Inconsistent RMS computation

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Serge
Newbie
Posts: 0
Joined: Thu Jul 16, 2020 1:34 pm

Inconsistent RMS computation

Post by Serge »

I recently discovered inconsistency in reported RMS for low signal. This was observed with 4262 and 4227 scopes running latest version of PicoScope for Windows on Win 10 64-bit PC. The RMS value for low signal (noise) reported in live view could be only half of the actual RMS value. If I save the data and replay, I would get different RMS value, which agrees with my calculations, but it is almost twice as large as reported in live view. Why such a difference? This doesn't happen with larger amplitude signals. For instance, If I input 2V Pk-Pk sine wave, the reported RMS in live view would be close to 700mV, which is correct.
I will try to attach screenshots with RMS computed in live view and RMS in replay mode. Please note the Peak-Peak values are similar, but RMS values are not.
Attachments
Replay
Replay
Live Data
Live Data

AndrewA
PICO STAFF
PICO STAFF
Posts: 283
Joined: Tue Oct 21, 2014 3:07 pm

Re: Inconsistent RMS computation

Post by AndrewA »

Hi,
For measurements the columns Min, Max, Average are calculated from the last N number of waveform buffers shown in the 'Capture Count' column, when the scope is capturing waveforms.
So these values don't update when opening saved files. The 'Capture Count' column changes to 1 when opening a saved file.
The value in the 'value' column is average measurement value for the waveform buffer displayed.
You can change the 'Capture Count' or number of waveform buffers used in the Preferences-
'Tools' menu -> 'Preferences' -> 'General' tab -> 'Statistics Captures' this can be set from 2 to 1000
The values in the columns Min, Max, Average won't updated if the change the setting above on a saved file, only if you capture new waveforms.

So I am not sure why the values in the 'value' column are different could be the live capture is using the wrong number of waveforms in its calculations.
I would try and capture 20 plus waveforms (you only have 15), to see if makes a difference.
And sure you running the latest version of PicoScope software. https://www.picotech.com/downloads/_lig ... icoscope-6
Regards Andrew
Technical Specialist

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