Author Topic: Oscilloscope input noise comparison  (Read 15240 times)

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Online maxwell3e10

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Oscilloscope input noise comparison
« on: October 13, 2018, 03:58:10 am »
Some time ago I collected shorted input data from several oscilloscopes to compare their noise. More discussion is in this topic https://www.eevblog.com/forum/testgear/owon-xds3062a-input-noise-(good)-and-glitches-(bad)/

Recently I got a Keysight EDUX1002 scope, so I thought I would compare it to the Chinese brands. As one might expect, the noise spectrum of the Keysight is very clean, but the absolute noise level is a little higher for this entry level scope of comparable cost.

Perhaps it would be interesting to compare higher-end scopes as well, but that's above my budget level :)
« Last Edit: October 13, 2018, 04:37:31 am by maxwell3e10 »
 
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Offline Sylvi

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Re: Oscilloscope input noise comparison
« Reply #1 on: October 13, 2018, 04:24:18 am »
Hi

That's very interesting

How did you measure the noise?

I recently bought an OWON XDS2102A (100MHz, 12-bit) which is definitely showing me things I could not see with either of my previous CRT scopes - one was 50MHz Philips and the other a 20MHz Instek.
 

Offline Hydron

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Re: Oscilloscope input noise comparison
« Reply #2 on: October 13, 2018, 07:45:56 am »
Perhaps it would be interesting to compare higher-end scopes as well, but that's above my budget level :)
If you could specify what data is required, then maybe people with higher end instruments could send in the files to analyse and add to the graph? I'd be happy to grab some data from my RTB2004.
 

Online maxwell3e10

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Re: Oscilloscope input noise comparison
« Reply #3 on: October 13, 2018, 08:02:53 am »
For this comparison I was using the longest time scale that gives 1 GHz sampling rate and the lowest voltage scale that allows full bandwidth. Then just save raw ascii file. To limit file size, use no more than 1M samples.  The input is terminated into 50 Ohm. No averaging or artificial high resolution modes.
 

Offline Hydron

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Re: Oscilloscope input noise comparison
« Reply #4 on: October 13, 2018, 08:23:22 am »
For this comparison I was using the longest time scale that gives 1 GHz sampling rate and the lowest voltage scale that allows full bandwidth. Then just save raw ascii file. To limit file size, use no more than 1M samples.  The input is terminated into 50 Ohm. No averaging or artificial high resolution modes.
I've made a couple of quick measurements at full and 20MHz BW, can't do exactly 1GSa/s but could do 1.25, is that OK? I've saved them as CSV (could do binary as another option if helpful), have sent you a link to the download via PM as it's too big to upload on the forum.
If you need different settings (e.g. due to the 1.25GSa/s thing) please let me know.
 

Online awallin

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Re: Oscilloscope input noise comparison
« Reply #5 on: October 13, 2018, 08:28:35 am »
Perhaps it would be interesting to compare higher-end scopes as well, but that's above my budget level :)
If you could specify what data is required, then maybe people with higher end instruments could send in the files to analyse and add to the graph? I'd be happy to grab some data from my RTB2004.

+1 for this.
Put the data-files (zipped?) and analysis scripts (numpy/matplotlib?) into a git repo, with instructions on how to collect data - and just wait for the pull-requests to roll in  8)
 

Online maxwell3e10

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Re: Oscilloscope input noise comparison
« Reply #6 on: October 13, 2018, 08:36:47 am »
Thanks, I got  the data for RTB2004! Looks nice, 3-4 nV/Hz^(1/2) noise. I will wait if any other data comes in before making another plot.
 

Offline nctnico

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Re: Oscilloscope input noise comparison
« Reply #7 on: October 13, 2018, 09:12:51 am »
I'll try to pull a file from an R&S RTM3004 later today. A small suggestion: if the new plots are going to contain more scopes then creating 2 graphs (full bandwidth and 20MHz bandwidth) may be a good idea.
« Last Edit: October 13, 2018, 09:15:36 am by nctnico »
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Offline Andreas

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Re: Oscilloscope input noise comparison
« Reply #8 on: October 14, 2018, 06:24:57 pm »
Hello,

interesting project.

For better comparison (and as I am not familiar with all scopes) it would be good to have also the max. bandwidth of the scope used and the max sample rate addditionally to the minimum sensitivity.
And if available the AC-rms (standard deviation) and peak-peak noise for the measurement.

I could part my PicoScope 5444A data but as .csv the amount of data for a 1Ms measurement is huge (even compressed).
With the proprietary internal Format it is below 1 MB so that I could even upload it here.
(You would need the PicoScope Software from PicoTech to export a .csv from here: https://www.picotech.com/downloads )

Also if I export it to Matlab-Format and compress it is below the forum limit.

So the question is if one of these formats would be a alternative.

with best regards

Andreas


 

Offline Hydron

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Re: Oscilloscope input noise comparison
« Reply #9 on: October 14, 2018, 10:22:41 pm »
I had the same issue with CSV size - I PM'd maxwell3e10 a download link rather than post it on the forum. A better solution might be to upload it on github or similar, that way nobody has to download proprietary software to convert the file format.
 

Offline cruff

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Re: Oscilloscope input noise comparison
« Reply #10 on: October 17, 2018, 12:27:44 am »
The input is terminated into 50 Ohm.

Do you mean the input connector was shorted and you used the built-in 50 ohm termination mode of the scope?  Or did you use the scope's high value termination mode and terminated with 50 ohms at the input connecotr?

So effectively you are measuring the thermal noise of the termination resistor(s)?  Or am I missing something?
 
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Online maxwell3e10

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Re: Oscilloscope input noise comparison
« Reply #11 on: October 17, 2018, 01:10:58 am »
Some scopes have a build-in 50 Ohm mode and some don't. I meant to add a 50 Ohm resistor outside to short the input. One could also use a shortening BNC. It wouldn't make much difference unless the noise is 2-3 nV/Hz^(1/2).
 

Offline nctnico

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Re: Oscilloscope input noise comparison
« Reply #12 on: October 17, 2018, 11:34:59 am »
Usually the 50 Ohm and 1M Ohm paths are different so the noise is also different.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online maxwell3e10

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Re: Oscilloscope input noise comparison
« Reply #13 on: October 18, 2018, 03:23:08 am »
Yes, that appears to be the case. I had always assumed in the past that a 50 Ohm input impedance setting on a scope simply switches in a 50 Ohm resistor at the input of the 1 MOhm amplifier.
 

Offline rf-loop

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Re: Oscilloscope input noise comparison
« Reply #14 on: October 18, 2018, 05:13:28 am »
Yes, that appears to be the case. I had always assumed in the past that a 50 Ohm input impedance setting on a scope simply switches in a 50 Ohm resistor at the input of the 1 MOhm amplifier.

It depends oscilloscope. In low price and low frequency oscilloscopes this is "typical" cheap  way to do it. They need only 50 ohm "DC" load resistance. Nothing good in this method for high frequencies - except price. Also noise in these kind of front ends are not at all excellent. Also very small internal noise currents can produce high noise voltages over these 1M pathway after input 50ohm load resistor.

I have seen these many times, connecting external 50ohm feed thru or terminator to 1M scope input it can some times  show even more noise than open or LF-RF shielded input BNC using metal caps.

Time ago one perhaps most worst case  was really amazing effect in some Owon models where lot of internal SMPS circuits was made like wide band radio jammer. "Every kid can design SMPS" so they put cheap unexperienced engineer kids to design these..or for make some modifications to reduce manufacturing costs.  These noise currents flood wild inside scope every part and flow thru different parts and if connect external 50ohm displayed noise level rise a lot of. Because 50 ohm resistor make possible to internal transmitted noise have now pathway to input BNC GND and this generated  noise current produce voltage over 1M.  But if input was open (example if use BNC metal cap without center connection) , less noise. But this was special case.

If scope have this cheap 50 ohm resistor method or if it have real 50ohm input impedance they may be quite different if think analog front end noise.
« Last Edit: October 18, 2018, 05:27:27 am by rf-loop »
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Offline David Hess

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Re: Oscilloscope input noise comparison
« Reply #15 on: October 19, 2018, 01:55:51 pm »
Broadband noise of 10 nV/SqrtHz is about right.  But noise at low frequencies increases due to 1/f or flicker noise which can be very high for integrated CMOS designs which are common now in DSOs.  There is also additional low frequency Johnson noise from the parallel RC network in series with the input which provides overload protection.
 

Online maxwell3e10

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Re: Oscilloscope input noise comparison
« Reply #16 on: October 19, 2018, 02:12:07 pm »
Yes, it would be interesting to have another set of data that spans the frequency range 10 Hz to 1MHz to measure 1/f noise. This would require using lower sample rate, about 1 MHz.
 

Online rsjsouza

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Re: Oscilloscope input noise comparison
« Reply #17 on: October 19, 2018, 06:05:30 pm »
Interesting comparison. I can provide Rigol's DS4014 information. I am pretty sure it will be a lot higher than the average, but still worth taking it.
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Offline Performa01

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Re: Oscilloscope input noise comparison
« Reply #18 on: October 19, 2018, 06:51:11 pm »
Yes, it would be interesting to have another set of data that spans the frequency range 10 Hz to 1MHz to measure 1/f noise. This would require using lower sample rate, about 1 MHz.
The problem with a low sample rate like 1MSa/s is that all the noise above Nyquist (500kHz) will fold back and add to the result. This would severely distort the measurements, since a major part of noise energy would come from that folded back frequency range above 500kHz.

Even with 20MHz bandwidth limit, it would still be 20MHz vs. 500kHz noise bandwidth.

So my suggestion would be to use at least 100MSa/s in order to get a reasonable attenuation of the aliased frequency band by means of the 20MHz bandwidth limit.

How low do we need (or want) to go with such a measurement? For 10Hz, we'd need 10ms/div timebase and at least 10Mpts of data, which we could not handle over this forum. You'd need to provide some server/cloud where we could upload it anyway.
« Last Edit: October 19, 2018, 06:53:33 pm by Performa01 »
 

Offline nctnico

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Re: Oscilloscope input noise comparison
« Reply #19 on: October 19, 2018, 07:12:52 pm »
How low do we need (or want) to go with such a measurement? For 10Hz, we'd need 10ms/div timebase and at least 10Mpts of data, which we could not handle over this forum. You'd need to provide some server/cloud where we could upload it anyway.
Wetransfer works excellent for sending big files to people.
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Offline Keysight DanielBogdanoff

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Re: Oscilloscope input noise comparison
« Reply #20 on: October 19, 2018, 09:53:32 pm »
Thanks for doing this! I'm also interested in how you measured noise. We generally use the smallest hardware V/div setting and measure V RMS (NOT Peak-Peak). This makes sure you don't have any issues with the quantity of acquisitions/update rate.
 
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Online rsjsouza

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Re: Oscilloscope input noise comparison
« Reply #21 on: October 19, 2018, 10:22:55 pm »
Thanks for doing this! I'm also interested in how you measured noise. We generally use the smallest hardware V/div setting and measure V RMS (NOT Peak-Peak). This makes sure you don't have any issues with the quantity of acquisitions/update rate.
Interesting, Daniel. Have you ever seen any manufacturers skew or butcher RMS measurements that could lead to incorrect results?
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Offline nctnico

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Re: Oscilloscope input noise comparison
« Reply #22 on: October 19, 2018, 10:34:20 pm »
Thanks for doing this! I'm also interested in how you measured noise. We generally use the smallest hardware V/div setting and measure V RMS (NOT Peak-Peak). This makes sure you don't have any issues with the quantity of acquisitions/update rate.
I'm wondering if that is the best way of doing it on an oscilloscope which uses decimated data for on-screen measurements.  Also the RMS peak measurement may be affected by any DC offset in the signal. Using the actually sampled data and doing a noise spectrum analysis seems like a much better way to me.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Rich@RohdeScopesUSA

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Re: Oscilloscope input noise comparison
« Reply #23 on: October 20, 2018, 03:07:22 am »
Thanks for doing this! I'm also interested in how you measured noise. We generally use the smallest hardware V/div setting and measure V RMS (NOT Peak-Peak). This makes sure you don't have any issues with the quantity of acquisitions/update rate.
I'm wondering if that is the best way of doing it on an oscilloscope which uses decimated data for on-screen measurements.  Also the RMS peak measurement may be affected by any DC offset in the signal. Using the actually sampled data and doing a noise spectrum analysis seems like a much better way to me.
We typically recommend the same method Daniel suggested (smallest volt/div setting, AC RMS or std deviation or a vertical histogram) with one addition (which I think Daniel would agree with) - you should really figure it as a percent of full scale as some oscilloscopes have 10 vertical divisions and some only have 8.  Of course the other key thing to keep in mind when measuring noise using this method is that noise is a function of BW.  So you may need to use filters to get an apples to apples compare.

-Rich
 

Offline David Hess

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Re: Oscilloscope input noise comparison
« Reply #24 on: October 20, 2018, 03:24:17 am »
you should really figure it as a percent of full scale as some oscilloscopes have 10 vertical divisions and some only have 8.

It is even more complicated than that.  The number of ADC counts per division varies between DSO designs and has little to do with the number of displayed divisions.  For instance 25 counts per division is (or was) common producing a 10.2 division "virtual" display of which either 10 or 8 vertical divisions might be visible.  My DSOs all use 5:4 displays so 8 vertical divisions with an extra division and then some at the top and bottom so clipping it usually outside the range of the display.

It should always be possible to convert to microvolts RMS within a given bandwidth and nV/SqrtHz versus frequency based on the noise measurement and vertical sensitivity.
 


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