this post was submitted on 13 Oct 2023
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Siglent SDS1104XE: slanted square wave (imgur.com)
submitted 11 months ago* (last edited 11 months ago) by andreyk0@discuss.tchncs.de to c/askelectronics@discuss.tchncs.de
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Hi!

I've asked Siglent support but after a couple of responses the thread went cold. Maybe I'm being dumb but I've noticed that there's a frequency (low, around 100Hz) where scope response changes a bit. Below and above it square input looks square. Right about it square input looks slanted.

I tried to do a very slow "sweep" and there's small but visible change in the envelope. So, e.g. with a constant 600mV p2p input lower frequencies measure exactly that while higher ones measure 612mV, so ~2% diff.

Terminated 50Ohm cables (not that it matters at such a low freq) to be sure. Latest firmware, after full self-cal. Siggen itself seems allright, I have an ancient Tek scope and the siggen output looks Ok there with same input/same cables. Scope seems happy and fully functional otherwise, few years old though, out of warranty.

Has anyone else seen anything like that? Is this a normal behavior within the expected margin of error?

Thanks!

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[–] irdc@derp.foo 3 points 11 months ago (3 children)

Kinda looks like an impedance mismatch.

[–] andreyk0@discuss.tchncs.de 2 points 11 months ago (1 children)

Thanks for taking a look! SG is set to 50Ohm output, cable is 50Ohm, terminated "through" 50Ohm at scope end. Also, SG output looks normal on a different scope (same cable/terminator arrangement). Not sure what else I could do to match the impedance here.

[–] OmnipotentEntity@beehaw.org 2 points 11 months ago (1 children)

Does this scope do Fourier analysis? If so, can you see which frequencies are being suppressed? Does this suppression at this frequency also occur with generated white noise?

[–] andreyk0@discuss.tchncs.de 1 points 11 months ago

Thanks! I tried but I don't see the effect with noise gen, maybe my measurement is not quite right. Simply punching in a couple of frequencies (60Hz and 200Hz) that I've already seen differ in amplitude with the Measure function also has different peak amplitudes in FFT output. Since the frequencies are so low I've verified stability of siggen amplitude output with an AC voltmeter and it's stable within 50uV or so, nowhere near the diff in magnitude measured by the scope. Hmm...

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