SOLVED: Oscilloscope Question: Two signals on one channel? (pictures fixed) - Page 1
Forum section: electronics / beginners

author: protobird
author_rank: Newbie
timestamp: October 03, 2021, 08:11:03 pm
content: Hello! I am new to the forums since I couldn't find an answer elsewhere. I'm having a strange thing happen on my new Rigol DS1054Z. This isn't my absolute first time using an oscilloscope, but I'm still pretty new to using them and I'm a bit perplexed.Well, they say a picture speaks a thousand words:This is a 1khz signal coming from my phone using an app, just hooked up as a basic test. Leaving the trigger in edge mode and mussing about with it can help a bit, as did switching to a 1x probe. Zooming in and out in either axis can sometimes get it a bit more stable, but I'm not sure what the conditions are. Even when I get it to stop showing two waveforms it wobbles around a lot. Using the slope trigger works much better but I'm not sure why it's necessary and it's still not perfect.The images after this were made with a tone from my computer instead, which was less noisy and had a bit more gain. I just used a pulseaudio module.Here's the better image:As far as troubleshooting goes, I know that the firmware is up to date, I ran the self cal (though it did fail on the first attempt), and the square wave test signal looks great. It also doesn't seem to be an issue with the particular probe or channel. I fiddled about with a whole bunch of other settings (especially with the trigger) and I can't remember it all, suffice it to say it didn't make much difference and it was not very enlightening, especially since I don't know what I'm doing yet.So what's going on here? I'm sure the problem is operator error since everything else seems to be working, but I didn't have the same issue with my old bitscope.EDIT: Sorry about not attaching the images properly the first time. Been a looong time since I've used forums really and I wasn't sure if the preview would actually embed the images. Somehow the files I uploaded were .desktop files? No idea how that happened.

author: StillTrying
author_rank: Super Contributor
timestamp: October 03, 2021, 08:43:22 pm
content: It's triggering correctly on the HF noise spikes.Put CH1's 20MHz filter on and the trigger's HF reject on and it will be much better.

author: protobird
author_rank: Newbie
timestamp: October 03, 2021, 09:34:11 pm
content: Quote from: StillTrying on October 03, 2021, 08:43:22 pmThat did help. It's still happening some though. I'm guessing there's no magic fix for such a noisy signal?

author: rstofer
author_rank: Super Contributor
timestamp: October 03, 2021, 10:27:10 pm
content: You could try Single Shot mode where just a single sweep is performed and the image is fixed.

author: Jwillis
author_rank: Super Contributor
timestamp: October 03, 2021, 10:33:47 pm
content: Did you try using average in the acquisition or High resolution mode . Either can reduce noise on repetitive signals .

author: bob91343
author_rank: Super Contributor
timestamp: October 03, 2021, 11:17:28 pm
content: This is not unusual. When there is noise, that will cause spontaneous triggering. Using external triggering with a low pass filter can fix it.

author: protobird
author_rank: Newbie
timestamp: October 04, 2021, 12:22:37 am
content: Thank you to all. I feel like I understand better what's going on now.

author: StillTrying
author_rank: Super Contributor
timestamp: October 04, 2021, 09:51:13 am
content: You've also gotNoise Rejection, which increases the edge trigger's hysteresis levels, the same as a schmitt trigger.Noise RejectionNoise rejection can reject the high frequency noise in the signal and reduce the possibility of miss-trigger of the oscilloscope.Press MENU → Setting → NoiseReject in the trigger control area (TRIGGER) onthe front panel to enable or disable noise rejection.There's waveformAveraging, which reduces the amount of HF noise you see on the screen trace.AverageIn this mode, the oscilloscope averages the waveforms from multiple samples to reduce the random noise of the input signal and improve the vertical resolution.Greater number of averages can lower the noise and increase the vertical resolution; while at the same time, it will slow the response of the displayed waveform to the waveform changes.When "Average" mode is selected, press Averages and use knob to set the desired number of averages. The number of averages can be set to 2, 4, 8, 16, 32, 64, 128, 256, 512 or 1024. The default is 2.That'swaveformaveraging, which means you need good triggering before you use it, else it would just average your 180 deg. double trace to a stright line.There'sHigh Resolutionwhich averages the ADC samples straight out of the ADC, so lowers the BW and noise of the whole scope.High ResolutionThis mode uses a kind of ultra-sample technique to average the neighboring points of the sample waveform to reduce the random noise on the input signal and generate much smoother waveforms on the screen. This is generally used when the sample rate of the digital converter is higher than the storage rate of the acquisition memory.Note: "Average" and "High Res" modes use different averaging methods. The former uses "Multi-sample Average" and the latter uses "Single Sample Average".You have to remember that you have these HF filters on, because when you come to look at a real HF signal you'll find the triggering barely works.

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2021, 03:04:21 pm
content: The usual solution is to set the trigger to reject either noise or high frequencies, but increasing the trigger holdoff duration will also correct the display.

author: radiolistener
author_rank: Super Contributor
timestamp: October 06, 2021, 08:05:02 pm
content: you can fix it by enable low pass filter for the trigger. Or use trigger with high enough threshold to avoid re-triggering on signal noise...