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Siglent SDG1032 Amplitude not constant when sweeping

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nctnico:

--- Quote from: eTobey on May 19, 2024, 01:47:38 pm ---I now zoomed it further. I deliberately picked a place where there was an artefact. It was a break in the sweep without a signals, but it is rather narrow, and it gets overlayed by the signals on the side. i could measure 3 break in about 13.4 ms interval after each other.

It is only visible in normal aqusition, and not in peak acq. Changing the acqs. mode does not change the sweep amplitude that is seen on the scope.

--- End quote ---
To me this looks like a display artefact. You have to keep in mind that a digital oscilloscope (in general, not just Siglent) can't use the entire memory length to produce the image for a trace because that would be way too slow. So what the oscilloscope does is called sub-sampling. It takes the value for every Nth sample (1 in 100 for example) and use that to produce a trace envelope outline. Typically this works but for frequency sweeps, you can get intermodulation products which then start to show as valeys which don't make sense.

TurboTom:
Please deepen your knowledge in sampling theory! If you sample a signal swept to 30MHz at 10MSa/s, these aliasing patterns are exactly what to expect. Please make sure you have sampling memory on your scope set large enough to match the Nyquist criteria, i.e. Fsample > 2*Fsignal! This is digital scope basics!

@nctnico beat me to it  :-//  ;)

P.S: I've got the impression your DG812 wasn't that much "rubbish" after all...  >:D

eTobey:

--- Quote from: nctnico on May 19, 2024, 03:09:36 pm ---To me this looks like a display artefact.

--- End quote ---


--- Quote from: TurboTom on May 19, 2024, 03:11:40 pm ---Please deepen your knowledge in sampling theory! If you sample a signal swept to 30MHz at 10MSa/s, these aliasing patterns are exactly what to expect.

--- End quote ---

Indeed it is, but it does not really matter here, as those valleys are very very narrow, and zoomed out you can barely see them. Using peakmode, those narrow valleys even completely dissappear.

nctnico:

--- Quote from: eTobey on May 19, 2024, 03:18:54 pm ---Indeed it is, but it does not really matter here, as those valleys are very very narrow, and zoomed out you can barely see them. Using peakmode, those narrow valleys even completely dissappear.

--- End quote ---
That is because peak-mode runs the ADC at full speed before doing the decimation so you won't get sampling related intermodulation products. The post-processing (to make the trace display) intermodulation will still be there though.

eTobey:
I have now made a final measurement, using proper connection, and using the maxhold function.

It shows clearly a asymmetry and the amplitude varation over the sweep in great detail. Too bad, that the math dont work with zoom mode (all of them?).

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