need and you'd have enough money left over to create an awesome lab.

Yes I know, this is one of my doubts. I will check some review of that scope but in the mean time what do you think about its bigger brother sds2000x-plus?
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #49 on: December 24, 2021, 08:20:51 pm »
Can you try again only set the vertical scale to 1mV/div and measure with no probe and with the shorted probe?  Then do the same at 100mV/div.

Here I am, I did the test you asked for, please let me know whath do you think about It.
Thank you very kind!

Photos:
1) 1mv/div, no probe, 20Mhz BW limit, 1x
2) 1mv/div, probe, 20 MHz BW limit, 1x
3) 100mv/div, no probe, all the same
4) 100mv/div, probe, all the same
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #50 on: December 24, 2021, 08:48:05 pm »
OTOH you could even go down to a 500 Euro oscilloscope like the Siglent SDS1204X-E. It's probably all you need and you'd have enough money left over to create an awesome lab.

Yes I know, this is one of my doubts. I will check some review of that scope but in the mean time what do you think about its bigger brother sds2000x-plus?

Me? For that money I'd start with the little brother plus 1000 Euros of other toys(!). When you know more about oscilloscopes and their limitations you can decide if you want to sell it and buy the bigger one.
« Last Edit: December 24, 2021, 09:09:08 pm by Fungus »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #51 on: December 24, 2021, 08:51:21 pm »
4) 100mv/div, probe, all the same


Looks flat enough to me.  Nothing to be unhappy about.

What about 1v/div with 10x probe?
« Last Edit: December 24, 2021, 08:56:25 pm by Fungus »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #52 on: December 24, 2021, 08:56:52 pm »
Also try averaging mode:



(image from section 4-5 of the manual)

Averaging can be used with any periodic waveform, ie. almost all waveforms where noise might be an issue, eg. measuring ripple.
« Last Edit: December 24, 2021, 09:01:04 pm by Fungus »
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #53 on: December 24, 2021, 09:12:21 pm »
With 1mv/div It gives about 160mV RMS and 1,2mV peak to peak. Whatching other scopes they seem to have about 4 time less noise.
Am I wrong?

About me I am not rich, but I would like to buy a quite good oscilloscope.

I dislike to spend a lot of money ofcourse, but in my experience with quality instruments you get the best results from what you are doing so maybe a big effort now could be a big help tomorrow, or maybe It Is better that now I spend a little amount of money and wait for more evolute oscilloscopes in the future....

If the 1000 series is good for me I take It, if It Is better the Rigol or the 2000 series i will go for It but at the moment I am unsure on which way to take.

You speack about other usefull instruments, what could I buy?
I have the solder station and some fixed voltage psu and one psu I have made by my self with 4 indipendent regulable output but not very precise....
 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #54 on: December 24, 2021, 09:17:11 pm »
Here I am, I did the test you asked for, please let me know whath do you think about It.
Thank you very kind!

So just as a quick apples-to-apples comparison with your first example, I set up an SDS1104X-E with the same no-probe, 20MHz BW, 5us/div and 1mv/div.  Then I connected a 1mVrms 100kHz signal so you can see the suitability of the scope for viewing such a signal.  The noise makes it a bit jumpy and jittery because it affects the trigger point.  I have some low-frequency noise that isn't from the scope but is from other stuff that I have around that I can't turn off right now, so I did both a live snapshot and a stopped picture, which is effectively a single-shot.  As you can see, the scope is perfectly usable on 1mV signals and your Rigol, which is over 3X as noisy, might have a tough time with the same signal.

I also took a snapshot with the input set to GND, which shows you the ADC noise, or so I think.  The LSB at this setting is 40uV, so the peak-to-peak noise is 1 bit. 

I can't tell you what scope works best for you, but if you work at all with small signals, the front-ends of the entry level Siglents is definitely the best for the buck.  Features like the ERES acquisition mode and the 10-bit mode of the SDS2000X+ series do make them better, but you shouldn't generally count on multi-capture averaging even though it can look very good, as this can lead to errors.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #55 on: December 24, 2021, 09:37:20 pm »
With 1mv/div It gives about 160mV RMS and 1,2mV peak to peak. Whatching other scopes they seem to have about 4 time less noise.
Am I wrong?

It's not a question of right/wrong, it's a question of whether it makes a difference.

There's a whole thread here saying "not as much as you think".

About me I am not rich, but I would like to buy a quite good oscilloscope.

Of course.

If the 1000 series is good for me I take It, if It Is better the Rigol or the 2000 series i will go for It but at the moment I am unsure on which way to take.

It's impossible to define "better" when there's almost 50% price difference.  :-//

Is a car with leather seats "better" than a car which doesn't have them? You can argue that it is but most people still do the shopping with ordinary seats and can fit just as much in the car.

You speack about other usefull instruments, what could I buy?
I have the solder station and some fixed voltage psu and one psu I have made by my self with 4 indipendent regulable output but not very precise....

What multimeters do you own? Do you have space for a bench multimeter? A nice bench multimeter will be far more useful than a Siglent 2000 series vs. a Siglent 1000 series. A good handheld meter is also good (and both together is even better).

A good power supply is also more useful. Not so much for providing power but for setting current limits and experimenting with "will this run at XX volts".

You can also afford to get the logic analyzer add-on for your Siglent 1000 series. That will be great for your digital work.

What solder station have you got? A 100 Euro solder station is much better than a 20 Euro station.

Hot air gun for desoldering?

Thermal camera? More useful than you might think...
« Last Edit: December 24, 2021, 09:57:21 pm by Fungus »
 

Offline tautech

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #56 on: December 24, 2021, 09:42:27 pm »

You can also afford to get the logic analyzer add-on for your Siglent 1000 series. That will be great for your digital work.

Don't, it's an abomination get the SDS2104X+ and SPL2016 instead. Totally different and better class of MSO probe.
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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #57 on: December 24, 2021, 09:47:49 pm »
Don't, it's an abomination

Tautech says something made by Siglent is an abomination? Avoid like the plague.  :scared:

(I haven't actually used one, I sorta assumed it would work for 300 Euros given that the rest of the 'scope is OK)

get the SDS2104X+ and SPL2016 instead. Totally different and better class of MSO probe.

And ... totally blows the budget. That's the tautech we know and love.  :)

Edir: I'm only kidding, OP did mention "2000 Euros" somewhere and it's not quite 2000...
« Last Edit: December 24, 2021, 10:28:26 pm by Fungus »
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #58 on: December 24, 2021, 10:06:26 pm »
Hahahaha you all good guys!
I like you
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #59 on: December 24, 2021, 10:16:52 pm »
Hahahaha you all good guys!
I like you

Have you tried averaging mode yet?  :popcorn:
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #60 on: December 24, 2021, 10:19:40 pm »
So, I did some test, just to learn and experiment.

Same crappy psu, two probes ON in DC mode, one disconnected and the pink one probe on the psu output.
First photo 10x attenuation, second photo 1x.

The ripple of the psu is about 150mv, now the front end noise seem to make a good part of the noise saw on the ripple wave, high resolution or avarage get rid of a big part of this noise. If I did not be aware of the noisy front end i could think that the noise in the ripple was from the psu itself.
Second if i work with the probe in 1x mode the ripple Is now very evident.
Is this the right way to check It? I mean in 1x mode.
Should i use AC or DC mode?
I have read that AC mode get rid of the DC bias so good to work in this way...
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #61 on: December 24, 2021, 10:24:16 pm »
Hahahaha you all good guys!
I like you

Have you tried averaging mode yet?  :popcorn:

Yes many times, and also hi-res mode.
I didn't used It in the photos because i wanted to show the real front end noise.

Actually i really like this rigol, Is a powerfull scope, but i have never put my hands on the siglents
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #62 on: December 24, 2021, 10:34:17 pm »
If I did not be aware of the noisy front end i could think that the noise in the ripple was from the psu itself.

Ripple can only be at the switching frequency of the PSU, never in the MHz range.

Second if i work with the probe in 1x mode the ripple Is now very evident.
Is this the right way to check It? I mean in 1x mode.

Yes. 1x mode with 20MHz bandwidth limiter and maximum averaging.

Should i use AC or DC mode?
I have read that AC mode get rid of the DC bias so good to work in this way...

AC will center the ripple waveform around zero so you measure RMS, etc.

(ie. it's good)

« Last Edit: December 24, 2021, 10:39:17 pm by Fungus »
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #63 on: December 24, 2021, 10:37:11 pm »
Here I am, I did the test you asked for, please let me know whath do you think about It.
Thank you very kind!

So just as a quick apples-to-apples comparison with your first example, I set up an SDS1104X-E with the same no-probe, 20MHz BW, 5us/div and 1mv/div.  Then I connected a 1mVrms 100kHz signal so you can see the suitability of the scope for viewing such a signal.  The noise makes it a bit jumpy and jittery because it affects the trigger point.  I have some low-frequency noise that isn't from the scope but is from other stuff that I have around that I can't turn off right now, so I did both a live snapshot and a stopped picture, which is effectively a single-shot.  As you can see, the scope is perfectly usable on 1mV signals and your Rigol, which is over 3X as noisy, might have a tough time with the same signal.

I also took a snapshot with the input set to GND, which shows you the ADC noise, or so I think.  The LSB at this setting is 40uV, so the peak-to-peak noise is 1 bit. 

I can't tell you what scope works best for you, but if you work at all with small signals, the front-ends of the entry level Siglents is definitely the best for the buck.  Features like the ERES acquisition mode and the 10-bit mode of the SDS2000X+ series do make them better, but you shouldn't generally count on multi-capture averaging even though it can look very good, as this can lead to errors.

Ahhahaah bdunham7 with the same settings i get almost 5 times your noise. The Rigol really like noise....
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #64 on: December 24, 2021, 10:40:29 pm »
If I did not be aware of the noisy front end i could think that the noise in the ripple was from the psu itself.

Ripple can only be at the switching frequency of the PSU, never in the MHz range.

Second if i work with the probe in 1x mode the ripple Is now very evident.
Is this the right way to check It? I mean in 1x mode.

Yes. 1x mode with 20MHz bandwidth limiter and averaging mode enabled.

Should i use AC or DC mode?
I have read that AC mode get rid of the DC bias so good to work in this way...

AC will center the ripple waveform around zero so you measure RMS, etc.

(ie. it's good)

Thank you fungus you are helping me very well.

And also merry Christmas in advance to everyone!!
 

Offline tautech

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #65 on: December 24, 2021, 10:41:51 pm »
Don't, it's an abomination

Tautech says something made by Siglent is an abomination? Avoid like the plague.  :scared:

(I haven't actually used one, I sorta assumed it would work for 300 Euros given that the rest of the 'scope is OK)
It works fine but it becomes another box on the bench instead of being inbuilt which would be difficult with the very compact SDS****X-E series.
get the SDS2104X+ and SPL2016 instead. Totally different and better class of MSO probe.

And ... totally blows the budget. That's the tautech we know and love.  :)

Edir: I'm only kidding, OP did mention "2000 Euros" somewhere and it's not quite 2000...
SPL2016 with a new scope are on special ATM until at least years end for the unbeatable price of $219 with MSO and FG licensing. That's one hell of a deal unless you want to DIY one with the info from that thread.

Christmas midday dinner calls.......
« Last Edit: December 24, 2021, 10:44:53 pm by tautech »
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #66 on: December 24, 2021, 10:47:50 pm »
Here I am, I did the test you asked for, please let me know whath do you think about It.
Thank you very kind!

So just as a quick apples-to-apples comparison with your first example, I set up an SDS1104X-E with the same no-probe, 20MHz BW, 5us/div and 1mv/div.  Then I connected a 1mVrms 100kHz signal so you can see the suitability of the scope for viewing such a signal.  The noise makes it a bit jumpy and jittery because it affects the trigger point.  I have some low-frequency noise that isn't from the scope but is from other stuff that I have around that I can't turn off right now, so I did both a live snapshot and a stopped picture, which is effectively a single-shot.  As you can see, the scope is perfectly usable on 1mV signals and your Rigol, which is over 3X as noisy, might have a tough time with the same signal.

I also took a snapshot with the input set to GND, which shows you the ADC noise, or so I think.  The LSB at this setting is 40uV, so the peak-to-peak noise is 1 bit. 

I can't tell you what scope works best for you, but if you work at all with small signals, the front-ends of the entry level Siglents is definitely the best for the buck.  Features like the ERES acquisition mode and the 10-bit mode of the SDS2000X+ series do make them better, but you shouldn't generally count on multi-capture averaging even though it can look very good, as this can lead to errors.

What circuits works with such a low intensity signals?
Sensors for example?
I could think about maybe a shunt, they gives very low output when measuring currents, i use many times shunts in my circuits....

Speaking instead about a signal amplifier, as suggested by some one in this thread, It was told that they are a better way to check low intensity signals than giving faith on a low noise front end.

Is this true all times?
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #67 on: December 24, 2021, 10:52:15 pm »
AC will center the ripple waveform around zero so you measure RMS, etc.

...which is of course the wrong way to do it, and was the big error in the now-infamous "Rigol vs. Siglent noise" video.

Using AC mode to calculate RMS assumes the 'scope is perfectly calibrated, ie. with zero offset voltage.

The correct way to measure RMS is with "Std.Dev", which removes the offset voltage mathematically in both AC and DC modes.



You should still use AC mode though because it centers the signal and allows you to crank up the vertical scale without going off screen.
« Last Edit: December 24, 2021, 10:57:51 pm by Fungus »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #68 on: December 24, 2021, 10:56:16 pm »
Have you tried averaging mode yet?

Yes many times, and also hi-res mode.
I didn't used It in the photos because i wanted to show the real front end noise.

Averaging mode isn't cheating! It's there for a reason...  :-+

(And even Siglents have it)
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #69 on: December 24, 2021, 10:58:52 pm »
Watch this photo attached.

If i don't ground the probe tip the oscilloscope shows a 50hz sinewave. I have also tried turning off every appliance connected near the scope and also the lights but the sine wave remain unchanged.
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #70 on: December 24, 2021, 11:12:54 pm »
I could think about maybe a shunt, they gives very low output when measuring currents, i use many times shunts in my circuits....

Speaking instead about a signal amplifier, as suggested by some one in this thread, It was told that they are a better way to check low intensity signals than giving faith on a low noise front end.
For such purposes you better use a pre-amplifier with a differential input. You can easely DIY these using an instrumentation amplifier chip or a specific current sensing amplifier chip. From there feed the amplified signal into your oscilloscope.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #71 on: December 24, 2021, 11:26:59 pm »
Don't, it's an abomination

Tautech says something made by Siglent is an abomination? Avoid like the plague.  :scared:

(I haven't actually used one, I sorta assumed it would work for 300 Euros given that the rest of the 'scope is OK)
It works fine but it becomes another box on the bench instead of being inbuilt which would be difficult with the very compact SDS****X-E series.
get the SDS2104X+ and SPL2016 instead. Totally different and better class of MSO probe.

And ... totally blows the budget. That's the tautech we know and love.  :)

Edir: I'm only kidding, OP did mention "2000 Euros" somewhere and it's not quite 2000...
SPL2016 with a new scope are on special ATM until at least years end for the unbeatable price of $219 with MSO and FG licensing. That's one hell of a deal unless you want to DIY one with the info from that thread.

Christmas midday dinner calls.......

Yes i really need a complete mso now because i am designing an e-bike controller from scratch and also working with the display circuitry and program that i am also designing from the bottom. I did similar things in the past, but without an oscilloscope and had to cope with the problems encountered only with the help of the theory and literature due to the fact that i couldn't check my circuits... But i got to the end fine everytime. Such a satisfaction...
Sorry for my bad english...

The offer from siglent is very appealing and i don not have the time now to make the probes by my self.
But i could instead buy a cheaper scope and an external LA to connect on the computer, maybe this  is the best way to proceed at the moment
Ummm... But i am not sure what way i will take hahaha....

Actually i saw that the AWG of the siglent and rigol are limited and an external signal generator could be better.... But i like that the Rigol has two outputs.
I should try a siglent but Amazon don't sell It here in Italy, only the lower models....
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #72 on: December 24, 2021, 11:28:18 pm »
I could think about maybe a shunt, they gives very low output when measuring currents, i use many times shunts in my circuits....

Speaking instead about a signal amplifier, as suggested by some one in this thread, It was told that they are a better way to check low intensity signals than giving faith on a low noise front end.
For such purposes you better use a pre-amplifier with a differential input. You can easely DIY these using an instrumentation amplifier chip or a specific current sensing amplifier chip. From there feed the amplified signal into your oscilloscope.

Thank you i will do It
 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #73 on: December 24, 2021, 11:48:31 pm »
What circuits works with such a low intensity signals?
Sensors for example?
I could think about maybe a shunt, they gives very low output when measuring currents, i use many times shunts in my circuits....

Speaking instead about a signal amplifier, as suggested by some one in this thread, It was told that they are a better way to check low intensity signals than giving faith on a low noise front end.

Yes--sensors, audio circuit, measuring circuits, even RF circuits.  And keep in mind as I said before that you may often want to use a 10X or even 100X probe or custom voltage divider to avoid loading the circuit you are testing and that will magnify the problem. 

Here's the scope with a 10X probe connected to a 50-ohm terminated signal generator with no signal running, no signal single-shot and then the same 1mV signal as before.  Note that I had to use the single-shot to capture the signal as it is so low that the scope won't trigger on it, however it is still clearly visible above the noise. 

As far as preamplifiers, yes they have a place but they are usually fairly specific and limited, as opposed to being general purpose.  For example they typically have a much more limited input voltage range or bandwidth, or both.

« Last Edit: December 25, 2021, 02:57:17 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #74 on: December 24, 2021, 11:54:09 pm »
Fiorenzo,

Can you post a screenshot of your PSU ripple with 20MHz limiter and maximum averaging?

 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #75 on: December 25, 2021, 12:40:56 am »
Meh, ringing on step responses isn't usually Gibbs unless something has been designed or set wrong.

It can be a mixture of both.

You can have ringing (and even pre-ringing) on an analog scope.

But you can't have Gibbs.   ::)

I have wondered about that.  Preshoot in analog oscilloscopes comes from the delay line implemented as either a lumped-element transmission line or counter braided differential coaxial line.  Both have a sharp cutoff frequency and faster propagation of high frequencies than low frequencies, but that sharp cutoff reminds me of the truncated Fourier coefficients which lead to the Gibb's phenomena.

With 1mv/div It gives about 160mV RMS and 1,2mV peak to peak. Whatching other scopes they seem to have about 4 time less noise.
Am I wrong?

It is possible but as I pointed out, noise is not the only consideration.  My preferred oscilloscope has about 120 microvolts RMS input noise because it comes with high input common mode range and differential inputs which are more useful, similar to the AM502 mentioned below.

Trying to do a recap: at this point It seem to me that a "low noise" oscilloscope is important when working with FFT analysis, power supply ripple, audio signal, and high impedance circuits?

Like I wrote earlier, except for FFTs where noise is a direct limitation, it is not very important.  Oscilloscopes are noisy because of the compromises they have to make.

When making measurements, other sources of error usually overwhelm noise.  Ground loops with single ended probes and the probe ground lead will pick up all kinds of noise in excess of an oscilloscope's front end noise.  This suggests that money is better spent on better probes than an oscilloscope with the lowest possible noise which cannot be taken advantage of anyway.

Power supply ripple is separate from power supply noise, and a DSO can be triggered and use averaging to remove the noise and keep the ripple.  Measuring power supply noise on the other hand will often require a low noise preamplifier which is not difficult to build.

In audio applications, an oscilloscope has so much distortion that only gross measurements will be accurate, so noise is not relevant.

If you are interested in general purposes low noise measurements within a 1 MHz bandwidth on any oscilloscope, then you might find a Tektronix AM502 differential amplifier to be useful.  They are easy to repair because unique parts can be sourced from the Tektronix 5A22 and 7A22, and they work well with 1x oscilloscope probes.  Using one does mean acquiring a TM500 series power supply mainframe though.

Quote
In regard of the mso5000 with its high sample rate of 8GSa/s I am not sure if in the balance It is an advantage due to its apparently noisy front end.
As an ignorant, at the beginning I thought: the Rigol is better because It has better specs, so I bought it. Could you suggest me a different model if you think It could be better?

Noise specifications are generally lacking so it is difficult to make a recommendation based on them.

Something to consider about the 70 MHz Rigol MSO5072/MSO5074 is that it is bandwidth upgradable to 350 MHz via firmware meaning that it has the higher noise 350 MHz front end whether the bandwidth is limited to 70 MHz or not.  So it must have a higher noise than the example I gave of 100 MHz oscilloscopes.

What is going on here is that for a given device technology, higher bandwidth yields higher noise density.  So for instance a JFET front end which supports a bandwidth of 100 MHz could have a noise density of 3.5 nV/Sqrt(Hz), while a JFET front end which supports 350 MHz would have a noise density several times higher, whether the bandwidth is limited or not.  For the same bandwidth, a lower bandwidth oscilloscope can have lower noise than a higher bandwidth oscilloscope.

I suspect that is a major part of what is going on with the relatively high noise of the MSO5072/MSO5074.  Its noise should be compared to other 350 MHz instruments even though it is limited to 70 MHz.

 
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Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #76 on: December 25, 2021, 01:36:00 am »
It is possible but as I pointed out, noise is not the only consideration.  My preferred oscilloscope has about 120 microvolts RMS input noise because it comes with high input common mode range and differential inputs which are more useful, similar to the AM502 mentioned below.

I suspect that is a major part of what is going on with the relatively high noise of the MSO5072/MSO5074.  Its noise should be compared to other 350 MHz instruments even though it is limited to 70 MHz.

I also would put up with a bit of noise to get differential inputs.  The isolated scopes that I have are also a bit noisier and I don't complain.

As for the Rigols, I really think the issue is how they manage the lowest ranges by using a digital expansion of a higher range.  Other scopes would have an additional 10X analog gain.  The scope I used for comparison is 200MHz+, the more comparable SDS2000X+ models are 500MHz+ and have slightly better noise levels than mine.  Comparing 300-500MHz class scopes with the old Tek 2465B that I have--which gets very close to what you said is the ideal noise level-- the better Siglent is maybe 50% noisier (hard to compare because the Tek doesn't have 1mV or 500uV/div settings), the cheap Siglent is 2X as noisy and the Rigol is about 8X.  It's like going from a fine-tip ball point pen to a Sharpie and then to a highlighter or magic marker.   Of course that is not using  ERES or 10-bit or any other DSO tricks. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #77 on: December 25, 2021, 11:24:56 am »
My background is RF so I generally don't use a scope very often but when I do it is nice to look at small signals.

I have a really old HP Infinium digital scope with 500MHz bandwidth and it has the noise performance I would expect. At full 500MHz bandwidth and with the input set to 50R termination and 1mV/div it shows about 100uV rms noise when the Vrms measurement is enabled. When fed with noise as the signal under test it can typically measure wideband or narrowband noise signals with acceptable results down to about 200uVrms. This scope is fairly limited in terms of features (and memory depth) compared to modern scopes but it does at least have the noise performance I'd expect from a scope like this.


« Last Edit: December 25, 2021, 11:32:10 am by G0HZU »
 

Offline knudch

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #78 on: December 25, 2021, 03:37:57 pm »
Random Noise is one thing..

If you @Fiorenzo would try on your Rigol a thing like this:
(done 50ohm termination)

Discrete "false signals is something else
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #79 on: December 25, 2021, 05:43:59 pm »
Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

My advice to Fiorenzo is to consider something else if the poor noise performance of the Rigol is bothering you. That level of noise would put me off buying a Rigol scope although I doubt I would ever buy one anyway.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #80 on: December 25, 2021, 06:11:48 pm »
Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.

Does your HP have waveform averaging mode?

At least it looks like your HP can zoom out.

 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #81 on: December 25, 2021, 07:19:24 pm »
I have a really old HP Infinium digital scope with 500MHz bandwidth and it has the noise performance I would expect. At full 500MHz bandwidth and with the input set to 50R termination and 1mV/div it shows about 100uV rms noise when the Vrms measurement is enabled. When fed with noise as the signal under test it can typically measure wideband or narrowband noise signals with acceptable results down to about 200uVrms. This scope is fairly limited in terms of features (and memory depth) compared to modern scopes but it does at least have the noise performance I'd expect from a scope like this.

When a low impedance input is used, higher bandwidth oscilloscopes bypass the high impedance buffer and without that, the input noise can be much lower.  The high impedance buffer has to be bypassed because at some point it will not have enough bandwidth.

An example of this in old oscilloscopes is the venerable Tektronix 485 which was built at a time when the fastest high impedance buffers were 250 to 300 MHz.  In 50 ohm mode, instead of inserting a 50 ohm feedthrough termination before the high impedance buffer, a coaxial relay directs the signal around the high impedance buffer.

Later oscilloscopes managed high input impedance up to 500 MHz and I think some now manage 1 GHz.
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #82 on: December 25, 2021, 07:25:44 pm »
Quote
What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.
Does your HP have waveform averaging mode?

I'm not quite sure what info you are asking for but this is a really old HP scope using very dated technology. The basic specs are 2GSa/s and 500MHz bandwidth across 4 channels. Yes it has an averaging mode but it isn't turned on. The 30MHz bandwidth limit is turned on for the screenshot in my previous post.
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #83 on: December 25, 2021, 07:30:59 pm »
Quote
When a low impedance input is used, higher bandwidth oscilloscopes bypass the high impedance buffer and without that, the input noise can be much lower.  The high impedance buffer has to be bypassed because at some point it will not have enough bandwidth.

An example of this in old oscilloscopes is the venerable Tektronix 485 which was built at a time when the fastest high impedance buffers were 250 to 300 MHz.  In 50 ohm mode, instead of inserting a 50 ohm feedthrough termination before the high impedance buffer, a coaxial relay directs the signal around the high impedance buffer.

Later oscilloscopes managed high input impedance up to 500 MHz and I think some now manage 1 GHz.

If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.
 

Offline oz2cpu

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #84 on: December 25, 2021, 07:41:25 pm »
some scopes are just badly designed, full of switchmode own noise..
the good old Rigol 1054 that we almost all owned ..
here i posted a few pictures, look and cry

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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #85 on: December 25, 2021, 07:51:23 pm »
If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.

There's no point. Everybody here knows (and freely admits) that lower noise oscilloscopes exist. They make for pretty screenshots and youtube videos.

The questions is: How much advantage does it give you in real life?

For digital signals? None at all. For that you need bandwidth and high sample rates which is where the MSO5000 shines.

For periodic signals in the mV range? I suspect the answer is "not much if you use averaging", hence me asking if anybody can post a picture of a Rigol MSO5000 showing power supply ripple with 1x probe, 20MHz limiter and waveform averaging. (Prove me wrong!)



For non-periodic signals in the mV range? You'd have an advantage there but they're few and far between and you're probably better off looking for them in FFT mode than trying to trigger on them and view them as a trace.

nb. For any signal in the mV range you can add a signal amplifier. They sell 30dB amplifiers on aliexpress for not much money.
« Last Edit: December 25, 2021, 08:03:50 pm by Fungus »
 

Offline Kleinstein

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #86 on: December 25, 2021, 08:34:41 pm »
The x1 probe is not always an option: it is slow (e.g. 5-10 MHz BW) and quite some load (e.g. 100 pF range).
With a x1 proble the probe will limit the BW, but it still makes sense to enable the 20 MHz limit to reduce amplifier noise, as there is essentially no signal > 20 MHz anyway.

With a non periodic signal the FFT is not an option. It would not give much (if any) useful information.
Averaging only works well if one has a good signal to trigger from - so if there is only a small / noisy signal this will not help.
Noise in the trigger signal can smoothen out the signal with averaging.

With the rigol scope still at hand, one could measure the noise, to see if it is really much worse, or just looking higher noise with higher sampling rate.
A point to compare would be with a short (input to GND), a time scale to get a comparable sampling rate (e.g. 1 Gs/s and thus BW limited by the sampling rate to 250 MHz) with only 1 channel active and than a high gain (e.g. 1 mV/div or 5 mV/div before the probe setting).

Some digital signals like LVDS are not that large: 400 mV at the input would be 40 mV after a 10:1 probe and thus may want 5 mV/div sensitivity.
 

Offline Kleinstein

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #87 on: December 25, 2021, 08:38:19 pm »
Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.

Does your HP have waveform averaging mode?

At least it looks like your HP can zoom out.

The picture show 10 Ms/s. So there is anditional BW limit (~ 5 MHz) there. To do a fair comparison one would have the switch the faster scope also a slower hirizontal rate to get the lower sampling rate.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #88 on: December 25, 2021, 08:42:21 pm »
Averaging only works well if one has a good signal to trigger from - so if there is only a small / noisy signal this will not help.

If the ripple is too small to trigger from then the power supply is probably OK and there's nothing to worry about.

 

Offline oz2cpu

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #89 on: December 25, 2021, 08:51:35 pm »
my siglent, looks like same signal as yours fungus
clearly alot less noise, but this is only a 2GS scope :-)
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Offline Kleinstein

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #90 on: December 25, 2021, 08:54:19 pm »
Averaging only works well if one has a good signal to trigger from - so if there is only a small / noisy signal this will not help.

If the ripple is too small to trigger from then the power supply is probably OK and there's nothing to worry about.

It is not just triggering, just a stable triggger to see fast parts too.
Often there is a stable trigger available, but one may have to find it. An extra trigger and than looking at the ripply would even separate contricbutions to the ripple if there are multiple asyncronous parts (e.g. mains and a SMPS).

It really depends on how much ripple you still care. Sometime 0.1 mV could be too much.
 

Offline oz2cpu

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #91 on: December 25, 2021, 09:06:44 pm »
1mV div, here is where you really see the internal noise :-)
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Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #92 on: December 25, 2021, 09:42:58 pm »
Quote
The picture show 10 Ms/s. So there is anditional BW limit (~ 5 MHz) there. To do a fair comparison one would have the switch the faster scope also a slower hirizontal rate to get the lower sampling rate.

I'm not sure there will be a 5MHz bandwidth limit. With the 30MHz limiter enabled I think the bandwidth limit for signals is still 30MHz on this scope even at low sample rates. One would have to be wary of aliasing but the signal being viewed here is noise.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #93 on: December 25, 2021, 09:43:36 pm »
It is not just triggering, just a stable triggger to see fast parts too.
Often there is a stable trigger available, but one may have to find it.

Yep. No arguments there. That's why I was wondering if OP can actually do it in practice.

He already posted a screenshot of his ripple here so let's see what the 'scope is capable of. It looks like a triggerable signal to me but I don't have an MSO5000 to play with..


 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #94 on: December 25, 2021, 09:49:05 pm »
If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.

There's no point. Everybody here knows (and freely admits) that lower noise oscilloscopes exist. They make for pretty screenshots and youtube videos.

The questions is: How much advantage does it give you in real life?

For digital signals? None at all. For that you need bandwidth and high sample rates which is where the MSO5000 shines.

For periodic signals in the mV range?
:palm: You keep taking the wrong turn when only focussing on small signals. Your screenshot clearly shows a wide band of noise where any detail on any signal level is lost. The noise is easely 40% of a division. With 8 divisions and 8 bits you have 32 LSB per division but with 40% noise you might as well have a 5 bit ADC and still get the same result. Needing to use averaging or high-res mode is just a crutch when the noise comes from the DSO itself.

There are so many lower noise alternatives out there from GW Instek, MicSig and (if you have the budget) R&S as well that it makes no sense to buy a noisy Rigol at all.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #95 on: December 25, 2021, 10:04:35 pm »
There are so many lower noise alternatives out there from GW Instek, MicSig and (if you have the budget) R&S as well that it makes no sense to buy a noisy Rigol at all.

Except for the Pesky Fact that OP says he mainly does digital stuff and nobody else makes a 4 channel, 350Mhz, 8GSample/sec 'scope for only $1000.

To me it seems like you're arguing that a car with leather seats will allow people to carry more shopping.
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #96 on: December 25, 2021, 10:10:33 pm »
I think it’s time to do some myth-busting – and providing some hard evidence instead of presenting just wild guesses.


Myth #1: “Noise is only important when using x1 probes. It is irrelevant when using the much more common x10 probes, because the high source impedance will exceed the noise of the DSO anyway.”

Nothing could be further from the truth. The only relevant effect is the attenuation of the probe, which requires an appropriate vertical gain on the DSO to compensate for it. And of course this won’t matter much at the low sensitivities, i.e. the noise will be about the same with a x1 probe at 10 V/div, a x10 probe at 1 V/div and a x100 probe at 100 mV/div. But if you happen to work with signals much lower than 80 Vpp, you will notice that 1 mV/div with a x100 probe will be noisier than 100 mV/div with a x1 probe and that in a scenario like this, a proper low noise frontend will be much more pleasant to work with. You sure don’t want the excessive noise of an e.g. Rigol MSO5000 when working with x100 probes (e.g. in vintage tube gear).

The output impedance of a x10 probe is dominated by an output capacitance of about 100 pF, therefore the noise bandwidth is only about 1.6 kHz. So except for very low frequencies <100 kHz, we won’t see any significant difference in a properly designed general purpose DSO frontend, whether the scope input is left open, terminated by 1 M or 50 ohms or shorted to ground.

For low frequencies, things are a lot more complex than just a FET buffer, because of the split path design of all contemporary wideband frontend designs. The practical consequence is, that general purpose (wideband) oscilloscopes generally aren’t well suited for low frequency tasks below about 10 kHz regardless of the probes used. There are specialized instruments for this.

Look at the first two screenshots attached. They show the noise spectrum up to 1 GHz of the Siglent SDS2354X (570 MHz bandwidth). First with the input left open in high impedance mode, then the input internally terminated by 50 ohms. There are minimal changes of the spurious signals (because of the different contributions of voltage- and current effects), but the noise changes by less than 1 dB within the 570 MHz bandwidth of the scope frontend.

SDS2354X Plus_FFT_Noise_1M_ BW570M_8bit
SDS2354X Plus_FFT_Noise_50_BW570M_8bit

Btw: please notice, that up to 1 GHz there are few spurious signals and no spur is exceeding -120 dBV, which is equivalent to 1 µVrms. This is another important aspect, because strong spurs near the signal frequency can be at least as annoying as excessive noise.


Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

In any modern DSO, the bandwidth limit is an integral function of the PGA – and it sits at its output. So all the input noise gets filtered before the ADC. Of course this is only a first order RC-filter, because other than some popular believe, there is also no such thing as an effective AA-filter (Anti Aliazing filter) in a serious DSO. The most important property of any DSO frontend that is not a toy is constant group delay, and this rules out any “effective” AA-filter.

But even with a humble first order filter, the effect of noise reduction is quite obvious.

Next screenshot shows the noise floor at 50 ohms input termination again, but this time with 200 MHz input bandwidth limit. Btw, Siglent scopes show all relevant information on the screen, so screenshots should be pretty much self-explanatory.

SDS2354X Plus_FFT_Noise_50_BW200M_8bit

Compare this with the previous screenshot. At 110 MHz, we already have a difference of 0.8 dB. At 340 MHz it is nearly 6 dB and 7 dB at 560 MHz. That is a difference, isn’t it?

The next screenshot demonstrates what happens if the common 20 MHz bandwidth limiter is activated.

SDS2354X Plus_FFT_Noise_50_BW20M_8bit

At 20 MHz, noise is 2.7 dB down, we get -10.9 dB at 110 MHz, -16 dB at 340 MHz and -17.6 dB at 560 MHz.

The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.

SDS2354X Plus_FFT_Noise_50_BW100M_10bit

With this setting, the noise floor fell below the -150 dBV above 200 MHz, so the reference level of the spectrum alanysis had to be adjusted accordingly. Little change up to 110 MHz, but -18.7 dB at 340 MHz and -26.8 dB at 560 MHz make this an excellent low noise mode in applications where 100 MHz bandwidth is sufficient.

Of course we can use the 20 MHz bandwidth limiter here as well, as the next screenshot demonstrates.

SDS2354X Plus_FFT_Noise_50_BW20M_10bit

At frequencies above 20 MHz, noise drops dramatically: About -13.5 dB at 110 MHz, -28.8 dB at 340 MHz and -31.6 dB at 560 MHz.

So this proves that it has nothing to do with the genuine bandwidth of the frontend or any other signal paths. I can easily demonstrate that e.g. a modern 2 GHz DSO like the SDS6204 behaves no different in this regard. The following screenshot shows a noise plot of the 2 GHz scope that can be compared to the very first screenshot in this posting.

SDS6204_FFT_Noise_1M_BW2G_D1G

This is in high impedance mode with open input. Once again, the noise with internal 50 ohms termination is very similar. More interesting is the comparison of this 2 GHz scope with the SDS2354X Plus. Even though the high bandwidth scope produces more spurs (but only very few of them slightly exceed 1 µVrms), the noise is comparable or mostly even better than on its 500 MHz counterpart – within the operating bandwidth of the latter, that is. Of course, at 840 MHz the SDS2304X Plus is already in the stopband of the frontend and noise drops significantly, whereas the SDS6000 has not even reached half its bandwidth, so its noise at that frequency has to be in the same ballpark as the other measurements before.

What these screenshots also reveal, is that the sample rate does not affect (excessive) frontend noise. If anything, higher sample rate helps to reduce noise. The 5 GSa/s, 2 GHz scope produces less noise than its 2 GSa/s, 500 MHz counterpart.

There is the ADC noise itself, which is the granular noise determined solely by the ADC resolution, not the sample rate. A higher sample rate will spread out the noise energy over a wider bandwidth, but the total energy will remain the same. So if only a limited part of that bandwidth is observed (i.e. FFT zoom feature), only a part of the noise is visible, hence will appear lower than the total noise actually is.

High sample rates also do not accentuate high frontend noise.

If the sample rate is excessive with regard to the bandwidth of the frontend, it will just produce redundant data and pointlessly eat up sample memory with little to no effect on the result.

If, on the other hand, the bandwidth of the frontend exceeds half the sample rate, the noise portion above Nyquist will be aliased back into the Nyquist bandwidth, hence we’ll still get all the frontend noise, even with inadequate low sample rates.

« Last Edit: December 26, 2021, 06:30:09 am by Performa01 »
 
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Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #97 on: December 25, 2021, 10:38:47 pm »
Quote
If, on the other hand, the bandwidth of the frontend exceeds half the sample rate, the noise portion above Nyquist will be aliased back into the Nyquist bandwidth, hence we’ll still get all the frontend noise, even with inadequate low sample rates.

Yes, that's what I find with my old HP scope here when the 30MHz bandwidth limit is selected. It still measures the noise level fine even at very low sample rates. It doesn't matter at all because the signal being measured is noise. The same applies if I deliberately feed wideband noise to the scope front end and use a very low sample rate to try and measure the Vrms of the noise. As long as the bandwidth of the external noise signal is less than the 30MHz bandwidth of the scope it should measure the Vrms quite well even at 100kSa/s.



 
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Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #98 on: December 25, 2021, 10:44:12 pm »
Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

I think the statement, at least the one I'm thinking of, was that the noise density was higher for higher-BW capable amplifiers.  The noise will still be a function of the noise density and the actual bandwidth, so limiting BW will  still reduce noise as expected.

Quote
In any modern DSO, the bandwidth limit is an integral function of the PGA – and it sits at its output. So all the input noise gets filtered before the ADC. Of course this is only a first order RC-filter, because other than some popular believe, there is also no such thing as an effective AA-filter (Anti Aliazing filter) in a serious DSO. The most important property of any DSO frontend that is not a toy is constant group delay, and this rules out any “effective” AA-filter.

I believe that many very-high-BW scopes now have more than a first-order roll off and thus a flatter frequency response but poorer step response. IIRC either Tek or HPAK or both had/have models where you specify which way you want it as a factory option.  I don't think this is specifically for anti-aliasing, but it will have that effect.  I can't be very specific because stuff like that doesn't get into my hands much--I have to read about it here on EEVBlog.
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Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #99 on: December 26, 2021, 07:12:27 am »
Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

I think the statement, at least the one I'm thinking of, was that the noise density was higher for higher-BW capable amplifiers.  The noise will still be a function of the noise density and the actual bandwidth, so limiting BW will  still reduce noise as expected.
Well, the FFT plots in my screenshots show nothing but the noise density. Of course, the total noise is actually higher for the 2 GHz instrument at full bandwidth than what it could ever be on the SDS2kX Plus.

I strongly suggest that, other than in the seventies of the last century, for modern semiconductors the noise density remains fairly constant over frequency. In my screenshots it can be seen that it gets rather lower at higher frequencies and four times the system bandwidth doesn’t mean higher noise density at all.

Quote
In any modern DSO, the bandwidth limit is an integral function of the PGA – and it sits at its output. So all the input noise gets filtered before the ADC. Of course this is only a first order RC-filter, because other than some popular believe, there is also no such thing as an effective AA-filter (Anti Aliazing filter) in a serious DSO. The most important property of any DSO frontend that is not a toy is constant group delay, and this rules out any “effective” AA-filter.

I believe that many very-high-BW scopes now have more than a first-order roll off and thus a flatter frequency response but poorer step response. IIRC either Tek or HPAK or both had/have models where you specify which way you want it as a factory option.  I don't think this is specifically for anti-aliasing, but it will have that effect.  I can't be very specific because stuff like that doesn't get into my hands much--I have to read about it here on EEVBlog.
Yes, the genuine bandwidth of most scopes is not first order gaussian – unless they are artificially bandwidth limited. This is also why we don’t get ideal pulse response characteristics and vendors have to specify some overshoot.

There can be all sorts of filters in high end scopes – mainly for pulse response equalization, but you can have higher order AA-filters as well. But this is limited to filters with Gaussian and Bessel characteristics, where the transition from the passband to the stopband is very smooth, so you still need substantial oversampling in order to get a useful attenuation to fight aliasing. Nothing gained for todays top models within a series, where the bandwidth is not at least five times lower than the sample rate.

Furthermore, higher order filters can get tricky with regard to their sensitivity to component tolerances (and their temperature stability) and they might need alignment in the first place. All this increases effort and costs and puts the risk of different frequency/phase response in different DSO channels.

This is probably why standard PGAs for “normal off the shelf” instruments have the bandwidth limiter integrated and designers have decided that a first order RC-filter is the best compromise.

 

Offline rf-loop

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #100 on: December 26, 2021, 07:19:18 am »


I believe that many very-high-BW scopes now have more than a first-order roll off and thus a flatter frequency response but poorer step response. I

As is also of course in not-very-high-BW  Siglent SDS6k.
"poorer" step response...  what can also turn to "less aliasing" step response.
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Offline jonpaul

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #101 on: December 26, 2021, 10:38:55 am »
Bonjour just seen this long thread now.

Suggest the OP check the many fine books and papers on noise, measurement, reduction, definition.

The noise "floor" is a function of the resistance, bandwidth, temperature.
Averaging is possible only on repetitive signals.
Noise may be irrelevant in some digital systems, but or primary importance in fine instrumentation, audio, etc.
Think of microphone preamps, seismic pickups, photomultipliers, etc.

A  digital scope and analog are "different animals" and have various benefits and downsides.

Many options for preamps, diff amps, etc. The best we have seen are the TEK 7000  plugin 7A22.

Finally the Chine scopes  may have misleading specs and hidden faults,   the cheapest implementation.

We have used the classic Tektronix scopes since 1967.

Just my reflections!

Bon Chance,

Jon




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Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #102 on: December 26, 2021, 11:09:48 am »
The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.

The spectrum reminds me on the typical frequency response of a 8-tap moving average filter (possibly in addition to other filters).
Is this the well-known HiRes mode, or yet a different mode?
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #103 on: December 26, 2021, 11:34:57 am »
Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”

I think the statement, at least the one I'm thinking of, was that the noise density was higher for higher-BW capable amplifiers.  The noise will still be a function of the noise density and the actual bandwidth, so limiting BW will  still reduce noise as expected.

Well, the FFT plots in my screenshots show nothing but the noise density. Of course, the total noise is actually higher for the 2 GHz instrument at full bandwidth than what it could ever be on the SDS2kX Plus.

I strongly suggest that, other than in the seventies of the last century, for modern semiconductors the noise density remains fairly constant over frequency. In my screenshots it can be seen that it gets rather lower at higher frequencies and four times the system bandwidth doesn’t mean higher noise density at all.

For a given transistor technology and construction, there is a tradeoff between bandwidth and noise density, so for instanced a 2N3822 JFET supporting a bandwidth up to 115 MHz (1) has a noise density of about 3.5 nV/Sqrt(Hz) while a 2N4416 JFET supporting a bandwidth up to 250 MHz has a noise density of about 6 nV/Sqrt(Hz).  If the later is used in a 100 MHz amplifier, it results in higher noise than the lower performance part even with the same bandwidth.

Further, in general MOSFETs are noisier than JFETs which are noisier than bipolar transistors, which may be an issue with modern instruments which are more likely to rely on RF MOSFETs instead of RF JFETs for their input buffer.  It is difficult to make an analytical comparison here even if we know what part is being used because the RF MOSFETs are not as well characterized for noise.  This also means that the noise from a bipolar stage following the high impedance input buffer should be of no significance.

The above does not apply to higher bandwidth instruments that use exotic and effective unavailable to us technologies.  Specialized transistors on exotic processes will have a completely different figure of merit for bandwidth and noise compared to silicon MOSFETs, JFETs, and bipolar transistors, but the general rule about the tradeoff between them still applies.  But these inexpensive DSOs up to 350 or maybe even 500 MHz are not using anything like that.

And of course none of the above says anything about poor design.  Noise could be in excess of the predicted front end noise for lots of different reasons.  Empirical measurement is king here and easy to do in this case if the oscilloscope can report peak-to-peak or AC RMS (standard deviation) measurements.  (2)

(1) As a source follower where Ft = Gm / (2 Pi C); the transistor used for the high impedance buffer needs high transconductance and low capacitance.  High transconductance reduces noise, up to a point where other internal noise sources dominate, but the construction for low capacitance increases it.

(2) Which reminds me to suggest being a little cagey about Rigol's RMS and standard deviation measurements on a noise waveform, or any instrument which makes measurements on the display record.  I have seen evidence in that past that the processing to produce the display record corrupts these measurements when applied to noise.
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #104 on: December 26, 2021, 11:38:41 am »
Quote
The picture show 10 Ms/s. So there is anditional BW limit (~ 5 MHz) there. To do a fair comparison one would have the switch the faster scope also a slower hirizontal rate to get the lower sampling rate.

I'm not sure there will be a 5MHz bandwidth limit. With the 30MHz limiter enabled I think the bandwidth limit for signals is still 30MHz on this scope even at low sample rates. One would have to be wary of aliasing but the signal being viewed here is noise.

At low sample rates the noise within the ADC input bandwidth simply gets aliased to lower frequencies.  The total noise remains the same.

For low frequencies, things are a lot more complex than just a FET buffer, because of the split path design of all contemporary wideband frontend designs. The practical consequence is, that general purpose (wideband) oscilloscopes generally aren’t well suited for low frequency tasks below about 10 kHz regardless of the probes used. There are specialized instruments for this.

Split path high impedance buffers started showing up not long after integrated low input bias current operational amplifiers in the 1970s.  The split path actually reduces low frequency noise because even a noisy operational amplifier has lower flicker noise than the RF FET used for the high impedance buffer.  Sometimes it is a lot lower.

The disadvantage of the split path design is that without careful consideration, overload recovery can be horrible.

The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.

The spectrum reminds me on the typical frequency response of a 8-tap moving average filter (possibly in addition to other filters).
Is this the well-known HiRes mode, or yet a different mode?

High resolution mode is usually or always implemented as a boxcar averaging filter for simplicity since it must operate at the maximum sample rate during decimation, so it should produce something like a sinc response which is what is shown.
« Last Edit: December 26, 2021, 11:42:17 am by David Hess »
 

Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #105 on: December 26, 2021, 12:38:08 pm »
High resolution mode is usually or always implemented as a boxcar averaging filter for simplicity since it must operate at the maximum sample rate during decimation, so it should produce something like a sinc response which is what is shown.

In the screeshot, obviously a 8-tap boxcar averaging filter (or similar) was applied, but without down-sampling, otherwise the 3 side-lobes were no longer visible in the spectrum, but already folded down to the first Nyquist zone of the lower sampling rate. So I was just wondering, whether this was really "HiRes" mode (in the sense of LeCroy's definition), or rather a different mode which just applies a post-acquisition filter.

Indeed, when it must run in real-time, during acquision, then a boxcar filter has of course the simplicity advantage that it can be implement as CIC filter, not requiring any multiplications.
Given the huge memory depths available today, a scope manufacturer may be tempted, though, to renounce capture-time DSP at all, and support only post-acquisition filters, for cost reasons.
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #106 on: December 26, 2021, 12:45:46 pm »
For low frequencies, things are a lot more complex than just a FET buffer, because of the split path design of all contemporary wideband frontend designs. The practical consequence is, that general purpose (wideband) oscilloscopes generally aren’t well suited for low frequency tasks below about 10 kHz regardless of the probes used. There are specialized instruments for this.

Split path high impedance buffers started showing up not long after integrated low input bias current operational amplifiers in the 1970s.  The split path actually reduces low frequency noise because even a noisy operational amplifier has lower flicker noise than the RF FET used for the high impedance buffer.  Sometimes it is a lot lower.

The disadvantage of the split path design is that without careful consideration, overload recovery can be horrible.
Yes, split path input buffer have been invented a long time ago – and it’s all the more baffling that most people don’t seem to be aware of it and make it sound as if an oscilloscope frontend still consists of a cascade of differential amplifiers. Maybe some even think it consists of just a high speed OpAmp…

If you actually think the LF noise in a split path design would be reduced, you’re forgetting that the LF path has to be attenuated quite a bit (usually up to 10 times) in order to get the desired input protection and a decent offset compensation range. This has to be compensated for by a corresponding gain in the OpAmp. Together with the high source impedance of the divider (which has to have a total resistance of 1 meg) this can raise the noise floor by more than 20 dB below the crossover frequency.

So there is no way around the sad fact, that the usual general purpose DSO isn’t well suited for precision work at low frequencies because of the steeply rising noise floor down there.
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #107 on: December 26, 2021, 12:57:16 pm »
The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.

The spectrum reminds me on the typical frequency response of a 8-tap moving average filter (possibly in addition to other filters).
Is this the well-known HiRes mode, or yet a different mode?

It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC.

 
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Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #108 on: December 26, 2021, 08:12:51 pm »
Indeed, when it must run in real-time, during acquision, then a boxcar filter has of course the simplicity advantage that it can be implement as CIC filter, not requiring any multiplications.
Given the huge memory depths available today, a scope manufacturer may be tempted, though, to renounce capture-time DSP at all, and support only post-acquisition filters, for cost reasons.

The implementations I have seen all used a power-of-2 number of samples so the filter could be implemented with only adds and shifts, and if promoting 8-bit acquisitions to a 16-bit record, only adds.  Modern low end DSOs usually only produce an 8-bit acquisition record but all of the old Tektronix DSOs promoted 8 and 10 bit samples to 16-bits immediately and did all processing in 16-bits.  Tektronix was very scrupulous at one time.
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #109 on: December 26, 2021, 09:13:48 pm »
Yes, split path input buffer have been invented a long time ago – and it’s all the more baffling that most people don’t seem to be aware of it and make it sound as if an oscilloscope frontend still consists of a cascade of differential amplifiers. Maybe some even think it consists of just a high speed OpAmp…

Differential amplifiers are still routine and the highest performance digitizers have differential inputs.  Usually the first stage after the low impedance attenuators converts from single ended to differential, and this stage is convenient for adding the combined position and offset signal is introduced.

The various modern PGAs used in oscilloscopes are differential so they follow the same pattern, but since they replace the low impedance attenuators, position and offset are added after.  DSOs with a separate offset control will add it before the PGA.  Old designs which do this have to somehow add the offset before some of the attenuation stages which means moving some of the attenuators to the differential part of the signal chain which is relatively expensive.

Quote
If you actually think the LF noise in a split path design would be reduced, you’re forgetting that the LF path has to be attenuated quite a bit (usually up to 10 times) in order to get the desired input protection and a decent offset compensation range. This has to be compensated for by a corresponding gain in the OpAmp. Together with the high source impedance of the divider (which has to have a total resistance of 1 meg) this can raise the noise floor by more than 20 dB below the crossover frequency.

That is a good point that I had forgotten, but the noise can still be lower even in old designs.

Old designs which have two separate x10 high impedance attenuators limit the input range to the buffer to 1/10th the level of new DSOs, so attenuation on the DC path is also lower.  The Tektronix 22xx series only attenuates by 1.33.

Luckily for the discussion here, low frequency noise is irrelevant because wideband noise at 20 MHz and higher bandwidths dominates.

Quote
So there is no way around the sad fact, that the usual general purpose DSO isn’t well suited for precision work at low frequencies because of the steeply rising noise floor down there.

I agree but if you include older instruments, then some general purposes DSOs are much better than others at low and/or high frequencies.  I have not tested enough modern low end DSOs to know if they all have subpar noise performance.  Even with older instruments though, I gave up on good low noise performance a long time ago with the exception of anything with the Tektronix 5A22/7A22/AM502.

At low frequencies it is relatively easy to make a low noise amplifier, but since oscilloscopes lack the noise marker function for their FFT, I would like to have a low noise dynamic signal analyzer instead.
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #110 on: December 26, 2021, 09:16:42 pm »
It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC.

Old Tektronix DSOs used 16-bit acquisition and processing memory so high resolution mode did not halve the record length.
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #111 on: December 27, 2021, 01:14:52 am »
Yes, split path input buffer have been invented a long time ago – and it’s all the more baffling that most people don’t seem to be aware of it and make it sound as if an oscilloscope frontend still consists of a cascade of differential amplifiers. Maybe some even think it consists of just a high speed OpAmp…

Differential amplifiers are still routine and the highest performance digitizers have differential inputs.  Usually the first stage after the low impedance attenuators converts from single ended to differential, and this stage is convenient for adding the combined position and offset signal is introduced.

The various modern PGAs used in oscilloscopes are differential so they follow the same pattern, but since they replace the low impedance attenuators, position and offset are added after.  DSOs with a separate offset control will add it before the PGA.  Old designs which do this have to somehow add the offset before some of the attenuation stages which means moving some of the attenuators to the differential part of the signal chain which is relatively expensive.
It doesn’t make much sense to get philosophic about obsolete designs. We are talking about general purpose DSOs here, which ranges from entry level (low end) up to the midrange, but excludes high end gear, which is specialized and definitely not general purpose. At one point, at least after the invention of the digital readout, T&M industry noticed that a minimum of DC accuracy and stability was expected. Users were no longer willing to permanently turn the offset control of their scopes just to center the trace, as they used to do with their ancient CROs, but expected a decently stable offset position and some accuracy. So, the split path design has long become universal for all general purpose DSOs – despite its drawbacks, where the most obvious is the overload recovery issue. And this is unavoidable, even by a good design.

Of course we find the cascaded differential stages in almost every HF IC, and in HF instruments like spectrum analyzers it might well be the only amplifier architecture required, but split path has become common in wideband general purpose oscilloscopes since they are supposed to work from DC up to the specified bandwidth.

Btw, there are folks who have managed to build a balanced version of the split path input buffer, so you can have this with balanced inputs too.


Quote
If you actually think the LF noise in a split path design would be reduced, you’re forgetting that the LF path has to be attenuated quite a bit (usually up to 10 times) in order to get the desired input protection and a decent offset compensation range. This has to be compensated for by a corresponding gain in the OpAmp. Together with the high source impedance of the divider (which has to have a total resistance of 1 meg) this can raise the noise floor by more than 20 dB below the crossover frequency.

That is a good point that I had forgotten, but the noise can still be lower even in old designs.

Old designs which have two separate x10 high impedance attenuators limit the input range to the buffer to 1/10th the level of new DSOs, so attenuation on the DC path is also lower.  The Tektronix 22xx series only attenuates by 1.33.

Luckily for the discussion here, low frequency noise is irrelevant because wideband noise at 20 MHz and higher bandwidths dominates.
It’s not “old designs” that utilize two input attenuators. Of course you cannot build a good scope with vertical gain settings from 500 µV/div up to 10 V/div with just one single attenuator. For instance, every contemporary Siglent DSO has two input attenuator stages. Offset compensation voltage has to be added to the input in order to be effective (otherwise the input stage would require a totally unrealistic high common mode range), so this is part of the LF path of a split path input buffer design and topologically sits between the attenuators and the PGA.

With low attenuation factors you either need high supply rails (old design) or you get only a very low offset compensation range. But does a Tek 22xx even have a split path design? The specifications of up to one division trace shift for variable gain and trace invert make me wonder. All the more so as the best sensitivity is not particularly high at 2 mV/div. Or maybe they use the cheapest FET-OpAmp with high Offset voltage and -drift without self-calibration in the LF path – but this would somehow scotch the whole idea of the split path approach?

Above some 100 kHz the situation eases a lot and at 10 MHz and above we get noise figures in the realm of 2 – 3.5 nV/sqrt(Hz) with proper designs at least from Rohde & Schwarz, LeCroy and Siglent.


Quote
So there is no way around the sad fact, that the usual general purpose DSO isn’t well suited for precision work at low frequencies because of the steeply rising noise floor down there.

I agree but if you include older instruments, then some general purposes DSOs are much better than others at low and/or high frequencies.  I have not tested enough modern low end DSOs to know if they all have subpar noise performance.  Even with older instruments though, I gave up on good low noise performance a long time ago with the exception of anything with the Tektronix 5A22/7A22/AM502.

At low frequencies it is relatively easy to make a low noise amplifier, but since oscilloscopes lack the noise marker function for their FFT, I would like to have a low noise dynamic signal analyzer instead.
I do not know what you mean by “low end” DSOs. We are talking about serious instruments here, so low end would be the entry level class. But the problem is not limited to these – all contemporary scopes up to the upper midrange have the very same problem: rising noise at very low frequencies because of the special conditions in a split path input buffer design.

If someone needs a superb instrument for low frequencies, then a Picoscope 4262 is one of the few options – apart from a DSA, that is. The 4262 only has 5 MHz bandwidth, but it is true 16 bits, has an SFDR of >96 dB and a near constant noise density from DC to its upper bandwidth limit.


It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC.

Old Tektronix DSOs used 16-bit acquisition and processing memory so high resolution mode did not halve the record length.
I was talking about a Siglent SDS2000X Plus, which provides 200 Mpts memory per channel pair, hence there is some headroom for this. How long was the memory in said old Tektronix DSOs?

« Last Edit: December 27, 2021, 01:16:41 am by Performa01 »
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #112 on: December 27, 2021, 03:36:58 am »
It doesn’t make much sense to get philosophic about obsolete designs. We are talking about general purpose DSOs here, which ranges from entry level (low end) up to the midrange, but excludes high end gear, which is specialized and definitely not general purpose.

The point is that the designs have not changed much.  The PGA has replaced the low impedance switched attenuator but engineers then and now are solving the same problems.  Early singed ended input digitizers were replaced with differential input digitizers, and differential signal paths were ultimately kept.  Everything is more integrated now of course.

Quote
At one point, at least after the invention of the digital readout, T&M industry noticed that a minimum of DC accuracy and stability was expected. Users were no longer willing to permanently turn the offset control of their scopes just to center the trace, as they used to do with their ancient CROs, but expected a decently stable offset position and some accuracy. So, the split path design has long become universal for all general purpose DSOs – despite its drawbacks, where the most obvious is the overload recovery issue. And this is unavoidable, even by a good design.

The move toward the split-path design was not driven by performance; it was about cost.  It happened as soon as low cost monolithic low input current operational amplifiers became available.  The cost savings came from replacing the discrete dual matched JFET with a single unselected JFET even though the split-path design requires trimming of the compensation or gain or both.

Quote
Btw, there are folks who have managed to build a balanced version of the split-path input buffer, so you can have this with balanced inputs too.

Haha, I am one of those folks, but it was much lower noise, impedance, and bandwidth for low level DC differential amplification.  I extended and improved an existing single ended design to fully differential and it worked perfectly on the first try, which pleasantly surprised me.

Quote
It’s not “old designs” that utilize two input attenuators. Of course you cannot build a good scope with vertical gain settings from 500 µV/div up to 10 V/div with just one single attenuator. For instance, every contemporary Siglent DSO has two input attenuator stages. Offset compensation voltage has to be added to the input in order to be effective (otherwise the input stage would require a totally unrealistic high common mode range), so this is part of the LF path of a split path input buffer design and topologically sits between the attenuators and the PGA.

Modern "budget" DSOs use only one input attenuator, which places much greater demands on the input buffer to handle larger signal levels.  The mid-tier models I have considered still use two input attenuators.  The presence of two input attenuators might be a good way to divide the lowest end budget DSOs from the next level up in performance.

Quote
With low attenuation factors you either need high supply rails (old design) or you get only a very low offset compensation range. But does a Tek 22xx even have a split path design? The specifications of up to one division trace shift for variable gain and trace invert make me wonder. All the more so as the best sensitivity is not particularly high at 2 mV/div. Or maybe they use the cheapest FET-OpAmp with high Offset voltage and -drift without self-calibration in the LF path – but this would somehow scotch the whole idea of the split path approach?

The Tektronix 22xx series does as shown below, and it might have been the first split-path design from them, but not all stages have balance adjustments in the 22xx series.  It is split-path but DC coupled and the operational amplifier controls the source current of the JFET to produce the DC and low frequency output.  Steve Roach discussed DC and AC coupled split-path designs in his article about oscilloscope signal conditioning.

The offset null is used for balance which is a terrible idea for precision, but probably good enough for an oscilloscope.  That might explain why one channel of one of mine has noticeable warmup drift.

When I studied the design in detail years ago with an eye toward noise analysis, I got the feeling that the Tektronix engineers paid attention to proper distribution of noise and gain.

Sensitivity was limited to 2 mV/div simply because greater sensitivity would require another preamplifier stage and noise was already greater than trace width, which seems funny now that modern oscilloscopes put up with even more noise.  It is not shown below but the basic sensitivity is 5 mV/div.  2 mV/div relies on increasing gain by 2.5 times in the preamplifier instead of removing attenuation which was pretty common at the time but has disadvantages.

The more modern AC coupled split-path amplifier allows AC and DC coupling to be implemented with the low frequency path instead of a high voltage RF relay which is a major advantage.

Quote
It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC.

Old Tektronix DSOs used 16-bit acquisition and processing memory so high resolution mode did not halve the record length.

I was talking about a Siglent SDS2000X Plus, which provides 200 Mpts memory per channel pair, hence there is some headroom for this. How long was the memory in said old Tektronix DSOs?

The maximum record length on those old DSOs is tiny by modern standards at only 4k, but even though fast RAM was expensive in both cost and size, they still made it twice as wide as needed.  Processing in the modern way would have doubled the record length without increasing the amount of installed memory.  Tektronix would later advertise this as a "no compromise" feature.
 
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Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #113 on: December 27, 2021, 11:31:39 am »
The move toward the split-path design was not driven by performance; it was about cost.  It happened as soon as low cost monolithic low input current operational amplifiers became available.  The cost savings came from replacing the discrete dual matched JFET with a single unselected JFET even though the split-path design requires trimming of the compensation or gain or both.
Well, of course cost might have been a major consideration, even though I cannot see why back then a dual matched FET should have been more expensive than an IC that contains basically the same plus a bunch of additional transistors and other components. Today it’s a different story of course, because these are hard to get and expensive spare parts now, but back in the seventies a dual FET was about as affordable (or rather expensive) as a JFET OpAmp (like LF356) as far as I remember.

The discrete differential stages usually did require trimming of the “offset balance”, as far as I remember the old circuit diagrams of up to 300 MHz frontends that did not use a split path topology.
Even though your circuit diagram shows three trimmers, I don’t think we’ve seen this in recent designs. Self calibration takes care of the offset error and with modern low tolerance parts in the input and feedback networks the balance between both paths and the transition at the crossover frequency are good enough even without adjustments.

Quote
Btw, there are folks who have managed to build a balanced version of the split-path input buffer, so you can have this with balanced inputs too.

Haha, I am one of those folks, but it was much lower noise, impedance, and bandwidth for low level DC differential amplification.  I extended and improved an existing single ended design to fully differential and it worked perfectly on the first try, which pleasantly surprised me.
Congrats – my hat goes off to you! This was (and still is) true design work, not very common anymore…

The Tektronix 22xx series does as shown below, and it might have been the first split-path design from them, but not all stages have balance adjustments in the 22xx series.  It is split-path but DC coupled and the operational amplifier controls the source current of the JFET to produce the DC and low frequency output.  Steve Roach discussed DC and AC coupled split-path designs in his article about oscilloscope signal conditioning.

The offset null is used for balance which is a terrible idea for precision, but probably good enough for an oscilloscope.  That might explain why one channel of one of mine has noticeable warmup drift.

When I studied the design in detail years ago with an eye toward noise analysis, I got the feeling that the Tektronix engineers paid attention to proper distribution of noise and gain.

Sensitivity was limited to 2 mV/div simply because greater sensitivity would require another preamplifier stage and noise was already greater than trace width, which seems funny now that modern oscilloscopes put up with even more noise.  It is not shown below but the basic sensitivity is 5 mV/div.  2 mV/div relies on increasing gain by 2.5 times in the preamplifier instead of removing attenuation which was pretty common at the time but has disadvantages.

The more modern AC coupled split-path amplifier allows AC and DC coupling to be implemented with the low frequency path instead of a high voltage RF relay which is a major advantage.
Thanks for the excerpt from the circuit diagram. It is quite interesting.

Yes, I’ve immediately noticed that it’s only DC coupled, which means a number of drawbacks, particularly the fact that the input goes open circuit in AC coupled mode, whereas good designs are supposed to have a constant input impedance regardless of the input coupling, or any other settings for that matter.

The LF-path also doesn’t provide the offset control usually found in DSOs – just because it really is best placed here. But yes, with the low division ratio of the LF input network, the compensation range could not be huge anyway. Nevertheless I have to assume that the offset adjustment is done at a later stage, which means that it actually relies on the usable common mode range of the input buffer – which will of course work to a certain degree because of the relatively high rail voltages of +/- 8.6 V.

A maximum sensitivity of 2 mV/div means 16 mVpp full scale. Even 5 mV/div is equivalent to 40 mVpp FS. Since this is hardly enough to drive the plates of a CRT, there has to be a lot of amplification after the programmable attenuator. In a DSO, the ADC would require at the very least several hundred millivolts (but usually up to two volts) full scale for proper operation. This is why integrated PGAs do not only provide attenuation, but amplification as well. Consequently, as the signal needs to be amplified anyway, there’s no need to stop at 2 mV/div. With 20 MHz bandwidth limit the total noise in a proper low noise design can be as low as 20 µVrms, so this should not be a problem for the trace width.

 
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Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #114 on: December 27, 2021, 02:40:05 pm »
Quote
If someone needs a superb instrument for low frequencies, then a Picoscope 4262 is one of the few options – apart from a DSA, that is. The 4262 only has 5 MHz bandwidth, but it is true 16 bits, has an SFDR of >96 dB and a near constant noise density from DC to its upper bandwidth limit.
Yes, I've seen these and there are also some alternatives. Very tempting. At the moment I sometimes use a Tek RSA3408A 8.5GHz RTSA for looking at low frequency stuff. This has a low noise floor and it has the advantage (for me at least) of having a 50 ohm input impedance. The Picoscope should be a bit better although it is limited to a 5MHz BW. The Tek analyser can capture 40MHz but it is only a 14bit system.

 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #115 on: December 27, 2021, 03:27:38 pm »
The move toward the split-path design was not driven by performance; it was about cost.  It happened as soon as low cost monolithic low input current operational amplifiers became available.  The cost savings came from replacing the discrete dual matched JFET with a single unselected JFET even though the split-path design requires trimming of the compensation or gain or both.

Well, of course cost might have been a major consideration, even though I cannot see why back then a dual matched FET should have been more expensive than an IC that contains basically the same plus a bunch of additional transistors and other components. Today it’s a different story of course, because these are hard to get and expensive spare parts now, but back in the seventies a dual FET was about as affordable (or rather expensive) as a JFET OpAmp (like LF356) as far as I remember.

Monolithic chips do not require hand grading for precision.  The dual matched parts were graded by hand.  Note that monolithic dual transistors will not work in this application because of parasitic coupling.

Tektronix kept the simpler dual stacked JFET buffer in the trigger circuits where precision was less important.

Quote
The discrete differential stages usually did require trimming of the “offset balance”, as far as I remember the old circuit diagrams of up to 300 MHz frontends that did not use a split path topology.
Even though your circuit diagram shows three trimmers, I don’t think we’ve seen this in recent designs. Self calibration takes care of the offset error and with modern low tolerance parts in the input and feedback networks the balance between both paths and the transition at the crossover frequency are good enough even without adjustments.

The designs Steve Roach shows (attached below) include automated trimming of the gain of the low frequency path.  He briefly mentions noise on page 70 where he discusses the shortcomings of RF MOSFETs.

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Btw, there are folks who have managed to build a balanced version of the split-path input buffer, so you can have this with balanced inputs too.

Haha, I am one of those folks, but it was much lower noise, impedance, and bandwidth for low level DC differential amplification.  I extended and improved an existing single ended design to fully differential and it worked perfectly on the first try, which pleasantly surprised me.

Congrats – my hat goes off to you! This was (and still is) true design work, not very common anymore…

The part of it that I really liked was adjusting the frequency breakpoint between the fast and slow path for lowest noise using a sampling DC voltmeter.  Low noise was my primary design goal.  Then I went back and measured the frequency of the breakpoint and it was exactly where the noise curves of the slow and fast path crossed, right where it should be.

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Yes, I’ve immediately noticed that it’s only DC coupled, which means a number of drawbacks, particularly the fact that the input goes open circuit in AC coupled mode, whereas good designs are supposed to have a constant input impedance regardless of the input coupling, or any other settings for that matter.

I do not know that one way is better than the other and oscilloscopes did it that way for decades without problems except where a DC return path was required.  AC coupled designs have to sink the gate current somehow which presents its own complications.  The reverse engineered Rigol DS1000Z front end that Dave made shows that the input resistance changes when coupling is switched, which has got to be incorrect, but maybe someone could measure it.  The big advantage of the AC coupled split-path buffer is that coupling can be switched on the low frequency side with a solid state switch.

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The LF-path also doesn’t provide the offset control usually found in DSOs – just because it really is best placed here. But yes, with the low division ratio of the LF input network, the compensation range could not be huge anyway. Nevertheless I have to assume that the offset adjustment is done at a later stage, which means that it actually relies on the usable common mode range of the input buffer – which will of course work to a certain degree because of the relatively high rail voltages of +/- 8.6 V.

The stage following the low impedance attenuator does single ended to differential conversion and that is where offset and position are inserted.  Since gain is fixed after that point, the scaling of the position control is fixed, but it was still also intended to operate as a limited range offset control.

Adjusting offset at the input buffer in this case would alter the transconductance changing the gain and frequency response, but maybe not enough to matter?  Later gain stages include first order correction of bandwidth and gain over temperature.

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A maximum sensitivity of 2 mV/div means 16 mVpp full scale. Even 5 mV/div is equivalent to 40 mVpp FS. Since this is hardly enough to drive the plates of a CRT, there has to be a lot of amplification after the programmable attenuator. In a DSO, the ADC would require at the very least several hundred millivolts (but usually up to two volts) full scale for proper operation. This is why integrated PGAs do not only provide attenuation, but amplification as well. Consequently, as the signal needs to be amplified anyway, there’s no need to stop at 2 mV/div. With 20 MHz bandwidth limit the total noise in a proper low noise design can be as low as 20 µVrms, so this should not be a problem for the trace width.

The worst case input signal range at 50 mV/div, where low impedance attenuation is maximum, is +/- 250 millivolts with overrange.  The peak-to-peak noise is only apparent in digital storage mode.  At the maximum sensitivity of 2 mV/div, the input noise is only just dominates the noise of the following stages.
« Last Edit: December 27, 2021, 03:30:25 pm by David Hess »
 
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Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #116 on: December 27, 2021, 05:13:18 pm »
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If someone needs a superb instrument for low frequencies, then a Picoscope 4262 is one of the few options – apart from a DSA, that is. The 4262 only has 5 MHz bandwidth, but it is true 16 bits, has an SFDR of >96 dB and a near constant noise density from DC to its upper bandwidth limit.
Yes, I've seen these and there are also some alternatives. Very tempting. At the moment I sometimes use a Tek RSA3408A 8.5GHz RTSA for looking at low frequency stuff. This has a low noise floor and it has the advantage (for me at least) of having a 50 ohm input impedance. The Picoscope should be a bit better although it is limited to a 5MHz BW. The Tek analyser can capture 40MHz but it is only a 14bit system.
I just had a closer look - and sadly my previous statement about near constant noise density isn't true. Even though it clearly is not a split path design and the 1/f corner frequency is significantly lower than for the 500 MHz and 2 GHz scopes that I have here, there is still some significant 1/f noise, slowly starting below some 25 kHz. Well, that's obviously the drawback of an 1 Mohms input impedance, requiring a FET input...

Other than the general purpose scopes, there is a major difference between open circuit and 50 ohms termination. Without termination, the noise raises significantly.

The noise density stays below 7 nV/sqrt(Hz) at and above 20 kHz, but gets as high as 102 nV/sqrt(Hz) down at 100 Hz. The first two attached screenshots show the noise spectrum at full sample rate up to 100 kHz and at full bandwidth. The noise density is generally higher than in the general purpose scopes (where it is in the range 2-3.5 nV/sqrt(Hz) at and above 1MHz), which might have to do with the higher sensitivity of these scopes. The Picoscope 4262 is limited to 20 mVpp full scale as the most sensitive range.
EDIT: Caution! this is for AC coupling with incomplete termination, which results in bad LF performance.

Pico_4262_Noise_50_5M_D100k
Pico_4262_Noise_50_5M

Next comes the noise density graph:
EDIT: Caution! this is for AC coupling with incomplete termination, which results in bad LF performance.

Pico_4262_ND_50_5M

A distortion test at 20 kHz

Signal_1V_20kHz

And finally a two tone intermodulation test, demonstrating the SFDR (just look at the cursor measurement; the automatic measurement failed because it obviously isn't intelligent enough to operate on the whole trace):

Signal_IMD_40mV_20-21kHz

EDIT: The noise measurements shown so far did not show the true performance, because they were flawed for two reasons:

1.   The input was AC coupled by accident, which of course increases LF-noise significantly.
2.   The input had a 50 ohm through terminator fitted, but since this scope is sensitive to the source impedance, an additional 50 ohm end terminator should be used to complete the 50 ohms setup.

So I've added the correct measurement results for spectral noise and noise density:

Pico4262_Noise_25_5MHz_D50kHz
Pico_4262_ND_25_5M



« Last Edit: December 28, 2021, 09:49:04 am by Performa01 »
 
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Online mawyatt

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #117 on: December 27, 2021, 05:34:53 pm »
The two tone IMD looks good as one would expect from a "True" 16 bit system. If you don't mind could you do this test at ~1MHz with the Picoscope 4262?

BTW one of the reasons almost everything analogish in complex chips is differential is you can't get a good ground reference on-chip for larger size chips. Later when analog type flip ball bond chips became available the on-chip ground reference was better than with traditional wire bonds since these ball bonds could be located within the chip boundaries as required and thus offered a lower ground impedance.

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Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #118 on: December 27, 2021, 06:08:44 pm »
Quote
I just had a closer look - and sadly my previous statement about near constant noise density isn't true. Even though it clearly is not a split path design and the 1/f corner frequency is significantly lower than for the 500 MHz and 2 GHz scopes that I have here, there is still some significant 1/f noise, slowly starting below some 25 kHz. Well, that's obviously the drawback of an 1 Mohms input impedance, requiring a FET input...

Thanks. The Tek3408A RTSA can be very laggy and frustrating to use at times but it is very powerful. The front end is 50 ohms and the noise figure at low frequencies is about 20dB. I've not looked to see how noisy it is below 1kHz but it's bound to get a bit noisier here.
« Last Edit: December 27, 2021, 06:11:02 pm by G0HZU »
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #119 on: December 27, 2021, 06:19:21 pm »
Monolithic chips do not require hand grading for precision.  The dual matched parts were graded by hand.  Note that monolithic dual transistors will not work in this application because of parasitic coupling.
Thanks for the explanation – this makes sense of course.

The designs Steve Roach shows (attached below) include automated trimming of the gain of the low frequency path.  He briefly mentions noise on page 70 where he discusses the shortcomings of RF MOSFETs.
Yes, this well known article is brilliant indeed! Yet we can see its age by looking at the first schematic, figure 7-1: the 50 ohms termination is accomplished by just a resistor, that is connected in parallel to the ordinary high impedance input with its high shunt capacitance – a solution that is barely suitable for scopes with a bandwidth exceeding some 100 MHz.

I do not know that one way is better than the other and oscilloscopes did it that way for decades without problems except where a DC return path was required.  AC coupled designs have to sink the gate current somehow which presents its own complications.  The reverse engineered Rigol DS1000Z front end that Dave made shows that the input resistance changes when coupling is switched, which has got to be incorrect, but maybe someone could measure it.  The big advantage of the AC coupled split-path buffer is that coupling can be switched on the low frequency side with a solid state switch.
It’s been quite some time, but I think I remember that this reverse engineered Rigol schematic has a number of errors in it. Some are more obvious than others. It is a nice means to get an overview, but certainly not suitable to study any circuit details.

Well, just because the AC block has been in the input path for a long time, especially when the bandwidth of a scope was rather low, this does not mean that it is a good thing to have to be prepared for unexpected changes in some major characteristics, when operating a switch that basically just alters the frequency response.

Consider a high impedance (100 Mohm), x100 high voltage probe connected to 2kV. If you now switch to AC coupling by accident, the input DC-block capacitor will charge up. Current is limited by the probe resistance, but after 10 seconds the capacitor might be charged to about 1.9 kV and this is equivalent to some 60 mJ of energy. So if the (supposedly) 400 volts rated capacitor doesn’t break down (and suffers damage or at least permanent degradation), it will send a potentially destructive pulse of electric energy into the frontend as soon as someone connects a low impedance source to the input after that incident.

Adjusting offset at the input buffer in this case would alter the transconductance changing the gain and frequency response, but maybe not enough to matter?  Later gain stages include first order correction of bandwidth and gain over temperature.
Figure 7-3 in your document shows the usual approach where to feed V_offset. In your circuit diagram of the Tek 22xx the offset voltage (delivered from an OpAmp with close to zero output impedance within the LF frequency range) would have to be fed into the lower leg of R98 (after disconnecting it from ground, that is). But with the low division ratio, which clearly is an attempt to keep the LF noise down, there is almost nothing gained, so I can completely understand why it’s done differently in this particular case.

EDIT: Sorry, only now i've checked what you mean. Of course, with the transistor output stage the original approach for offset compensation cannot be used.
« Last Edit: December 27, 2021, 07:17:08 pm by Performa01 »
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #120 on: December 27, 2021, 06:41:52 pm »
The two tone IMD looks good as one would expect from a "True" 16 bit system. If you don't mind could you do this test at ~1MHz with the Picoscope 4262?
Here you go - this instrument is not sensitive enough - you can barely see the IM3 products at 990 and 1020 kHz.

Signal_IMD_40mV_1000-1010kHz

 
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Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #121 on: December 27, 2021, 08:17:11 pm »
The designs Steve Roach shows (attached below) include automated trimming of the gain of the low frequency path.  He briefly mentions noise on page 70 where he discusses the shortcomings of RF MOSFETs.

Yes, this well known article is brilliant indeed! Yet we can see its age by looking at the first schematic, figure 7-1: the 50 ohms termination is accomplished by just a resistor, that is connected in parallel to the ordinary high impedance input with its high shunt capacitance – a solution that is barely suitable for scopes with a bandwidth exceeding some 100 MHz.

It works well with my 150 MHz 2445 and 300 MHz 2440 but they use hybrid construction so the parasitic elements are much less than with a surface mount printed circuit board.  I think the later TDS series did it up to 500 MHz but maybe not because the 1 GHz models obviously could not have.  As mentioned earlier, the old Tektronix 485 with printed board construction did *not* use a switchable termination but instead an RF relay to direct the input to either the high impedance buffer or a separate 50 ohm input, and the specifications reflect it with lower bandwidth in high impedance mode.  At the time I do not think they had a faster JFET high impedance buffer or they would have used it.  I consider the 485 to be a "heroic" engineering effort.

Based on context, I think Steve Roach worked on the 500 MHz and 1 GHz TDS series of oscilloscopes so his article gives an idea about what was going on in the late 1990s and early 2000s.  I believe this makes it particularly useful for emulation in modern amateur designs.

Quote
Consider a high impedance (100 Mohm), x100 high voltage probe connected to 2kV. If you now switch to AC coupling by accident, the input DC-block capacitor will charge up. Current is limited by the probe resistance, but after 10 seconds the capacitor might be charged to about 1.9 kV and this is equivalent to some 60 mJ of energy. So if the (supposedly) 400 volts rated capacitor doesn’t break down (and suffers damage or at least permanent degradation), it will send a potentially destructive pulse of electric energy into the frontend as soon as someone connects a low impedance source to the input after that incident.

Tektronix made high voltage 10x and 100x probes with a built in parallel resistance to avoid that problem.  They can be identified by having a lower than expected input resistance.  Probes like this are still made but they are difficult to find and come with a premium price.

Quote
Figure 7-3 in your document shows the usual approach where to feed V_offset. In your circuit diagram of the Tek 22xx the offset voltage (delivered from an OpAmp with close to zero output impedance within the LF frequency range) would have to be fed into the lower leg of R98 (after disconnecting it from ground, that is). But with the low division ratio, which clearly is an attempt to keep the LF noise down, there is almost nothing gained, so I can completely understand why it’s done differently in this particular case.

EDIT: Sorry, only now i've checked what you mean. Of course, with the transistor output stage the original approach for offset compensation cannot be used.

The 22xx series also did not need a different method because it is based on a traditional analog design where that was a solved problem.  It just represents the last fully documented oscilloscope design along with the 24xx series of analog and digital storage models.

Even so, I consider the 2232 to be the first "modern" DSO design with a recognizable user interface.  It's predecessor, the 2230, has an archaic albeit interesting user interface and really bridges the gap between analog and digital designs.

I do not recommend duplicating the 22xx design, but a lot can still be learned from it.

I wish we had better data on available RF MOSFET noise characteristics.  What is available is intended for RF amplifier applications.
« Last Edit: December 27, 2021, 08:20:27 pm by David Hess »
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #122 on: December 27, 2021, 08:28:48 pm »
I would like to thank you everybody for the many replies.
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.
I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have.

Thank you again.
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #123 on: December 27, 2021, 08:42:30 pm »
Quote
I wish we had better data on available RF MOSFET noise characteristics.  What is available is intended for RF amplifier applications.
Can you measure the noise parameters yourself at audio frequencies? I've done this stuff up at RF and recently measured the s-parameters for the BF998 MOSFET at various bias points across a frequency range of a few MHz up to 3GHz and I also created some noise data for it up at VHF. This noise data gets included in the s-parameter file. I did the same for the old BF981 a few years back with good results when designing amplifiers for low noise figure. I've never tried to do this at audio frequencies though.
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #124 on: December 27, 2021, 08:46:35 pm »
I would like to thank you everybody for the many replies.
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.
I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have.

Thank you again.
Sounds good! My first digital scope (Tektronix) was noisy and it spoiled the experience a bit. It is still possible to do good work with a noisy scope but I don't think I'd want to buy another one. Especially if it as noisy as that Rigol.
 

Online mawyatt

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #125 on: December 28, 2021, 12:56:48 am »
The two tone IMD looks good as one would expect from a "True" 16 bit system. If you don't mind could you do this test at ~1MHz with the Picoscope 4262?
Here you go - this instrument is not sensitive enough - you can barely see the IM3 products at 990 and 1020 kHz.

Signal_IMD_40mV_1000-1010kHz

This looks good, but the tone levels are -6dBV below the level used at 20KHz so one would expect the 3rd order IMD to be significantly down from the 20KHz case.

Anyway, thanks for the test.

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Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #126 on: December 28, 2021, 01:34:06 am »
Note also that the other spurious terms are limiting the spurious free dynamic range on the 1MHz IMD test. A really good (old school swept) microwave spectrum analyser can achieve a typical IP3 limited SFDR of about 112dB with a very narrow RBW on the first frequency range up to a few GHz. However, 105dB is more realistic at (say) 10Hz RBW. I wouldn't expect to see those other spurious terms either when using a conventional spectrum analyser. The Pico will have a much faster refresh rate though!

On narrow spans the phase noise will slightly limit the SFDR of the swept analyser so usually stuff like this is done at a wider frequency spacing with a conventional analyser.
« Last Edit: December 28, 2021, 01:38:41 am by G0HZU »
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #127 on: December 28, 2021, 02:52:19 am »
This looks good, but the tone levels are -6dBV below the level used at 20KHz so one would expect the 3rd order IMD to be significantly down from the 20KHz case.
Sorry, this was not indended – somehow I did not pay attention to the levels.

Please find attached the correct measurement. The result is the same as at 20 kHz.

Signal_IMD_80mV_1000-1010kHz

Note also that the other spurious terms are limiting the spurious free dynamic range on the 1MHz IMD test. A really good (old school swept) microwave spectrum analyser can achieve a typical IP3 limited SFDR of about 112dB with a very narrow RBW on the first frequency range up to a few GHz. However, 105dB is more realistic at (say) 10Hz RBW. I wouldn't expect to see those other spurious terms either when using a conventional spectrum analyser. The Pico will have a much faster refresh rate though!

On narrow spans the phase noise will slightly limit the SFDR of the swept analyser so usually stuff like this is done at a wider frequency spacing with a conventional analyser.
Of course you are right – and just for others to put this into perspective, I would like to add:

Swept spectrum analyzers only “see” their resolution bandwidth at any point in time (ok, only true for the last IF), whereas the DSO always works at full bandwidth (5 MHz in this particular case). Under these conditions, some 96 dB dynamic is all you can expect from a 16 bit system – everything beyond that is just a lucky incident based on the specific conditions and the results cannot be trusted any longer.

In the previous example with the 6 dB lower level, it has been perfectly possible to measure an IMD of 109.6 dBc, since none of the spurs got in the way of this measurement. According to the textbook theory, it should have been 115 dBc though, so the measurement was flawed anyway. As mentioned before, we cannot expect great accuracy once far outside the first order dynamic range of the acquisition system.

« Last Edit: December 28, 2021, 02:56:13 am by Performa01 »
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #128 on: December 28, 2021, 03:39:49 am »
That really is very impressive from the Picoscope in terms of SFDR.

I just turned on the RSA3408A to look at the LF noise floor up to 25kHz and I've added a plot below. This analyser is FFT only and can't do a swept measurement at any frequency. Below 40MHz it feeds direct to a 14 bit ADC and the IMD performance isn't that good. It's much worse than the Picoscope in this respect. However, the LF noise floor is quite good considering this isn't a dedicated AF analyser. I've used it to measure the noise figure of AF amplifiers a few times. As long as I provide enough gain to overcome the noise figure of the 3408A it can make fairly good noise figure measurements. I guess not many people make AF amps with 50 ohm ports but this type of amplifier is popular in direct conversion receivers.

You can see the noise floor is a fairly flat -154dBm/Hz across the AF band.



« Last Edit: December 28, 2021, 03:41:24 am by G0HZU »
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #129 on: December 28, 2021, 05:05:01 am »
Quote
I wish we had better data on available RF MOSFET noise characteristics.  What is available is intended for RF amplifier applications.

Can you measure the noise parameters yourself at audio frequencies? I've done this stuff up at RF and recently measured the s-parameters for the BF998 MOSFET at various bias points across a frequency range of a few MHz up to 3GHz and I also created some noise data for it up at VHF. This noise data gets included in the s-parameter file. I did the same for the old BF981 a few years back with good results when designing amplifiers for low noise figure. I've never tried to do this at audio frequencies though.

Up through audio frequencies would not be sufficient because RF MOSFETs can have a flicker noise corner frequency in the MHz range.

I am just not setup to make that kind of measurement easily.  I can make spot noise measurements up to 1 MHz but even that would not be high enough.  I would have to build something custom and I would prefer a more general solution.
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #130 on: December 28, 2021, 09:28:32 am »
You can see the noise floor is a fairly flat -154dBm/Hz across the AF band.
Yeah - bipolar technology, using rf-transistors (with very low intrinsic base resistance) makes for a good noise matching at low impedances like 50 ohms – and a low 1/f corner frequency.

High impedance FET input stages are noisy under such conditions. On the other hand, high impedance inputs are much more versatile. We can adapt them to any impedance we like by means of a pass through terminator (at least at low frequencies).

Meanwhile I’ve experimented a bit further and detected at least two flaws in my previous noise measurement:

1.   The input was AC coupled by accident, which of course increases LF-noise significantly.
2.   The input had a 50 ohm through terminator fitted, but since this scope is sensitive to the source impedance, an additional 50 ohm end terminator should be used to complete the 50 ohms setup.

Now look at the screenshot attached.

Pico4262_Noise_5MHz_D50kHz

I’ve tried to resemble your settings as close as possible but still kept the total FFT bandwidth at 5 MHz in order to keep the high frequency noise out of the LF region. Display units are dBm now for better comparability. Frequency step is 38.15 Hz, which is equivalent to a RBW of 112 Hz with the Flat-Top window – so noise levels will read slightly higher than in your setup.

With a noise level of -134.7 dBm this is very comparable to your RSA 3408A above some 30 kHz.
At 1 kHz, the FET input goes up by 17.8 dB to -116.9 dBm, but obviously stops at -114 dBm with this RBW.

So I’m confident to claim that the Pico 4262 has the same low noise in a 50 ohm system, as long as you keep the input DC-coupled and stay above 30 kHz.

EDIT: I have updated my original posting, where you can also see the updated noise density plot.
« Last Edit: December 28, 2021, 09:51:00 am by Performa01 »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #131 on: December 28, 2021, 09:50:30 am »
I would like to thank you everybody for the many replies.
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.
I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have.

Still waiting for the screenshots of your ripple with averaging turned on...  :popcorn:

 

Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #132 on: December 28, 2021, 10:50:02 am »
I would like to thank you everybody for the many replies.
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.
I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have.

Still waiting for the screenshots of your ripple with averaging turned on...  :popcorn:

Looking at this post again, I can believe that the first image shows noise from the scope (although it is quite a lot). But I rather cannot believe that the 40mVpp "noise band" on top of the sawtooth in the second image is scope noise as well (apparently scope settings are the same as in the first image). I guess the latter is already present in the input signal. I don't feel able to assess whether it is random noise, or rather a high-frequency oscillation. FFT should help to reveal it.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #133 on: December 28, 2021, 11:46:11 am »
Still waiting for the screenshots of your ripple with averaging turned on...  :popcorn:

Looking at this post again, I can believe that the first image shows noise from the scope (although it is quite a lot). But I rather cannot believe that the 40mVpp "noise band" on top of the sawtooth in the second image is scope noise as well (apparently scope settings are the same as in the first image). I guess the latter is already present in the input signal. I don't feel able to assess whether it is random noise, or rather a high-frequency oscillation. FFT should help to reveal it.

I'm just interested in what an MSO5000 can do with a signal like that when a user really uses all the provided features.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #134 on: December 28, 2021, 11:52:00 am »
Also... the color gradient mode as mentioned on the first page. How would the ripple appear if you enable that?


 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #135 on: December 28, 2021, 12:06:35 pm »
For color grading to be usefull you need a perfectly repetitive signal. After all color grading is a form of averaging. Power supply ripple isn't perfectly repetitive so color grading won't help at all.

Just face it: you will want to use an oscilloscope with the least amount of internal noise to look at any signal. After all the purpose of an oscilloscope is to look at the shape of a signal and the less an oscilloscope distorts that signal, the better. It seems Rigol dropped the ball where it comes to noise reduction and at some point cheap doesn't make up for poor performance.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #136 on: December 28, 2021, 12:28:32 pm »
For color grading to be usefull you need a perfectly repetitive signal. After all color grading is a form of averaging. Power supply ripple isn't perfectly repetitive so color grading won't help at all.

I'd still like to see a screenshot of it.
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #137 on: December 28, 2021, 04:42:52 pm »
I am going to do the photos you asked, maybe this evening or tomorrow
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #138 on: December 28, 2021, 09:25:39 pm »
Frequency step is 38.15 Hz, which is equivalent to a RBW of 112 Hz with the Flat-Top window – so noise levels will read slightly higher than in your setup.

With a noise level of -134.7 dBm this is very comparable to your RSA 3408A above some 30 kHz.
At 1 kHz, the FET input goes up by 17.8 dB to -116.9 dBm, but obviously stops at -114 dBm with this RBW.

If I did my math right, that comes out to  3.9 nV/SqrtHz so similar to a well designed 100 MHz JFET input, and consistent with the specified 15 picofarad input capacitance.  (1) For a lower frequency singled ended JFET input instrument, 1 nV/SqrtHz is possible (LSK170) but the input capacitance would be 2 or 3 times higher.  So why is the input capacitance low and noise high for such low bandwidth?

Given their dynamic range and distortion requirements for 16-bits, a simple FET source follower would have too much distortion; feedback is required to lower the distortion.  So they probably used a JFET operational amplifier, and that would be consistent with higher noise, 5 MHz bandwidth, and a 15 picofarad input capacitance.

That also places this instrument into a different class than an oscilloscope, although similar to the old Tektronix oscilloscopes which used the 5A22 or 7A22 differential amplifier.

(1) There is a close relationship with input capacitance, bandwidth, and input noise.  Lower bandwidth FETs have lower noise and higher input capacitance.
« Last Edit: December 28, 2021, 10:08:35 pm by David Hess »
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #139 on: December 28, 2021, 09:34:08 pm »
I would like to thank you everybody for the many replies.

Hopefully the discussion was of some help.

Quote
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.

I do not think low noise and high sample rate are mutually exclusive because the digitizer's input noise should be insignificant compared to the noise from earlier stages and especially from the amplified noise of the high impedance input buffer.  The instruments in question seem to suffer from higher noise in general rather than because of sample rate.
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #140 on: December 28, 2021, 09:57:59 pm »
So i did a lot of photos.
First two comparison: no probe, 1mv/div, 20mhz BW limit. There is a huge difference

 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #141 on: December 28, 2021, 10:01:03 pm »
Same no probe
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #142 on: December 28, 2021, 10:03:35 pm »
4x avarage no probe
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #143 on: December 28, 2021, 10:05:33 pm »
Eres 3db on siglent and hi-res on rigol
no probe
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #144 on: December 28, 2021, 10:06:46 pm »
Normal mode
Probe connected 1x and grounded
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #145 on: December 28, 2021, 10:08:00 pm »
Ripple same settings on both oscilloscope
AC, normal mode, 1x probe
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #146 on: December 28, 2021, 10:13:42 pm »
There is a huge difference between this two scopes.
The Rigol has a faster update of the image, almost double... and the quality of the rappresentation, i mean the graphics, of the signal is a lot better than the Siglent. It seem like the Siglent had a lower resolution. There is not a huge difference in the speed of the user interface and also the Rigol appear to have a better quality of the material of the scope. But the front end noise is a lot different.
Judge by your self from the photos attached.
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #147 on: December 28, 2021, 10:15:02 pm »
Normal mode
Probe connected 1x and grounded

Shorting the probe tip can be tricky.  For lowest noise it is not sufficient to simply clip the probe's ground lead to the probe's tip because the loop will pick up ambient noise.  Best is to short the tip out with a coaxial probe tip to BNC adapter plugged into a BNC short or 50 ohm termination, but winding wire around the probe tip also works.
« Last Edit: December 28, 2021, 10:37:18 pm by David Hess »
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #148 on: December 28, 2021, 10:19:06 pm »
This is even worse than I expected ( :wtf: ). I don't care about the open / shorted inputs at the most sensitive V/div (Rigol does digital zoom there so it is not an apples for apples comparison) but I do care about the display of an actual signal. On the Siglent you can clearly see spikes on the signal which are completely obscured on the Rigol.
« Last Edit: December 28, 2021, 11:07:43 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #149 on: December 28, 2021, 11:11:03 pm »
So i did a lot of photos.
First two comparison: no probe, 1mv/div, 20mhz BW limit.

Irrelevant. Only real signals count.

4x avarage no probe

Averaging only works when there's a periodic signal.
Ripple same settings on both oscilloscope
AC, normal mode, 1x probe

No averaging?

That's the only thing that counts - if averaging mode can better show the underlying signal or not.

Normal mode
Probe connected 1x and grounded

Shorting the probe tip can be tricky.  For lowest noise it is not sufficient to simply clip the probe's ground lead to the probe's tip because the loop will pick up ambient noise.

Yep. Connecting the ground clip to the probe actually creates an antenna.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #150 on: December 28, 2021, 11:14:05 pm »
Ripple same settings on both oscilloscope
AC, normal mode, 1x probe

I disagree that there's a huge difference in useful information. Sure, the Siglent line is thinner but The Rigol is showing the ripple just fine.

You should also be able to change the displayed part of the signal (the red bit) by twisting the selection knob on the Rigol (ie. intensity setting).

And again: What does averaging mode do if you turn it on in this situation? Why is this being avoided?




« Last Edit: December 28, 2021, 11:52:21 pm by Fungus »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #151 on: December 28, 2021, 11:32:30 pm »
This is even worse than I expected ( :wtf: ). I don't care about the open / shorted inputs at the most sensitive V/div (Rigol does digital zoom there so it is not an apples for apples comparison) but I do care about the display of an actual signal. On the Siglent you can clearly see spikes on the signal which are completely obscured on the Rigol.

I dunno what those "spikes" are but they're not ripple. To me it seems liek the Rigol is perfectly capable of showing the ripple from that power supply. Is there anybody here who can't see the ripple in this image or thinks that the 14.003mV displayed value is somehow massively different than the 14.048mV displayed by the Siglent?


(and this is without averaging, apparently, averaging can only improve this)

Is the Siglent display or the number displayed by the Siglent worth 400 Euros more? I dunno. It's all relative, but I could buy all sorts of useful stuff for 400 Euros. :-//

However you look at it: You'll have a hard time convincing me that the Rigol is a disaster. Sure the Siglent's line is thinner but the Rigol is perfectly capable of showing the ripple on screen and measuring it. That's what really counts, and is the subject of this thread.
« Last Edit: December 28, 2021, 11:48:49 pm by Fungus »
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #152 on: December 29, 2021, 12:30:05 am »
Just stop trying to make right what is clearly wrong. The small spikes that the trace shows on the Siglent (or any other modern DSO other than Rigol mentioned in this thread) can point to other problems. An FFT will tell more but again, on the Rigol the HF component riding on the lower frequency ripple likely gets drowned in the noise. The Rigol MSO5000 is outright horrible. Claiming anything else is delusional. Edit: pay close attention to the excellent example G0HZU posted below. It may save your bacon one day trying to find an illusive problem in a circuit.

At some point cheap doesn't make up for poor performance. In the end you'll need to buy an extra instrument to make the measurement cheaper gear can't do. Been there, done that and wasted enough money on cheap gear which in the end didn't deliver.
« Last Edit: December 29, 2021, 12:40:14 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline G0HZU

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #153 on: December 29, 2021, 12:38:06 am »
Quote
And again: What does averaging mode do if you turn it on in this situation? Why is this being avoided?

OK I'll bite...If you want to see the effects of averaging then see the example below.

I've just set up a couple of function generators and summed the waveforms into my old HP Infinium scope. I've set the scope to 50R input and done everything in x1.

The scope is set to 1mV/div and I've turned on the 30MHz bandwidth limit. Waveform 1 is a triangle wave at about 500Hz and it is about 5mV pkpk. Waveform 2 is a series of narrow positive pulses each of amplitude 1mV and they occur every 768us. The plots below show the low noise performance of this old scope and also show how averaging can cause information to be lost.

See below for a single shot capture and see also for what happens with averaging. The information about the pulses is totally lost in the averaged screenshot because the period of the triangle wave and the pulses is different. I hope this helps?


 
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Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #154 on: December 29, 2021, 01:09:08 am »
However you look at it: You'll have a hard time convincing me that the Rigol is a disaster. Sure the Siglent's line is thinner but the Rigol is perfectly capable of showing the ripple on screen and measuring it. That's what really counts, and is the subject of this thread.

What are you going to do when the ripple is even smaller?  For example, here is a ~150uVrms 10MHz signal being clearly triggered and displayed.  This signal was displayable--triggerable and above the noise threshold on a Tek 2465B (shown), a Tek 2221A (digital and analog) and the Siglent 1104X-E (although just barely and not as reliably).  I was able to do the same thing with a 1MHz and 100Hz signal of approximately the same amplitude.  What would it look like on the Rigol? 

You can't average your way out of this when you are looking for noise--possibly non-periodic--in the first place.  On the Tek 2221A and the Sig 1104X-E, averaging made the signal look nicer but I'm not convinced that means better. 

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #155 on: December 29, 2021, 04:05:33 am »
There is a huge difference between this two scopes.
The Rigol has a faster update of the image, almost double... and the quality of the rappresentation, i mean the graphics, of the signal is a lot better than the Siglent. It seem like the Siglent had a lower resolution. There is not a huge difference in the speed of the user interface and also the Rigol appear to have a better quality of the material of the scope. But the front end noise is a lot different.
Judge by your self from the photos attached.

Could you try the ripple and maybe no probe (like the first photos) with the Siglent in the 10-bit acquisition mode?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #156 on: December 29, 2021, 06:10:52 am »
Normal mode
Probe connected 1x and grounded

Shorting the probe tip can be tricky.  For lowest noise it is not sufficient to simply clip the probe's ground lead to the probe's tip because the loop will pick up ambient noise.  Best is to short the tip out with a coaxial probe tip to BNC adapter plugged into a BNC short or 50 ohm termination, but winding wire around the probe tip also works.


Yes I have put the wire of the probe in the less noisy position
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #157 on: December 29, 2021, 06:16:54 am »
So i did a lot of photos.
First two comparison: no probe, 1mv/div, 20mhz BW limit.

Irrelevant. Only real signals count.

4x avarage no probe

Averaging only works when there's a periodic signal.
Ripple same settings on both oscilloscope
AC, normal mode, 1x probe

No averaging?

That's the only thing that counts - if averaging mode can better show the underlying signal or not.

Normal mode
Probe connected 1x and grounded

Shorting the probe tip can be tricky.  For lowest noise it is not sufficient to simply clip the probe's ground lead to the probe's tip because the loop will pick up ambient noise.

Yep. Connecting the ground clip to the probe actually creates an antenna.

I know but i put the probes in the same manner with the lowest noise captured, i also did the photos with avarage on the signal of the ripple i didn't had the time to upload
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #158 on: December 29, 2021, 06:23:48 am »
There is a huge difference between this two scopes.
The Rigol has a faster update of the image, almost double... and the quality of the rappresentation, i mean the graphics, of the signal is a lot better than the Siglent. It seem like the Siglent had a lower resolution. There is not a huge difference in the speed of the user interface and also the Rigol appear to have a better quality of the material of the scope. But the front end noise is a lot different.
Judge by your self from the photos attached.

Could you try the ripple and maybe no probe (like the first photos) with the Siglent in the 10-bit acquisition mode?

Yes i will do. The 10 bit more reduce the noise as i checked yesterday evening. I will post some photos.
In regard of the Rigol It Is in my opinion a very good scope but It has the problem of the noisy front end
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #159 on: December 29, 2021, 08:11:57 am »
Ripple same settings on both oscilloscope
AC, normal mode, 1x probe

I disagree that there's a huge difference in useful information. Sure, the Siglent line is thinner but The Rigol is showing the ripple just fine.

You should also be able to change the displayed part of the signal (the red bit) by twisting the selection knob on the Rigol (ie. intensity setting).

And again: What does averaging mode do if you turn it on in this situation? Why is this being avoided?





The spikes you can clearly see in the Siglent i think are switching noise  coming from the next stage of this psu. I am going to do some test about It to check if the front end noise of the Rigol is covering such interference
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #160 on: December 29, 2021, 08:32:56 am »
As asked some photos with the avarage mode

4x average

In my opinion the avarage, even if It conserve almost the shape of the signal, destroys "the low intensity" informations
« Last Edit: December 29, 2021, 08:35:19 am by Fiorenzo »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #161 on: December 29, 2021, 10:22:10 am »
The spikes you can clearly see in the Siglent i think are switching noise  coming from the next stage of this psu. I am going to do some test about It to check if the front end noise of the Rigol is covering such interference

Thanks for the photos! This is all very useful information.

The one remaining test is to see the effect of turning up the display intensity on the Rigol to see if it reveals those spikes better (without averaging).
 

Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #162 on: December 29, 2021, 10:22:45 am »
I 'm reading some of these posts and  :-//

It is obvious MSO5000 has much more noise, and that it is a problem, unless you only look at digital signals and just want to look at general shape and some timing information.
It is shame, really, because new Rigol scopes are much more powerful processing wise than the old ones, and generally held great promise but analog front end/ADC noise performance is not very good.

Scope with low noise is always going to be better instrument than the one with high noise. Why is that even a discussion?
Is this some audiofool discussion how this huge noise is pleasant to look at because it's pretty?  |O

I don't use bandwidth limiting, averaging or any "signal cleanup" features when I'm looking into a signal I want to understand. You would want to look at this switcher signal with a full 1GHz bandwidth and with as low noise scope you can.
To really see what is there... Switching ripple is most of the time least interesting part of switching PSU. We expect it to be there, and most of the time it will be roughly what we calculated. Other, higher frequency stuff (those little hairs on top) is much more problematic and most of the time those will give you headaches.. Nanovolts of those will already be seen on any EMI test...

You filter, limit and "cleanup" signal in circumstances where you understand your signal and you want to ignore noise and other parts of signal on purpose. If your signal is buried inside the noise, you average.
But is that noise part of signal you're measuring or your scope is not irrelevant. If it comes from DUT I want to know that. I want to see it..
Only way to do that is to have low noise scope.

Of course, like OP correctly asked, there is a point of diminishing returns..
Is scope with 5 uV of RMS noise so much better than one with 50uV RMS noise for measuring this switcher signal from this example? Probably not.
It would be definitely better but probably not usefully so in this case. But one with 50uV of RMS noise is definitely better than one which has trace that is whooping 20 mV wide... On a signal that is 60mV P-P...
On this test I would call MSO5000 from Rigol useless for this measurement. And averaging this not autocorrelated signal ( it doesn't repeat cleanly and doesn't retrace it's waveform exactly but varies slightly all the time) will not extract more detail but will hide even more information about signal..

OTOH Siglent shows pretty much perfect representation of the signal, big peaks, ripple AND little hairs. That is your switcher output. That is useful information..

Little Micsig STO1104C/E, or Siglent SDS1104X-E could do equally good job here.

Sad part is that little Rigol DS1054Z would be much better for this signal than MSO5000.. DS2000A had excellent low noise front end .. But new series of Rigol scopes is very powerful in processing power but analog performance is worse than older series. Shame really, otherwise they are very nice scopes.



 
« Last Edit: December 29, 2021, 10:25:11 am by 2N3055 »
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #163 on: December 29, 2021, 10:29:23 am »
The spikes you can clearly see in the Siglent i think are switching noise  coming from the next stage of this psu. I am going to do some test about It to check if the front end noise of the Rigol is covering such interference

Thanks for the photos! This is all very useful information.

The one remaining test is to see the effect of turning up the display intensity on the Rigol to see if it reveals those spikes better (without averaging).

Sorry fungus i have been very busy. I have more photos. Check in the next messages
 

Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #164 on: December 29, 2021, 10:30:13 am »
16x average
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #165 on: December 29, 2021, 10:32:04 am »
Max average possibile
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #166 on: December 29, 2021, 10:36:03 am »
Fungus actually i think in my modest opinion that the Rigol is a greate scope, for sure i will miss it due to its graphics and fast acquisition, comparing both the scope you see the difference a lot. But It has also a lot more noise.... I have an idea to show you how much the signal degrade due to this problem.
 

Offline tautech

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #167 on: December 29, 2021, 10:42:11 am »
Fungus actually i think in my modest opinion that the Rigol is a greate scope, for sure i will miss it due to its graphics and fast acquisition, comparing both the scope you see the difference a lot. But It has also a lot more noise.... I have an idea to show you how much the signal degrade due to this problem.

Study the screenshots you've just posted to see just how fast it actually is with averaging engaged.

Which initiates another question; what would the MSO5000 sampling drop to if one more channel was activated ?
« Last Edit: December 29, 2021, 10:55:40 am by tautech »
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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #168 on: December 29, 2021, 11:07:21 am »
Just stop trying to make right what is clearly wrong. The small spikes that the trace shows on the Siglent (or any other modern DSO other than Rigol mentioned in this thread) can point to other problems.

I'm not trying to make anything "right", I'm just trying to see if the Rigol would prevent somebody from doing their job with this signal.

Questions:
(1) Can the Rigol display/measure the ripple? It clearly can.


(2) Can the Rigol see the high frequency spikes? Maybe not as well as the Siglent but they're clearly visible.


(and the jury is out until we know the effect of turning the intensity up/down to enhance them - just like you had to do on old Analog 'scopes to see artifacts like that)

Bottom line: The Rigol can see what's going on in that signal.

What are you going to do when the ripple is even smaller?

Get an amplifier?  :-//

Does ripple/noise/electronics magically stop at 1mV? There's plenty of signals that the Siglent can't see either...

 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #169 on: December 29, 2021, 11:11:48 am »
Which initiates another question; what would the MSO5000 sampling drop to if one more channel was activated ?

It's not difficult: With two channels you get 4GHz/channel, with three or four channels you get 2GHz/channel.

At no point is it less than the Siglent with just a single channel.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #170 on: December 29, 2021, 11:40:23 am »
But It has also a lot more noise.... I have an idea to show you how much the signal degrade due to this problem.

All 'scopes degrade the signal, none are perfect.

The real problem with this Rigol vs. Siglent comparison is that you're comparing two devices side by side where one of them happens to work better at 1mV then the other one does.

ie. When you turn the Siglent's vertical control to "max" then it makes the Rigol look bad at that level. If you were comparing 1V signals or 5V signals in your screenshots then you wouldn't see the same difference between them.

Guess what? Electronics doesn't magically stop at the exact place where the Siglent's vertical control does. There's plenty of signals below 1mV. You'll need an amplifier to see them and the exact same same amplifier would work with a Rigol, too.

Plus: You originally said you do digital stuff, so...  :-//
 

Offline tautech

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #171 on: December 29, 2021, 11:40:55 am »
Which initiates another question; what would the MSO5000 sampling drop to if one more channel was activated ?

It's not difficult: With two channels you get 4GHz/channel, with three or four channels you get 2GHz/channel.

At no point is it less than the Siglent with just a single channel.
It is when you overlook the full picture of each scopes acquisition system.

With channels 1 and 3 of each scope activated a very different picture may emerge at some timebase settings.
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Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #172 on: December 29, 2021, 11:46:37 am »

I'm not trying to make anything "right", I'm just trying to see if the Rigol would prevent somebody from doing their job with this signal.


Get an amplifier?  :-//

Does ripple/noise/electronics magically stop at 1mV? There's plenty of signals that the Siglent can't see either...

This not a stupid debate club where we debate whether it is more appropriate to say "apply the law" or "apply the letter of law"..
This is physics and measurements and math. There is no negotiation with these guys.
You are wrong and stop being a spoiled child. You. Are. Wrong. Take it as an adult.

Worse is worse, and if you can chose, chose better. Why on Earth would you take worse device and then spend years "figuring out" how to deal with fundamental shortcomings (that you really can't fix because of it's nature) than simply take something that actually works much better.

Nothing does anything magically. Usually more than good enough is when we know scope contributes less than few percent of the error. That also coincides with what can be easily seen with naked eye and 8 bits of a scope.
When scope shows 20 mV noise on top of 65mv P-P signal that is definitely bad. Unusable for that measurement. Fact that it shows something is not that useful. It needs to show it accurate enough so person looking into scope can make something with it..

Also you keep repeating about some magical amplifiers. Amplifiers that have DC-100 Mhz bandwith and less noise as even a little Micsig or Siglent SDS1104X-E cost as much as a good scope from Keysight. There are amplifiers for audio frequency range that can be used for low noise measurements. Those are useless for measuring a things mentioned here.. So no, some mythical preamps are not a solution.
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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #173 on: December 29, 2021, 11:48:21 am »
With channels 1 and 3 of each scope activated a very different picture may emerge at some timebase settings.

Yes, because if you enable to adjacent channels the Siglent only has 1GS/sec to look at 350Mhz signals, ie. it's getting uncomfortably close to Nyquist.

Turning a channel off can bump the sample rate to 2GS/sec and give a different picture.

The Rigol MSO5000 series is one of the few oscilloscopes which can maintain a comfortable Nyquist margin with any combination of channels.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #174 on: December 29, 2021, 11:51:05 am »
Also you keep repeating about some magical amplifiers. Amplifiers that have DC-100 Mhz bandwith and less noise as even a little Micsig or Siglent SDS1104X-E cost as much as a good scope from Keysight.

Yes, but amplifiers from DC to 1MHz are incredibly cheap (ie. a $2 OP-amp plus power supply) and would be perfectly adequate for audio work and looking at power supply ripple.

Amplifiers from 10kHz to 2GHz are also incredibly cheap.
« Last Edit: December 29, 2021, 12:06:01 pm by Fungus »
 

Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #175 on: December 29, 2021, 11:55:50 am »
But It has also a lot more noise.... I have an idea to show you how much the signal degrade due to this problem.

All 'scopes degrade the signal, none are perfect.

The real problem with this Rigol vs. Siglent comparison is that you're comparing two devices side by side where one of them happens to work better at 1mV then the other one does.

ie. When you turn the Siglent's vertical control to "max" then it makes the Rigol look bad at that level. If you were comparing 1V signals or 5V signals in your screenshots then you wouldn't see the same difference between them.

Guess what? Electronics doesn't magically stop at the exact place where the Siglent's vertical control does. There's plenty of signals below 1mV. You'll need an amplifier to see them and the exact same same amplifier would work with a Rigol, too.

Plus: You originally said you do digital stuff, so...  :-//


Your thinking:

Siglent that has 5-10X times better noise and performance and sensitivity but cannot measure properly something at 200uV levels,
Rigol MSO5000 cannot measure something properly even at 10mV
That makes them equal. Because they both have something they cannot measure.

By the analogy:
You are same as a gorilla. You both have eyes.. Let's ignore other, irrelevant, details..

Get a grip..
"Just hard work is not enough - it must be applied sensibly."
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Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #176 on: December 29, 2021, 11:59:33 am »
Also you keep repeating about some magical amplifiers. Amplifiers that have DC-100 Mhz bandwith and less noise as even a little Micsig or Siglent SDS1104X-E cost as much as a good scope from Keysight.

Yes, but amplifiers from DC to 1MHz are incredibly cheap (ie. a $2 OP-amp) and would be perfectly adequate for power supplies and audio work.

Amplifiers from 1MHz to 2GHz are also incredibly cheap.

Switching power supplies are measured by convention in DC-20MHz range. And that was for old switchers switching at up to some 100ths of kilohertz. Today switchers are in MHz range, and you better be looking at them up to 300-400 MHz range because of EMI problems...

Also those RF amplifiers are not replacement for a proper scope front end.
"Just hard work is not enough - it must be applied sensibly."
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #177 on: December 29, 2021, 12:07:28 pm »
I 'm reading some of these posts and  :-//

It is obvious MSO5000 has much more noise, and that it is a problem, unless you only look at digital signals and just want to look at general shape and some timing information.
It is shame, really, because new Rigol scopes are much more powerful processing wise than the old ones, and generally held great promise but analog front end/ADC noise performance is not very good.

Scope with low noise is always going to be better instrument than the one with high noise. Why is that even a discussion?
Is this some audiofool discussion how this huge noise is pleasant to look at because it's pretty?  |O

I don't use bandwidth limiting, averaging or any "signal cleanup" features when I'm looking into a signal I want to understand. You would want to look at this switcher signal with a full 1GHz bandwidth and with as low noise scope you can.
To really see what is there... Switching ripple is most of the time least interesting part of switching PSU. We expect it to be there, and most of the time it will be roughly what we calculated. Other, higher frequency stuff (those little hairs on top) is much more problematic and most of the time those will give you headaches.. Nanovolts of those will already be seen on any EMI test...

You filter, limit and "cleanup" signal in circumstances where you understand your signal and you want to ignore noise and other parts of signal on purpose. If your signal is buried inside the noise, you average.
But is that noise part of signal you're measuring or your scope is not irrelevant. If it comes from DUT I want to know that. I want to see it..
Only way to do that is to have low noise scope.

Of course, like OP correctly asked, there is a point of diminishing returns..
Is scope with 5 uV of RMS noise so much better than one with 50uV RMS noise for measuring this switcher signal from this example? Probably not.
It would be definitely better but probably not usefully so in this case. But one with 50uV of RMS noise is definitely better than one which has trace that is whooping 20 mV wide... On a signal that is 60mV P-P...
On this test I would call MSO5000 from Rigol useless for this measurement. And averaging this not autocorrelated signal ( it doesn't repeat cleanly and doesn't retrace it's waveform exactly but varies slightly all the time) will not extract more detail but will hide even more information about signal..

OTOH Siglent shows pretty much perfect representation of the signal, big peaks, ripple AND little hairs. That is your switcher output. That is useful information..

Little Micsig STO1104C/E, or Siglent SDS1104X-E could do equally good job here.

Sad part is that little Rigol DS1054Z would be much better for this signal than MSO5000.. DS2000A had excellent low noise front end .. But new series of Rigol scopes is very powerful in processing power but analog performance is worse than older series. Shame really, otherwise they are very nice scopes.

Thank you for the great explenations, I also agree with you
« Last Edit: December 29, 2021, 12:21:20 pm by Fiorenzo »
 
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Offline Vestom

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #178 on: December 29, 2021, 12:09:00 pm »
The lower sampling frequency of the SDS2000X+ is much less of a problem than it sounds due to the excellent ETS-like trigger implementation. There are plenty of examples showing the Leo Bodnar pulse with very good fidelity on the SDS2k+.

However frame-by-frame averaging is really a kludge one should be careful about using, since it hides transients, glitches etc. Also with modern low-amplitude digital signals measured using x10 probe, low noise performance has become more important for digital signals than you might think...

Of course you can add external amplifiers - but why not buy a proper scope instead?
 
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #179 on: December 29, 2021, 12:18:04 pm »
With channels 1 and 3 of each scope activated a very different picture may emerge at some timebase settings.

Yes, because if you enable to adjacent channels the Siglent only has 1GS/sec to look at 350Mhz signals, ie. it's getting uncomfortably close to Nyquist.

Turning a channel off can bump the sample rate to 2GS/sec and give a different picture.

No. Actually i did many experiment about It. If you set the Rigol so it has a lower sampling speed the noise doesn't become lower.

I checked It in many ways.

For example if you turn on all the channels together the noise stay the same.

If you lower the memory buffer the sampling rate becomes lower but the noise stay again similar until you force It to work with a very very little Memory like 20k or so.

I have tried all i could think to get a better sampling from the Rigol and It was not possible.

In regard of me now I am doing digital stuff but i am going to work a lot with analoge signals and circuits so i preferred to switch to the Siglent.

As i said i think the Rigol is a very great scope but noisy
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #180 on: December 29, 2021, 12:19:46 pm »
I 'm reading some of these posts and  :-//

It is obvious MSO5000 has much more noise, and that it is a problem, unless you only look at digital signals and just want to look at general shape and some timing information.

Fiorenzo originally said his main work was digital.

Scope with low noise is always going to be better instrument than the one with high noise. Why is that even a discussion?

Because: "Price"

(ie. This isn't a 100% technical discussion, if it was we'd all be driving 10-bit R&S 'scopes)

On this test I would call MSO5000 from Rigol useless for this measurement.

The Rigol displayed "14.003mV" on screen and the Siglent displayed "14.0482mV"

That's 0.3% difference between them.

Little Micsig STO1104C/E, or Siglent SDS1104X-E could do equally good job here.

Yep. My original recommendation was to save 1000 Euros and get the SDS1104X-E.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #181 on: December 29, 2021, 12:25:42 pm »
No. Actually i did many experiment about It. If you set the Rigol so it has a lower sampling speed the noise doesn't become lower.

That's because it won't actually change the ADC clock speed, it just discards samples.

What's being discussed is the sample rate to bandwidth ratio. When it approaches a ratio of 2 you'll start to see artifacts in the display.

The Siglent has 350Mhz bandwidth and can drop to 1Ghz sample rate if you enable all channels. This can produce visibly different results at maximum zoom. You'll see it most on digital signals.

The Rigol doesn't drop below 2GHz sample rate so it should never have a problem.
 

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #182 on: December 29, 2021, 12:35:40 pm »
No. Actually i did many experiment about It. If you set the Rigol so it has a lower sampling speed the noise doesn't become lower.

That's because it won't actually change the ADC clock speed, it just discards samples.

What's being discussed is the sample rate to bandwidth ratio. When it approaches a ratio of 2 you'll start to see artifacts in the display.

The Siglent has 350Mhz bandwidth and can drop to 1Ghz sample rate if you enable all channels. This can produce visibly different results at maximum zoom. You'll see it most on digital signals.

The Rigol doesn't drop below 2GHz sample rate so it should never have a problem.
In the end neither is suitable for looking at 350MHz signals using 4 channels. The Siglent SDS2k due to low samplerate, the Rigol MSO5000 due to excessive noise. Also note what David Hess wrote: Rigol typically performs math on decimated data which can give the wrong results when doing measurements on noise.

All in all, if you venture into the HF arena, you'll need to look at more expensive scopes. For a general purpose daily driver scope, low noise is king all day long.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #183 on: December 29, 2021, 12:38:56 pm »
No. Actually i did many experiment about It. If you set the Rigol so it has a lower sampling speed the noise doesn't become lower.

That's because it won't actually change the ADC clock speed, it just discards samples.

What's being discussed is the sample rate to bandwidth ratio. When it approaches a ratio of 2 you'll start to see artifacts in the display.

The Siglent has 350Mhz bandwidth and can drop to 1Ghz sample rate if you enable all channels. This can produce visibly different results at maximum zoom. You'll see it most on digital signals.

The Rigol doesn't drop below 2GHz sample rate so it should never have a problem.
In the end neither is suitable for looking at 350MHz signals using 4 channels. The Siglent SDS2k due to low samplerate, the Rigol MSO5000 due to excessive noise. Also note what David Hess wrote: Rigol typically performs math on decimated data which can give the wrong results when doing measurements on noise.

All in all, if you venture into the HF arena, you'll need to look at more expensive scopes. For a general purpose daily driver scope, low noise is king all day long.

MSO5000 actually can use all data.it has propper implementation  of math but it is let down by noise.. Shame.
"Just hard work is not enough - it must be applied sensibly."
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Offline FiorenzoTopic starter

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #184 on: December 29, 2021, 12:48:20 pm »
Yes fungus I think so.
In regard of your suggestion to buy a cheaper scope my thinking was in line with you but i needed some function that are only in the Siglent sds2000.

About this topic my need was to understand how more important is sample rate against front end noise.

Because of my bad english maybe i could not explain well my doubts.

In the end if you or other have suggestions i can do some other test with the limited equipment i have until I have both the scopes at home..... because i am going to send back the Rigol as soon as possible.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #185 on: December 29, 2021, 12:49:52 pm »
In the end neither is suitable for looking at 350MHz signals using 4 channels. The Siglent SDS2k due to low samplerate, the Rigol MSO5000 due to excessive noise. Also note what David Hess wrote: Rigol typically performs math on decimated data which can give the wrong results when doing measurements on noise.

All in all, if you venture into the HF arena, you'll need to look at more expensive scopes.

Probing a 350MHz signal with passive probes is also a minefield, the artifacts from the probe will usually be bigger than the signal.

I wouldn't buy either of these for the bandwidth, I'd buy them for the big touch screens, large memory, etc.
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #186 on: December 29, 2021, 12:52:24 pm »
Yes fungus I think so.
In regard of your suggestion to buy a cheaper scope my thinking was in line with you but i needed some function that are only in the Siglent sds2000.

What function?

Siglent has added more functions to the SDS1104X since launch. Maybe they added it but you're looking at an old manual.

In the end if you or other have suggestions i can do some other test with the limited equipment i have until I have both the scopes at home..... because i am going to send back the Rigol as soon as possible.

The only other interesting test is the effect of intensity on the color-graded display.

For that a photo of the screen isn't really good enough though. You'll need to put in a USB stick and press the 'print' button.
 

Offline Martin72

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #187 on: December 29, 2021, 02:01:54 pm »
I´ve changed from MSO5354 ( ;)) to the sds2k+ - After I did the very first ripple-measurement with the mso.
The noiselevel is "horrible", useless for doing low noise measurements - By the way, Dave mentioned it also in his first rigol mso 5000 video.
I got the sds2k+ over a year now, before the rigol over a year, there is nearly nothing the rigol can do better.
Display is worser, much worser ( same resolution btw), UI is mickey mouse style and sometimes really confusing, the response itself is slower...
It´s a shame, because the hardware itself is powerful, except the noisy frontend.
And as I´ve asked the rigol support, if there is a chance to get it better by buying the 7000, their answer to me kills everything.
Frontend are the same and that was it, I´ve changed to siglent without any regrets so far, except the missing of 4 math-traces at the same time..

 
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Online mawyatt

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #188 on: December 29, 2021, 03:53:23 pm »
We had to sift thru all this a year and half ago. After a few months decided to "listen" to those that had actual experience with the scopes and for comparisons actually had used various brands. Dave's videos are a great resource, and should be reviewed many times.

Couple folks here (think 2N3055 & Martin72) have actually used and worked with both the Rigol and Siglent scopes, so value these experiences and responses.

BTW we decided on the Siglent SDS2000X+, the deciding factor was the Rigol noise. Now have 2 Siglent scopes and maybe getting a 3rd, so that alone says enough about our experiences.

Best, and Happy Scope Hunting :-+
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 
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Online mawyatt

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #189 on: December 29, 2021, 04:20:30 pm »
Not to discredit Rigols' attempt and deployment of integrating the entire front end and ADC, actually we should applaud them :clap:

Coming from a career that often involved low noise, wide bandwidth and high dynamic range requirements, designing such at the discrete level is extremely difficult, and speaking from experience, at the Integrated IC level, this becomes exceedingly difficult and quite expensive. Things that work at the discrete level may not work so well on an IC, and an entire different design approach is often required. Modern IC processes that feature amazing small devices, and very fast, are dictated by the digital requirements. The analog use of such is "you get what you get" and you must figure out how to achieve the analog performance goals, which often means a complete departure from a conventional discrete approach.

Anyway, hopefully Rigol (Siglent, maybe some other mid-level players) will continue with an integrated scope front-end and ADC solution because we'll all benefit from this integration.

Best,
Curiosity killed the cat, also depleted my wallet!
~Wyatt Labs by Mike~
 
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Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #190 on: December 29, 2021, 05:47:54 pm »
Your thinking:

Siglent that has 5-10X times better noise and performance and sensitivity but cannot measure properly something at 200uV levels,

Are you sure? See reply #130 in the following thread ;)

https://www.eevblog.com/forum/testgear/suggestions-for-a-dmm/msg2766948/#msg2766948

 
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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #191 on: December 29, 2021, 06:10:48 pm »
Rigol MSO5000 cannot measure something properly even at 10mV

This, in a thread full of screenshots showing the Rigol measuring things to sub-millivolt resolution with the same accuracy as a Siglent.


 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #192 on: December 29, 2021, 07:01:39 pm »
Rigol MSO5000 cannot measure something properly even at 10mV

This, in a thread full of screenshots showing the Rigol measuring things to sub-millivolt resolution with the same accuracy as a Siglent.
It appears hard to understand for some (well, actually I know only one), but even though the amplitude measurement for the ripple is correct - which doesn't come as a surprise since noise averages out to zero - engineers still want to see the details apart from the very predictable ripple, which includes switching noise and other potential RFI components, as well as any other glitches that might hint on hidden problems with the circuit.

EDIT: And the result is only equal for the Vrms measurement. Has anyone noticed the huge difference in the Vpp measurment, where the noise does not average out to zero?
« Last Edit: December 30, 2021, 05:50:40 pm by Performa01 »
 
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Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #193 on: December 29, 2021, 08:18:23 pm »

Now let's see if you can understand this:

Imagine I have a sub mV signal for you to look at ... what are you Siglent owners going to do?

Now let's see if you can understand this:
Rigol cannot see anything less than 10mV.

Siglent users will see this, with stable triggering:


That's almost two orders of magnitude better.

I rest my case. You're trolling now, nothing else... Stop wasting our time. Please.

« Last Edit: December 29, 2021, 08:22:57 pm by 2N3055 »
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Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #194 on: December 29, 2021, 08:50:02 pm »
R&S owners are equally blessed with low noise  8)



And the option to use a filter on the signal.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #195 on: December 29, 2021, 09:01:14 pm »
R&S owners are equally blessed with low noise  8)

Compared to the previous Siglent screenshots, the sine wave looks a bit distorted, though. Or is the signal generator to blame here?
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #196 on: December 29, 2021, 09:10:24 pm »
R&S owners are equally blessed with low noise  8)

Compared to the previous Siglent screenshots, the sine wave looks a bit distorted, though. Or is the signal generator to blame here?
There is some noise riding on top and keep in mind the bottom trace is not averaged but filtered.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #197 on: December 29, 2021, 09:24:48 pm »
What are you going to do when the ripple is even smaller?  For example, here is a ~150uVrms 10MHz signal being clearly triggered and displayed.  This signal was displayable--triggerable and above the noise threshold on a Tek 2465B (shown), a Tek 2221A (digital and analog) and the Siglent 1104X-E (although just barely and not as reliably).  I was able to do the same thing with a 1MHz and 100Hz signal of approximately the same amplitude.  What would it look like on the Rigol? 

You can't average your way out of this when you are looking for noise--possibly non-periodic--in the first place.  On the Tek 2221A and the Sig 1104X-E, averaging made the signal look nicer but I'm not convinced that means better.

I have run across that problem a couple times, where only a lower noise oscilloscope would do when looking for subtle anomalies.  The usual solution is to use averaging or high resolution mode and trigger off of a different channel from a signal which is synchronous to the signal of interest, but this is not always possible.  See below about differential probes.

My 10 bit digitizing but not a DSO Tektronix 400 MHz 7854 can reveal signals that my 40 uV RMS over 100 MHz 2232 or 2230 cannot even see in digital storage mode, and do it with an RF sampling front end if needed, but an equivalent instrument today would be 10s of thousands of dollars and we are not discussing those.

Which initiates another question; what would the MSO5000 sampling drop to if one more channel was activated ?

Lower cost DSOs do that because they either have only one bank of acquisition memory with limited bandwidth shared between 4 digitizers, or because the digitizer is interleaved between the 4 channels.  The later is practically universal in lower cost DSOs.

Either can be seen as a cost saving measure or a way to maximize performance when only a single channel is used, and more expensive instruments may use completely separate digitizing and storage for each channel so sample rate, and usually record length, does not depend on the number of channels.  Tektronix used to refer to a DSO as being "real time" if its maximum sample rate, and record length, did not change with the number of channels and for many years they maintained a separate line of DSOs which worked this way because some applications demand it.

Also you keep repeating about some magical amplifiers. Amplifiers that have DC-100 Mhz bandwith and less noise as even a little Micsig or Siglent SDS1104X-E cost as much as a good scope from Keysight.

Yes, but amplifiers from DC to 1MHz are incredibly cheap (ie. a $2 OP-amp plus power supply) and would be perfectly adequate for audio work and looking at power supply ripple.

Amplifiers from 10kHz to 2GHz are also incredibly cheap.

What is not cheap or easy are getting good flatness and settling time over the bandwidth of interest.  However the high noise is a problem of design and not cost.  Up to 350 MHz, there is no excuse for such a noisy implementation and obviously Siglent is doing something right that Rigol is not.  The noise on the Siglent is high compared to what it could be, but that makes the noise on the Rigol much worse than high.

In the end neither is suitable for looking at 350MHz signals using 4 channels. The Siglent SDS2k due to low samplerate, the Rigol MSO5000 due to excessive noise. Also note what David Hess wrote: Rigol typically performs math on decimated data which can give the wrong results when doing measurements on noise.

It is not the decimated data which is the problem but performing math on the display record which has already been processed for the display.  More expensive instruments maintain a separate full resolution record for processing what is essentially the "raw" data, that is separate from the processed display record.

Probing a 350MHz signal with passive probes is also a minefield, the artifacts from the probe will usually be bigger than the signal.

Passive high impedance probes are more difficult to use at higher frequencies than active probes, but if this is taken into account, the usual problem is noise from the ground loop with a singled ended probe, which even an active probe does not solve.  A differential probe solves this but at the expense of greater noise, and this tradeoff is almost always worth it if their higher cost can be accepted.  Several times I have probed signals approaching the limit of the input noise of my oscilloscope where differential probing was a solution because it removed common mode noise.

Quote
I wouldn't buy either of these for the bandwidth, I'd buy them for the big touch screens, large memory, etc.

I was thinking that about the Rigol MSO5000 series before I saw how much better the Siglent is for noise.  They are both noisier than the general state of the art of more than 2 decades ago, but for most applications that is good enough.  By preference I do not even use the lowest noise oscilloscope that I have available.  Fiorenzo did the right thing by inquiring.
« Last Edit: December 29, 2021, 09:34:21 pm by David Hess »
 
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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #198 on: December 29, 2021, 09:30:58 pm »
Fiorenzo did the right thing by inquiring.

And has provided some valuable data.
 

Offline masterx81

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #199 on: December 29, 2021, 09:36:33 pm »
How did you get so low noise waveforms? Did all you live in a faraday cage?  :-DD
I can't go so low with my 1104x-e as it pickup always random noises also with all things turned off in the room except the scope. It get also noises from the cooking gas sparks for starting the flame two rooms away from me (kitchen). I get the 34khz from the fluorescent lamp on the desk when powered on. The scope itself send a decent ammount of noise from the lcd panel (i think the lcd backlight buck is causing it), and you need to be very careful to the routing of the wire of the probe. Also if i connect the output of a battery powered signal generator, i can't reach those so low level of noise (still better than the fg fy6900, correctly grounded and with all-linear psu). In any case before the siglent i had an owon 7102... and hell, that thing was horrible in noise.
 

Online bdunham7

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #200 on: December 29, 2021, 10:26:25 pm »
Well if we're having a noise contest, the new models are going to have to answer to the antiques.

In the DSO category, here's a Tek 2221A with a 150uVrms signal:

And to take the cake in the CRO category, here is a Kikusui COS5100A with a 20uVrms (-81dBm) signal at 1mV/div.  You can barely see it, but amazingly the trigger locks on steady.  I've never seen another full-bandwidth scope (100MHz in this case) that can do this.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline 0culus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #201 on: December 29, 2021, 10:30:10 pm »
Piece of cake for a 7A22 amplifier. 10 µV/div, adjustable bandpass from DC-1 MHz.
 

Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #202 on: December 29, 2021, 10:48:03 pm »
How did you get so low noise waveforms? Did all you live in a faraday cage?  :-DD
I can't go so low with my 1104x-e as it pickup always random noises also with all things turned off in the room except the scope. It get also noises from the cooking gas sparks for starting the flame two rooms away from me (kitchen). I get the 34khz from the fluorescent lamp on the desk when powered on. The scope itself send a decent ammount of noise from the lcd panel (i think the lcd backlight buck is causing it), and you need to be very careful to the routing of the wire of the probe. Also if i connect the output of a battery powered signal generator, i can't reach those so low level of noise (still better than the fg fy6900, correctly grounded and with all-linear psu). In any case before the siglent i had an owon 7102... and hell, that thing was horrible in noise.

Good question..
Everything coaxial, 50Ohm, attenuators.. And relatively quiet lab. No WiFi or phones within 5 meters... And signal source was Picoscope 4262 internal low distortion AWG.
"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 
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Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #203 on: December 29, 2021, 10:53:52 pm »
Well if we're having a noise contest, the new models are going to have to answer to the antiques.

In the DSO category, here's a Tek 2221A with a 150uVrms signal:

And to take the cake in the CRO category, here is a Kikusui COS5100A with a 20uVrms (-81dBm) signal at 1mV/div.  You can barely see it, but amazingly the trigger locks on steady.  I've never seen another full-bandwidth scope (100MHz in this case) that can do this.

20uV RMS at full 100 MHz BW ? That is good!

Piece of cake for a 7A22 amplifier. 10 µV/div, adjustable bandpass from DC-1 MHz.

7A22 is special no doubt..

Picoscope 4262 has noise floor of 8uV if we talk 5 MHz BW max..
It can come close to 7A22.

But original discussion was for at least 20MHz BW...
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Offline tautech

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #204 on: December 29, 2021, 10:56:39 pm »
Which initiates another question; what would the MSO5000 sampling drop to if one more channel was activated ?

Lower cost DSOs do that because they either have only one bank of acquisition memory with limited bandwidth shared between 4 digitizers, or because the digitizer is interleaved between the 4 channels.  The later is practically universal in lower cost DSOs.
Yet this is not the case for the 2 Siglent models that have been brought to this discussion whereas for the MSO5k AFAIK its 8GSa/s is shared by all 4 channels. OTOH SDS1104X-E and SDS2104X+ both have dual ADC's that give the user the opportunity to maintain high sampling rates when 2 channels are active by assigning them to each ADC that BTW each have their own memory support.

In the case of the screenshots previously posted with a single channel active the MSO5k is displaying 500MSa/s at the timebase selected whereas the SDS2104X+ displayed 400MSa/s.
Therefore activating any other channel should drop the MSO5k to 250MSa/s whereas if a second channel was assigned the the 2nd ADC on the SDS2104X+ its sampling rate and memory depth will remain unchanged.

As yet nobody has provided this info for MSO5k.  :popcorn:
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Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #205 on: December 29, 2021, 11:11:20 pm »
I duplicated your test as best as I could on my 2232.  Note that front end noise dominates digitizer noise at 2mV/div.  The third example shows peak detection with noise reduction applied.  I think the noise reduction algorithm is a noise gate.
 
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Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #206 on: December 29, 2021, 11:18:53 pm »
How did you get so low noise waveforms? Did all you live in a faraday cage?  :-DD
I can't go so low with my 1104x-e as it pickup always random noises also with all things turned off in the room except the scope. It get also noises from the cooking gas sparks for starting the flame two rooms away from me (kitchen). I get the 34khz from the fluorescent lamp on the desk when powered on. The scope itself send a decent ammount of noise from the lcd panel (i think the lcd backlight buck is causing it), and you need to be very careful to the routing of the wire of the probe. Also if i connect the output of a battery powered signal generator, i can't reach those so low level of noise (still better than the fg fy6900, correctly grounded and with all-linear psu). In any case before the siglent i had an owon 7102... and hell, that thing was horrible in noise.

Good question..
Everything coaxial, 50Ohm, attenuators.. And relatively quiet lab. No WiFi or phones within 5 meters... And signal source was Picoscope 4262 internal low distortion AWG.

I did the same but of course my 2232 has no 50 ohm input.  My signal source was a Tektronix FG502 11 MHz function generator set for minimum output with 50 dB of attention attached to its output.

If you look carefully, my examples show a discontinuity in the sine function output from common mode noise which I should have removed by using a higher output level and more attenuation.
« Last Edit: December 29, 2021, 11:20:35 pm by David Hess »
 
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Offline normi

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #207 on: December 29, 2021, 11:35:57 pm »
@Fiorenzo
You face the common issue most new scope buyers face; you want a scope that will do as much as possible for the amount of money you spend, unfortunately that is not the case. The MSO5000 will do things the Siglent can't and visa versa, although the Siglent is 40-50% more expensive. So lets look at the difference, noise floor Vs Sample rate.

Sample Rate:
It is horizontal resolution, it allows you to see finer details horizontally (time). It is an invisible feature, meaning that if your sample rate is low and causing an issue most cases you won't know. Aliasing is a visible issue if you are experienced enough to see it, but other critical flaws will be hidden. (You won't know that you don't know)

When most needed:
If there is a non repetitive and infrequent glitch in a signal that would not be sampled by the scope because the spacing between the samples is too wide (AKA low sample rate).

Impact:
You could go weeks trying to figure out a problem and will not realize that your circuit's failure is caused by the glitch that your slow scope is unable to find, this can be very costly in time.

Work around:
1. Obtain a high sample rate scope, all are very expensive (except the MSO5000).

Noise floor, small voltage scales:
 It is the vertical resolution (volts). It is very visible, which is why you came to this forum, so unlike sample rate you will be fully aware of what you can't see.

When most needed:
Looking at very low voltage signals.

Impact:
If your system is susceptible to very small noise signals or you work with tiny signals then you want as low noise as possible. Since you can clearly observe your scopes noise level you won't spend as much time searching as you would be aware that you can't see signals cleanly below a certain level. So not likely to have a dramatic impact as the unseen glitch, because you can quickly proceed to a work around if you want to investigate low noise.

Workaround:
1. Use averaging to clean up noisy signal
2. use an amplifier - some a very cheap and can be modified for Oscilloscopes or Spectrum analyzers, or can be built. Professional ones are very expensive but are usually used for differential measurements and are needed by even the scopes with low noise floor to see much smaller signals.
3. Obtain a scope with lower noise, in some cases these lower noise scope can be cheaper than the MSO5000

Outside of the fact that the RIGOL MSO5000 has a much lower price than the Siglent SDS - Plus, you could still buy a cheaper Siglent scope which would have the same noise performance as the SDS -Plus, however it is impossible to find another non Rigol 8G/S scope for under a $1000, or under $2000. This is the reason why the scope is attractive to many buyers.


 
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Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #208 on: December 29, 2021, 11:58:34 pm »
To challenge Rigol, here are 1.5mVpp on a real toy scope >:D
Relatively clean trace despite no averaging (not supported), but admittedly just 2MHz BW.
Frontend buffer seems to be just an off-the-shelf JFET opamp like LF356 or LF357, or similar.
 

Online nctnico

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #209 on: December 30, 2021, 12:07:48 am »
Noise floor, small voltage scales:
 It is the vertical resolution (volts). It is very visible, which is why you came to this forum, so unlike sample rate you will be fully aware of what you can't see.

When most needed:
Looking at very low voltage signals.
Buuzzzzz wrong! As I wrote before: noise floor simply scales along with the V/div settings. In that perspective using low level signals is not representative for regular scope usage. The Rigol MSO5000 also sucks for higher level signals because the noise will still drown details of the signal. Averaging won't help because that also obscures the details that you want to catch.

And having a higher samplerate gives you nothing if it is far beyond the bandwidth of your scope; it only wastes valuable memory. It just becomes a ridiculous number like having an 8000kW engine in a go-kart.
« Last Edit: December 30, 2021, 12:10:04 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #210 on: December 30, 2021, 12:45:16 am »
And having a higher samplerate gives you nothing if it is far beyond the bandwidth of your scope

It helps insofar, as you can you turn on HiRes acquisition, and still end up with a relatively high decimated sample rate.
E.g. 8-tap HiRes boxcar averaging @ 8GSa/s gives a decimated sample rate of still 1GSa/s, and noise is reduced by a factor of ~2.8.
If the primary sample rate were only 1GSa/s, then the decimated rate were already as low as 125MSa/s.
Edit: And enabling more than 1 channel reduces the sample rater further.
« Last Edit: December 30, 2021, 12:47:43 am by gf »
 

Offline David Hess

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #211 on: December 30, 2021, 05:28:15 am »
As I wrote before: noise floor simply scales along with the V/div settings. In that perspective using low level signals is not representative for regular scope usage. The Rigol MSO5000 also sucks for higher level signals because the noise will still drown details of the signal. Averaging won't help because that also obscures the details that you want to catch.

In most cases, the noise of the high impedance buffer at the input will dominate at the highest V/div sensitivities where attenuation immediately after the high impedance buffer is lowest.  For this not to be the case, their must be a gain stage following the attenuators which has higher noise.  This is not impossible, and could even be likely if the following gain stage is integrated CMOS instead of bipolar.

For the Rigol MSO5000 we could learn something from measuring the noise at all V/div settings.  If the high impedance input buffer noise is greater than the following stages, then there should be a jump to higher noise at about 0.1 V/div where the input attenuator is switched in, and the low impedance attenuators are switched out.
 

Offline Performa01

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #212 on: December 30, 2021, 07:47:05 am »
While we would like the noise at low frequencies (<100 kHz) to be lower, yet it is certainly not high at 1 MHz and above. Very comparable with old analog scopes.

Here's a demonstration, what an SDS2000X Plus can show with an emulated ripple with asynchronous spikes riding on it if optimal probing (without additional noise pickup) is applied. 2.5 mVpp 1 MHz ramp with 300 µV 6.000001 MHz 10 ns wide spikes riding on it.

DSO Sensitivity is 500 µV/div, 10 bits mode, 100 MHz bandwidth. No averaging of course, in order to keep the spikes clearly visible.

SDS2354X Plus_Ramp_2.5mV_1M_Pulse_300uV_6000001Hz
 
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Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #213 on: December 30, 2021, 09:02:38 am »
And having a higher samplerate gives you nothing if it is far beyond the bandwidth of your scope
It helps insofar, as you can you turn on HiRes acquisition, and still end up with a relatively high decimated sample rate.

The Rigol doesn't do HiRes mode though.

(unless they've added it and missed the memo)
 

Offline gf

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #214 on: December 30, 2021, 09:30:53 am »
The Rigol doesn't do HiRes mode though.

I don't remember that I had seen a HiRes screenshot from the MSO5000 in this thread.

But the MSO5000 User Guide claims that it does:

Quote
High Resolution

This mode uses an over-sample technique to average the neighboring points of the sample waveform. This reduces the random noise on the input signal, generates a much smoother waveform on the screen and improves the vertical resolution. This is generally used when the sample rate of the digitalconverter is greater than the storage rate of the acquisition memory.

Note:
* The"Average"and "High Res"modes use different averaging methods. The former uses "Multi-sampleAverage"and the latter uses "Single-sampleAverage".
* In "High Res"mode,the signal bandwidth does not exceed 1/32 of the sampling rate.
* In "High Res"mode,the highest waveform refresh rate mode is not supported.

Edit: It does not tell the actual decimation factor or the number of averaged neighbor samples, though. A boxcar filter with 16 taps had a -3dB cut-off of ~fs/35, with 8 taps it were ~fs/17, and in order to get the documented fs/32, the closest number of required taps were 15. But this is pure speculation now and I think the "truth of the actual implementation" can only be determined experimentally by an owner.

Btw: One disadvantage of a boxcar averaging filter (and thus disadvantage of HiRes, if based on boxcar averaging) is that its sinc frequency response starts rolling off already beyond DC, i.e. the passband has no pronounced "flat top". If this matters for a particular use case, the filter cut-off should be rather chosen several times higher than the highest frequency of interest.
« Last Edit: December 30, 2021, 11:06:51 am by gf »
 

Offline Fungus

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #215 on: December 30, 2021, 09:56:13 am »
The Rigol doesn't do HiRes mode though.

I don't remember that I had seen a HiRes screenshot from the MSO5000 in this thread.

But the MSO5000 User Guide claims that it does:

Quote
High ResolutionThis mode uses an over-sample technique to average the neighboring points of the sample waveform. This reduces the random noise on the input signal, generates a much smoother waveform on the screen and improves the vertical resolution. This is generally used when the sample rate of the digitalconverter is greater than the storage rate of the acquisition memory.

Note:
* The"Average"and "High Res"modes use different averaging methods. The former uses "Multi-sampleAverage"and the latter uses "Single-sampleAverage".
* In "High Res"mode,the signal bandwidth does not exceed 1/32 of the sampling rate.
* In "High Res"mode,the highest waveform refresh rate mode is not supported.

STOP THE THREAD!

I just downloaded the latest manual from Rigol and it says they've now added "High Res" mode.

HiRes will make a huge difference to the noise level by leveraging that massive 8Ghz sample rate.



Fiorino, we need another test with "HiRes" mode enabled. 
« Last Edit: December 30, 2021, 10:03:47 am by Fungus »
 

Offline 2N3055

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Re: How much noise floor and other things matter in oscilloscope usability
« Reply #216 on: December 30, 2021, 10:46:13 am »

STOP THE THREAD!

I just downloaded the latest manual from Rigol and it says they've now added "High Res" mode.

HiRes will make a huge difference to the noise level by leveraging that massive 8Ghz sample rate.



Fiorino, we need another test with "HiRes" mode enabled.

Yes, please, stop...  :-DD

He already did it. It didn't help, it simply low pass filtered out signal of interest. It hid it.

First thing, HiRes on MSO5000 has been around for some time now, and it's not big help...
They had to downsample 32 times to get something and then not much. HiRes makes it 250Msps/s single channel, down to 62.5Msps/s 3 or 4 channels enabled. 32 MHz max bandwidth at max sample rate, and if you grab longer sequences even lower.


You keep ignoring one fact (probably deliberately by now, because to you it's not about facts but about winning the argument even with lies and misdirection, apparently) that it is not about small signals per se but about details in signal that can be quite large.

High noise makes ENOB (effective number of bits) even less than already not very big 8 bits. So you see thick big sinewave on MSO5000, and a sinewave with a small squarewave superimposed on top on other scope that had low noise and had retained it's resolution better. Hi res scope would do even better, and that is why every manufacturer now is trying to go 10 or 12 or even more bits. It is not irrelevant.

MSO5000 is not useless. It's a scope that you can do work with. There are some usage scenarios when it's high noise won't stop you from doing your job. For some people that might be good enough. Mostly because if you don't know you have a problem, you cannot worry about it..

But, putting head in a sand doesn't make it better or equally good as some other equipment that actually have better specifications and can show signals MSO5000 cannot. Other instruments are literally much better for this kind of measurements. If you only need scope to decode 4 decodes at the same time, yeah, then MSO5000 will be better because other scopes mentioned don't have 4 decode channels. But mediocre analog performance didn't go away and suddenly became perfect. No, it is still mediocre but you don't care. Which is OK if you are FULLY AWARE of all pros and cons.

But fanboying like yours tries to make it like MSO5000 is equally good in EVERY parameter as other scopes. It is NOT.
If you're doing analog, there are better choices.
"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 
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Offline Fungus

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