voltage waveform changes shape with time despite constant settings

[Jun_l]

Hello,

I am using USB 6366 to measure a superimposed voltage (50 Hz + harmonics) and current simultaneously, the measurements seem to work fine and the results are plausible, but thre's a problem that I don't understand:

I noticed that when I start my measurements my waveforms do not resemble the ideal waveform - to be exact, they aren't symmetrical >> for example my 50Hz + 550Hz voltage waveform in black  

However, as time passes (5 minutes ish, I just let my programm run and I observe the shape), my waveforms slowly resembles their 'ideal' shape and become symmetrical >> see black colour waveform below

Does anyone have any idea why? Software, hardware? The phase of the signals do not happen to have an influence of the shape, phases of both signals aren't similar even if I get my 'ideal' waveform.

Would appreciate it if someone has an idea! My vi is attached below.

Jun

[GerdW]

Hi Jun,

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@Jun_l wrote:

Would appreciate it if someone has an idea! My vi is attached below.

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Unfortunately all your subVIs are missing.

Can you put all VIs (and typedefs) into a project folder, and then attach the ZIPped folder?

[Jun_l]

Hello Gerd,

I tried aligning and resampling my signals (your method of adding both waveform works, thanks!), but my end waveform is unfortunately still unsymmetrical. Maybe it's my programm, i however do not see where the error could be...

attached is now a zip folder with all the necessary files.

[Jun_l]

sorry...missed one subvi..

[RTSLVU]

subvi_FFT_Spectrum.vi is missing

Also the two pictures look pretty darn close to me.

I wonder if it's just an onscreen aliasing problem just making it look wrong?

Try expanding the size of your Charts/Graphs and see if is looks the same.