Scope Triggering/Sampling Question - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: October 15, 2025, 10:58:54 pm
content: I have an Agilent 54831 scope and always seem to have issues with triggering/sampling that I don't understand.Sometimes triggering issues can be attributed to auto trigger, but often times regardless of how I trigger, the waveform is always jumping.Regardless, earlier I pieced together a simple circuit to explain a bridge rectifier circuit. It was just a function generator feeding 60Hz to a 1N4004, a 50k resistor, and a 220uF. The input and half wave signals looked good, I added the cap, selected AC on the scope, and expanded. The saw tooth looked horrible, full of noise.I went into the menus and found 'sampling mode' was set to 'normal'. After I changed it to "high resolution' the wave seemed to look normal.I also tinkered with memory depth and other stuff, but none of those settings seemed to make a difference.As for the sampling mode, I would think 'normal' would have looked like acleaner waveformsome sort of decent waveform than 'high resolution', but apparently I'm wrong.Which settings should I pay attention to in order to get cleaner looking waveforms?

author: aeg
author_rank: Regular Contributor
timestamp: October 15, 2025, 11:14:08 pm
content: Trigger source is line, it's not coherent to the function generator output. Trigger source should be something with a fixed phase relation to the signal of interest. I'm not sure why the high resolution traces look OK, maybe the mains cycle drifted into sync with the function generator while you were in the menus.

author: aeg
author_rank: Regular Contributor
timestamp: October 15, 2025, 11:19:17 pm
content: Hm, scratch that, the channel 2 amplitude in normal mode is also implausably high. Maybe your scope is broken?

author: Someone
author_rank: Super Contributor
timestamp: October 15, 2025, 11:50:18 pm
content: Quote from: aeg on October 15, 2025, 11:19:17 pmAmplitudes seem to be excessively low??? 200mV/divPretty sure those scopes had the ability to scale probes.Are those pdfs showing just an image? Use the forums image handling to make it easier to view/quote/comment on.

author: bostonman
author_rank: Super Contributor
timestamp: October 17, 2025 12:56:36 am
content: QuoteI'll need to check, one probe should have automatically have set for x10, but I was more focused on trying to trigger the scope along with why I couldn't see a nice saw tooth.Triggering on 'line' was while playing with the trigger because the wave wasn't remaining still. Sometimes I see waveforms jumping and try triggering at all points and can't seem to lock it. That's when I turn off auto trigger, set to line, etc...The more confusing part is the blurry saw tooth that cleared up after I messed with the sampling mode. I'm not use to such scopes; most scopes I've used were always relatively low cost due to cheap companies not wanting to pay for expensive ones.Now I'm using one that (I consider) pretty good and it seems very touchy. I would think higher sampling rate would improve the wave forms, but I'm not sure what's going on. I have two of the same models and in the past I've tried all four channels along with the other scope with similar results.

author: Someone
author_rank: Super Contributor
timestamp: October 17, 2025 02:02:31 am
content: Quote from: bostonman onYesterdayat 12:56:36 amIf you show some examples of the problems that radically change with small differences then that would help (not downloading and opening up n pdf files and trying to link them back to your text).But perhaps first get a handle on selecting an appropriate trigger, trigger level, on obvious waveforms before working through the particulars of sampling/decimation/aliasing etc.https://www.tek.com/en/documents/primer/oscilloscope-systems-and-controls

author: moffy
author_rank: Super Contributor
timestamp: October 17, 2025 05:21:38 am
content: I had similar problems when using my signal generator, an FY6600, with its original power supply, it was generating up to 80V AC on its -ve terminal wrt ground on the scope. I had to upgrade the PSU in the unit to eliminate the noise, now it's just fine.

author: StillTrying
author_rank: Super Contributor
timestamp: October 17, 2025 07:10:23 am
content: Are you sure the trigger is on the clean mid level of the sine, and not the 3 mV of noise on the sawtooth?

author: bostonman
author_rank: Super Contributor
timestamp: October 17, 2025 01:20:33 pm
content: QuoteThe reason I did PDFs (ignoring that my scope either prints to a default virtual PDF printer or saves as an image) is that it includes all the scope settings allowing anyone deciphering the waveform to see exactly how the scope was setup.QuoteI didn't consider noise from the generator, however, recently I measured a full wave circuit in a stereo amp going through a diff probe, had a good ground connection, etc... and got the typical noise (see attached).I don't remember if I had scaled the v/div correctly in the attachment because I was going through the diff probe, but I was paying more attention to how the waveform looked.As mentioned, I'm so use to low grade scopes that produce somewhat pencil thick waveforms that using this higher end model raises questions on what I'm doing wrong. Since all four channels and two same model scopes produce similar looking waveforms, I somewhat ruled out a broken scope.Typically triggering issues in one case may be not using another channel to trigger on a data bus, then another time it's that I triggered correctly, but then the wave decides to jump phase for some reason, now it was changing the sample mode to high resolution gave me a waveform I am familiar with.Seems I haven't provided enough information, so let me take more measurements maybe this weekend and I'll post them. It's probably easier to examine this question with the scope present rather than a single image, so I understand the difficulty in assessing the images.

author: makebit
author_rank: Unknown
timestamp: October 17, 2025 04:31:55 pm
content: I wonder if some previous user to yourself was using it extensively with the earth cut out of the plug and wrecked it, such as it now has your said problem.

author: bostonman
author_rank: Super Contributor
timestamp: «Reply #10 on:Todayat 02:53:50 am »
content: I have a few more captures, but posting two for now.The grounding statement in the previous message gave me the idea to measure the ground wire on the probe to check whether it's open. I have different probes, one being an Agilent probe that automatically puts the scope in x10 mode; and the one I use for my main probe. It's in great shape, but oddly enough, measures approx. 2.1ohms from the ground lead to BNC shell whereas my other probes measure extremely low (approx. 0.3ohms).One reason for only posting a few of my captures is: I measured a (semi good) 9V battery and got more noise on channel 1 than channel 2 when put on AC coupling; something I just realized. Before posting, I want to check whether channel 1 has more noise.The measurements in the attached two looked the same (or very similar) on my other exact model Agilent scope (they were bought from different people a few years apart, so not one person could have applied similar damaging measuring tactics - or most likely didn't).Also, I realized my generator was set for 50ohms output impedance which took me a minute to figure out why I initially got higher than 1V RMS, but now it's set for high Z.Note the measurements in the attachments are in with the scope set to high resolution and not normal. In normal resolution mode the noise is much worse. As mentioned above, the wave forms look the same on both scopes.