Author Topic: Rigol DHO800 memory depth / bandwidth upgrades and noise level!  (Read 18016 times)

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Online ebastler

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #125 on: January 17, 2024, 08:20:27 am »
When I try to repeat the same test using the same settings, even when testing only a single channel (leaving the probe disconnected as Fungus apparently did), I get 60Hz noise that makes it impossible to detect the true noise floor.  Noise remained unchanged with or without the ground cable attached to wall socket Earth Ground.

But your probe is not disconnected! Your photo shows that it is connected to the scope, and its tip is left "floating", not connected to any defined potential. It is normal that the probe essentially acts as an antenna then, picking up all kinds of noise from the environment.

I don't know how Fungus had configured the inputs for his earlier measurements, but it am pretty sure it was not the configuration you show:
  • When people refer to "open inputs", they mean that nothing at all is connected to the scope's BNC jack.
  • To really measure the internal noise of the scope, the inputs should be shorted directly at the BNC jack, or a 50 Ohm termination installed there.
  • If you want to measure the background you can expext in a "real world" scenario, you can connect the probe to the scope, and then connect the probe's little ground strap to the tip. Even that little loop can pick up magnetic fields from the environment; we had reports from one user who could receive nearby FM radio stations that way.

 
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Offline JDW

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #126 on: January 17, 2024, 08:50:37 am »
When I try to repeat the same test using the same settings, even when testing only a single channel (leaving the probe disconnected as Fungus apparently did), I get 60Hz noise that makes it impossible to detect the true noise floor.  Noise remained unchanged with or without the ground cable attached to wall socket Earth Ground.

But your probe is not disconnected! Your photo shows that it is connected to the scope, and its tip is left "floating", not connected to any defined potential. It is normal that the probe essentially acts as an antenna then, picking up all kinds of noise from the environment.

I don't know how Fungus had configured the inputs for his earlier measurements, but it am pretty sure it was not the configuration you show:
  • When people refer to "open inputs", they mean that nothing at all is connected to the scope's BNC jack.
  • To really measure the internal noise of the scope, the inputs should be shorted directly at the BNC jack, or a 50 Ohm termination installed there.
  • If you want to measure the background you can expext in a "real world" scenario, you can connect the probe to the scope, and then connect the probe's little ground strap to the tip. Even that little loop can pick up magnetic fields from the environment; we had reports from one user who could receive nearby FM radio stations that way.

Thank you for explaining.  I choose to be more clear in my own personal wording in that if I mean for somebody not to connect any probe at all, I will tell them "no probe attached."  That's 100% clear, with zero guesswork for veterans or newbies alike.  Words like "disconnected" can mean multiple different things.  Even "open input" isn't necessarily clear if you've not learned the meaning.  "No probe attached" is just that.

I don't have a 50Ω terminator, and I am basically trying to replicate what Fungus did because that is established data we can go by.  No, his work is not a gold standard, but it is a nice frame of reference that other people can try to see if we get the same thing.

Currently, my scope's "Probe" settings for each channel are set to 10x, and that's because I normally use my Probes mechanically switched to the 10x setting.  But we're doing this particular test with "no probe attached" so...

  • When displaying only CH1 and with CH1 set to Probe: 10X:  The "Current" column shows about 600uV (Fungus was about 200uV)
  • When displaying only CH1 and with CH1 set to Probe: 1X:  The "Current" column shows about 60uV (Fungus was about 200uV)

I assume Fungus must have used 1x or 10x, so in either of those cases, my numbers are not close to his.

When I attach my probe to CH1 and mechanically set its switch to 10x, then set the scope's CH1 setting to Probe: 10x too, and then connect the probe's GND to the probe's tip (as per your bullet point 3): The "Current" column shows about 476uV (Fungus was about 200uV)  But you will likely explain that as being a matter of the ground strap picking up noise and radio stations.

I then tried your bullet point #2, shorting the inner gold pin of the CH1 BNC to its ground using the shortest wire possible.  I get a "Current" column value of 256uV, which is higher than the values Fungus showed, but we don't know exactly how he measured either.  Even so, it least this test was in the ballpark of his.  So I am largely satisfied with that result.

Thank you for your kind assistance.
« Last Edit: January 17, 2024, 08:55:30 am by JDW »
 
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Online 2N3055

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #127 on: January 17, 2024, 09:15:25 am »
It depends on what are you measuring.

Putting a 50Ω on 1MΩ input "simulates" what noise would be if input were 50 Ω.

Putting a shielding cover over BNC better "simulates" noise at 1MΩ input impedance.
Setting probe ratio to 1:1 will measure "native" noise.
Setting probe to 10x (which is really 1/10 but that is another story) will measure probe tip referred noise, i.e. what is your baseline noise floor when using 10x probe.

A probe "simulator" could be made, with low noise 9 MΩ resistor to ground, in a nice shielded box, connected to input BNC directly. That would simulate connected probe with grounded tip, with no interference. WIth scope set to 10x it would best simulate what a baseline nose would be at 1 MΩ with the ideal 10x probe.

Like I said before, just measuring something without establishing clear protocols and rules ends up with bunch of measurements that means very little because they cannot be directly compared and have no correlation any real world scenario.

And unlike BW measurements that need expensive and calibrated signal generators, some things require little investment, but some rigor and attention and some set rules.
"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 
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Online ebastler

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #128 on: January 17, 2024, 09:41:15 am »
  • When displaying only CH1 and with CH1 set to Probe: 10X:  The "Current" column shows about 600uV (Fungus was about 200uV)
  • When displaying only CH1 and with CH1 set to Probe: 1X:  The "Current" column shows about 60uV (Fungus was about 200uV)
I assume Fungus must have used 1x or 10x, so in either of those cases, my numbers are not close to his.

When you are interested in the noise generated internally in the scope, you should always use 1x as the probe setting. In the 10x setting the scope will "assume" that the signal it sees has been divided by 10 by an external probe. So e.g. 60 µV of noise from its internal input amplifiers will be reported as 600 µV, which is misleading.

If you don't have a termination resistor handy, leaving the BNC input unconnected but shielded is probably the most meaningful test you can do. As 2N3055 explained, that will give you the noise generated by the scope's input section when set to 1 MOhm (i.e. ready for an external probe), but without the "antenna" contributions from an actual probe. You can try and make some improvised shielding from copper or aluminim foil around the BNC jack.

When comparing noise figures, you will also notice that they depend on the scope's time base setting and sampling rate. Going to slower time bases lets the scope capture the low-frequency components of the noise -- which are significant, since there are noise cmponents where the amplitude follows a 1/f curve. Increasing the sampling rate (by increasing the memory depth used, in the Acquisition menu) lets the scope capture higher-frequency contributions, which also add more noise. So when comparing to measurements made by others, be sure to use the same acquisition settings.
 
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Online Aldo22

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #129 on: January 17, 2024, 11:06:49 am »
Interesting topic (noise measurement).
I only have a Hantek at the moment, but the problem will probably remain the same.
Without a checklist, you always forget a setting.

- Acquisition Mode: Normal - Average - Peak - HR give different results.
- 20MHz bandwidth limitation
- Probe attenuation settings
- V/div and s/div
- Input coupling GND
- With the Hantek, the sampling rate also makes a difference.

Have I forgotten something?
All these settings give me different noise values. Is it the same with the Rigol?

The "calibration kit" (Open, Short, Load) of the NanoVNA is quite handy for testing with a BNC-SMA adapter (Attachment) ...and cheap..  ;)
« Last Edit: January 17, 2024, 11:11:03 am by Aldo22 »
 
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Online Fungus

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #130 on: January 17, 2024, 08:09:46 pm »
I don't have a 50Ω terminator, and I am basically trying to replicate what Fungus did because that is established data we can go by.

Just don't connect anything to it. Bare BNCs. 50Ω terminator isn't necessary.

I tried covering them with tin foil to prevent radio interference but it made no difference here.

YMMV, it depends where you live and what's around you.

No, his work is not a gold standard

Gee, thanks...

Currently, my scope's "Probe" settings for each channel are set to 10x, and that's because I normally use my Probes mechanically switched to the 10x setting.  But we're doing this particular test with "no probe attached" so...

The ONLY thing probe 10x setting does is divide all the numbers shown on screen by 10 because the probe has a 10x resistor divider inside it. It makes no changes to the hardware.

The setting should obviously be at 1x to show correct values for this test.
« Last Edit: January 17, 2024, 08:12:00 pm by Fungus »
 

Online Bud

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #131 on: January 17, 2024, 09:48:16 pm »
Have I forgotten something?
Yes. A cookie tin box large enough for the scope to fit inside, strapped with a gage 8 wire to the cold water pipe.
Facebook-free life and Rigol-free shack.
 

Offline JDW

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #132 on: January 17, 2024, 10:08:45 pm »
The setting should obviously be at 1x to show correct values for this test.

Really?? 1x? 
As mentioned in the closing sentence of my previous post, I get a "Current" column value of 256uV when the scope is set to 10x.  Changing the scope setting for CH1, which is the channel I was testing) to 1x would then show a value of 25.6uV.  Your measured values were approximately 200uV.  Therefore, I do believe the 10x setting on the scope must be correct unless one wishes to argue that a measured value of 25.6uV is correct, which would then means my scope is significantly lower noise than yours.

Note that in that final test, I had no probes attached.  I merely shorted the middle BNC pin to its outer ground via the shortest wire possible.
 

Online Bud

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #133 on: January 17, 2024, 10:14:36 pm »
Changing the scope setting for CH1, which is the channel I was testing) to 1x would then show a value of 25.6uV.
No "would". Switch and check. You were advised to do the right thing.
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Offline JDW

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #134 on: January 18, 2024, 01:30:37 am »
Changing the scope setting for CH1, which is the channel I was testing) to 1x would then show a value of 25.6uV.
No "would". Switch and check. You were advised to do the right thing.

 :palm:

Based on your strongly worded remarks above, I suspect that if I had said "When you wet your fingers and then short them across bare wires in your wall socket with active AC power flowing, you would receive a shock" you would have told me: "try it and see." 

Here's the video proof you asked for, which again, is only common sense and is but an extension of the same testing I did yesterday...

https://youtu.be/OEXGotlZeiI

For those too lazy to watch the video evidence, the fact is that the numbers shown in the "Current" column are 10x larger when you set your channel to 10x (scope setting) as compared to the numbers in the "Current" column when you set the scope to 1x, for the particular channel being used at the time, which in my case is CH1.  This is exactly what I said previously.

Now I hope that settles it.
 

Online Aldo22

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #135 on: January 18, 2024, 12:17:52 pm »
Have I forgotten something?
Yes. A cookie tin box large enough for the scope to fit inside, strapped with a gage 8 wire to the cold water pipe.
To shield the lab from the noise of the oscilloscope or the other way around? ;)
 

Online ebastler

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Re: Rigol DHO800 memory depth / bandwidth upgrades and noise level!
« Reply #136 on: January 18, 2024, 06:31:32 pm »
Based on your strongly worded remarks above, [...]

That's just Bud. He is very much into short and strongly worded remarks. ::)
 


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