this post was submitted on 13 Oct 2023
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Siglent SDS1104XE: slanted square wave (imgur.com)
submitted 2 years ago* (last edited 2 years 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 2 points 2 years ago (1 children)

Kinda looks like an impedance mismatch.

[–] andreyk0@discuss.tchncs.de 2 points 2 years ago

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.

[–] Saigonauticon@voltage.vn 1 points 2 years ago* (last edited 2 years ago) (1 children)

Yup, seen that for sure.

Did you try turning the little adjustment knob in your probe to calibrate it? Sometimes needs a small screwdriver. Here's a reference:

https://www.elecrow.com/download/HowToCalibrate10xProbe.pdf

What I'm referring to is labelled 'Cap Trimmer'. The document also has some waveform images that match your problem.

I have a Siglent and it looked like this at the dealership, then they adjusted the probe a bit, and then it was 100% fine.

[–] andreyk0@discuss.tchncs.de 1 points 2 years ago

Thanks but unfortunately this is not a probe cal problem. Note that the issue is seen even in a 50Ohm environment, no probes.