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Oscilloscope input noise comparison

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maxwell3e10:

--- Quote from: Andreas on October 20, 2018, 07:14:53 pm ---how many samples are you actually using for calculation for the 1 GS/s sample rate?

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I am using all the samples, about 1M, but I set the linewidth of the FFT to 1 MHz. One can use smaller linewidth, but then the scatter is bigger.

Keysight DanielBogdanoff:

--- Quote from: Rich@RohdeScopesUSA on October 20, 2018, 03:07:22 am ---
--- Quote from: nctnico on October 19, 2018, 10:34:20 pm ---
--- Quote from: Keysight DanielBogdanoff 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.

--- End quote ---
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.

--- End quote ---
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

--- End quote ---

Everything I learned about measuring scope noise I learned from Rich, back before he went to the dark side  :box:

I second this comment.


--- Quote from: rsjsouza on October 19, 2018, 10:22:55 pm ---Interesting, Daniel. Have you ever seen any manufacturers skew or butcher RMS measurements that could lead to incorrect results?

--- End quote ---

What we've seen some manufacturers do is measure pk-pk and not have the same number of captures. Generally as your quantity of captures go up, so will your max pk-pk measurement.

Rich@RohdeScopesUSA:

--- Quote from: Keysight DanielBogdanoff on October 22, 2018, 04:15:22 pm ---Everything I learned about measuring scope noise I learned from Rich, back before he went to the dark side  :box:

--- End quote ---
  >:D LOL

-Rich

Andreas:

--- Quote from: maxwell3e10 on October 21, 2018, 02:28:53 am ---
--- Quote from: Andreas on October 20, 2018, 07:14:53 pm ---how many samples are you actually using for calculation for the 1 GS/s sample rate?

--- End quote ---
I am using all the samples, about 1M, but I set the linewidth of the FFT to 1 MHz. One can use smaller linewidth, but then the scatter is bigger.

--- End quote ---

Ok,
now I think I found a way: if I remove the time-stamps from the data (only the voltages) it compresses down below 1 MB.
Per default my output is in mV: is this a problem or should I scale it to Volts.

with best regards

Andreas

maxwell3e10:
Instek scopes make the best csv files from space point of view. They save only the waveform data in bits, so its typically only a single integer number for shorted input. It compresses down to 147 kB for 1 M data points. The only problem is that the preamble doesn't specify the volts/bit conversion in a very clear way. Its easy enough to scale the y axis and make the x axis.

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