Rigol 1054Z DSO vs Analog scope waveform/triggering question - Page 1
Forum section: electronics / testgear

author: Tony_G
author_rank: Super Contributor
timestamp: May 15, 2016, 12:05:09 am
content: Hi All,I'm hoping that someone has run into something like this before and can help me learn what the issue/solution is because I have no idea.I'm trying to repair an old HP 419A DC Null Voltmeter and the fine folks over at the HP/Agilent Yahoo group have really been helping out (fantastic source of help and information if you've never visited).The particulars of what I'm scoping may not be germane to the question but it is the return voltage from the photochopper in the unit. I'm supposed to see a waveform that has a larger and smaller wave repeating.Like this:(the bottom trace)but what I'm now seeing is this:the sudden rise halfway through the waveform seems suspicious:and several members think it may actually be an issue with capturing the waveform on a DSO instead of a true analog scope.Assuming this isn't actually the 'real' waveform, does it ring any bells with anyone in terms of what options I may need to set on the Rigol to capture it correctly?Thanks,TonyG

author: bitseeker
author_rank: Super Contributor
timestamp: May 15, 2016, 03:52:49 am
content: It is odd that on the Rigol the larger waveform is peaking late.Is the output that's shown on the analog scope from the same null meter as the anomalous one shown on the Rigol (i.e., is the problem at the scope or the DUT)?

author: vk6zgo
author_rank: Super Contributor
timestamp: May 15, 2016, 04:22:10 am
content: I'm very dubious about the idea that this is caused by the DSO----& I'm an analog person!How about duplicating the two channel display,to see if there are any weird timing or other artifacts in the large waveform?

author: tautech
author_rank: Super Contributor
timestamp: May 15, 2016, 07:35:07 am
content: What we can't see is if the OP has used Autoset on his CRO and CRO trigger level settings.So avalidcomparison the images arenot.His DSO however seemingly has been setup with Autoset and the trigger level has unfortunately been Autoset right near the peak of the waveform and we don't know if there's been an attempt to adjust it for what might be a better triggering and a more correct displayed waveform.In cases like these it always best todrivethe scope manually, to rely on Autoset is to invite errors especially on complex waveforms.A DSO is a powerful tool in the hands of a competent operator but can be a complete mind f*** in the hands of a novice.Dave's advice to purchase a cheap CRO as one's first scope holds very strong merit in the way a CRO teaches correct scope operation, this experience sets one up for life for any further scope work.

author: Fungus
author_rank: Super Contributor
timestamp: May 15, 2016, 09:46:15 am
content: Quote from: Tony_G on May 15, 2016, 12:05:09 amNope. It's more likely to be the other way around - the DSO is the on that's telling the truth.I don't really see how that image can be a problem with the scope in itzself. It must be a PEBKAC.Dave had a similar problem in this week's blog:(start at 21:35)

author: EI6JA
author_rank: Newbie
timestamp: May 15, 2016, 10:29:54 am
content: Hi, I am assuming from your post that the analog screenshot is taken from measurement on a working system by someone else, and the RIGOL screenshot was a measurement of the system you are faultfinding. To rule out triggering problems, I would do a single acquisition capture and have a look at the results. With single acquisition you will not have any additional sweeps overlaid on each other so should be able to rule out triggering issues. If you can provide some details of your scope setup such as trigger options including hold-off, and acquisition mode etc someone may spot an error in your setup. I suspect the scope is setup OK and the resultant waveform is a result of a fault in your circuit, but double check with single acquisition and some different scope settings.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: May 15, 2016, 10:58:18 am
content: Quote from: Tony_G on May 15, 2016, 12:05:09 amIs this supposed to be the analog screenshot? Because this looks very much like the screen of a HP 54600 Series scope - which is a DSO

author: nctnico
author_rank: Super Contributor
timestamp: May 15, 2016, 11:13:44 am
content: Quote from: Tony_G on May 15, 2016, 12:05:09 amThat is nonsense. If the waveform is not as it should be then there is something wrong with the device you are measuring.

author: Howardlong
author_rank: Super Contributor
timestamp: May 15, 2016, 11:14:52 am
content: Quote from: Wuerstchenhund on May 15, 2016, 10:58:18 amIndeed, those were my thoughts, although I still can't explain away the difference in traces.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 15, 2016, 11:31:47 am
content: Quote from: Wuerstchenhund on May 15, 2016, 10:58:18 am"After my first cursory look at the screenshot,I started to question the quoted "analog" nature of the 'scope,too.

author: Howardlong
author_rank: Super Contributor
timestamp: May 15, 2016, 11:47:31 am
content: The Rigol display almost looks like the trace is showing a runt. Is the Rigol probing shorting a couple of pins perhaps?

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: May 15, 2016, 01:11:44 pm
content: Quote from: tautech on May 15, 2016, 07:35:07 amNo, it doesn't hold s"strong merit".Dave has long been saying, repeatedly, that this advice (which I think was made back in the days when the Rigol DS1052E was hot stuff, i.e. half a decade or so ago) is no longer valid, and that these days a beginner's first scope should really be a DSO.It's also a bit far fetched to suggest a CRO would necessarily teach "correct scope operation" (unless of course the aim is learn how to correctly operate test equipment of bygone years). In fact, learning on an analog scope teaches many techiques which are obsolete, ineffective or counterproductive with a digital scope. Instead of learning how to curate antiques, a beginner is much better off by not foregoing the advances in scopes made in the last 30 years and learning how to correctly operate a contemporary instrument.

author: Earendil
author_rank: Regular Contributor
timestamp: May 15, 2016, 03:06:59 pm
content: Quote from: Tony_G on May 15, 2016, 12:05:09 amHmm, interesting.Have you calibrated your probes? What happens if you use the second channel to probe the same node at the same time as the first?(So you use two probes to probe the same node). Does the waveform change?

author: ebastler
author_rank: Super Contributor
timestamp: May 15, 2016, 03:35:17 pm
content: I am with EI6JA here: The "as expected" screenshot is probably from another voltmeter unit, measured by someone else. I don't see any reason to suspect the Rigol measurement to be flawed; this would be a highly unlikely kind of artefact. Most likely, the Rigol shows a correct measurement taken on a voltmeter that does not work correctly.

author: Howardlong
author_rank: Super Contributor
timestamp: May 15, 2016, 03:47:51 pm
content: Quote from: ebastler on May 15, 2016, 03:35:17 pmNow I've re-read the OP, I think you are right. The way I originally read it was that the OP had used two different scopes to measure the exact same device on the same bench. It would seem that isn't the case.Like you, I now have no reason to suspect the Rigol's measurements in this case. Whether the waveform is right for the DUT given that it's likely two different devices on two different benches, quite possibly with different test criteria, I can't say.If the OP could clarify the situation that would help.

author: Fungus
author_rank: Super Contributor
timestamp: May 15, 2016, 03:59:10 pm
content: Quote from: tautech on May 15, 2016, 07:35:07 amIn much the same way as reading an analog watch will help you to operate a digital watch, yes.

author: Tony_G
author_rank: Super Contributor
timestamp: May 17, 2016, 12:24:52 am
content: Thanks everyone for the replies (and my apologies for not getting back sooner - life interrupted)I must apologize for a lack of clarity - The "like this" image was meant to show what the traces should look like in a working system and not what the same DUT would look like using an analog scope - I'm not sure exactly what scope was used to capture the trace but a working 419A should show those waveforms on A2Pin12 and A2TP2.My scope is the Rigol and my 419A is certainly not working - I would not be remotely surprised if this was a PEBKAC issue - I just can't work out what I screwed up for the life of me.I'll try the single shot trigger and see if still reproduces.Thanks for the suggestions.

author: bitseeker
author_rank: Super Contributor
timestamp: May 17, 2016, 12:35:50 am
content: You should also try a different scope on the DUT. If you see the same waveform on two (or more) scopes, the problem is more likely with the device, not the scope.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 18, 2016, 04:13:26 am
content: Quote from: Fungus on May 15, 2016, 03:59:10 pmAs the vast majority of watches these days have digital "guts" & an analog display,operation is very much the same.Digital watches with a numeric display are pretty much creatures of the 1980s/early '90s.In a lot of applications,people just want to look at a waveform to see if it is approximately the right shape,duration,& amplitude.In those cases,analog 'scopes & DSOs are used in the same manner,without using any of the advanced features of the latter.