How to trigger a combined signal - Page 1
Forum section: electronics / general

author: RAPo
author_rank: Super Contributor
timestamp: October 12, 2022, 06:56:33 pm
content: Recently I acquired a 3-channel isolating amplifier switch.Basically it generates from 3 signals a new wave signal, where the first signalrides on the top of the wave, the second signal is in the middle and the third signal rides on the bottom.See the attached images.The question is: how can one best trigger such a combined signal?The apparatus has a selectable trigger-out signal, but that achieves triggering of the input signal, not the combined signal.

author: EPAIII
author_rank: Super Contributor
timestamp: October 13, 2022, 03:49:35 am
content: I have used oscilloscopes for over 56 years. I have never seen anything like that. What is the purpose/use of such a thing? Your "3-channel isolating amplifier switch" seems to have three Voltage steps and each of your input signals is SWITCHED on during and superimposed upon one of these three levels. It appears to be a type of combined time and Voltage level multiplexing. But WHY? WHY would you want to do that??? Or, if that is not what it is doing, then WHAT?As for triggering, you do not give enough information to allow an answer. I see several possibilities:1. You have access to all three of the initial signals, which appear to be phase locked together as well as the combined signal from your "3-channel isolating amplifier switch" which also appears to be phase locked to the input(s). In that case, I would simply choose one of the input signals, the triangle waveform seems like a good choice, and use that for your trigger signal. But this ASSUMES that the input signals are what you show and not something else (like stereo audio and composit video, perhaps or maybe R, G, B video or whatever).2. Your input signals are not always what you show (sine, triangle, and) but can be any arbitrary waveforms including random noise. In that case, you need to look to your "3-channel isolating amplifier switch" for the possibility of a trigger signal. Perhaps as a separate output. Or you may need to modify it's circuitry to bring out such a trigger signal. (Side question: How is the frequency of the three steps generated? From one of the input waves? Which one? From some combination of all three? How are they combined? More questions along this line.)3. Your input signals are not available and you do not have a trigger from the "3-channel isolating amplifier switch" and you can not modify it's circuitry to obtain one so you must trigger from that mess of a combined waveform. In that case and if your input signals do not change over time, then PERHAPS a low frequency filter or band pass filter can produce a trigger signal. I have been in similar situations and getting a good trigger was NOT EASY.4. Oh heck, there are just too many more possibilities to keep going through all of them. Perhaps you can be more SPECIFIC.But I have heard that things will be different, things will be BETTER when we have digital oscilloscopes..................Oh, wait, wait ... we DO have digital oscilloscopes. So, what's the next iteration? Analog, digital, then _____! And when do we get IT?I can't wait.

author: EPAIII
author_rank: Super Contributor
timestamp: October 13, 2022, 04:10:29 am
content: OK, upon rereading I see another interpretation here. You ask, "how can one best trigger such a combined signal?" Does this mean what it says? Are you looking for a trigger that PRODUCES the input signals in some other apparatus? Is that how they are generated in a phased locked condition with the three steps of the "3-channel isolating amplifier switch"?But you seem to have those signals already. I see them on the scope's display.And if it IS triggering the generation of the input signals, then why can't be also used to trigger ----- but here I do not know what to say because your words, once again, seem not to make sense.You say, "The apparatus has a selectable trigger-out signal, but that achieves triggering of the input signal, not the combined signal." By that do you mean that it triggers the beginning of the steps in your "3-channel isolating amplifier switch"? That is what it seems to say. But that would be obvious. Something must trigger those steps. Or do you mean that it is a trigger that is available to use on a scope to view the combined signal? But, in that case, aren't your photos showing scope waveforms that are properly triggered on/in the scope?What exactly are you trying to trigger? The input waveforms? The step waveform in the "3-channel isolating amplifier switch"? The scope display? But all of them seem to be nicely triggered in your photos. So WHAT?And in what piece of equipment do you wish to trigger that? In the "3-channel isolating amplifier switch"? In the waveform generator that generates the inputs? In a scope? In some other piece of equipment? Be specific please.Frankly your photos seem to show that everything IS being well triggered! So I really must ask.Quote from: RAPo on October 12, 2022, 06:56:33 pm

author: jonpaul
author_rank: Super Contributor
timestamp: October 13, 2022, 04:59:28 am
content: schematic or block diagram no idea what you are doingj

author: RAPo
author_rank: Super Contributor
timestamp: October 13, 2022, 07:23:10 am
content: Quote from: EPAIII on October 13, 2022, 03:49:35 amThe apparatus is just in. I'm exploring it now. As for the why, it seems to do two things: a poor man's extension to an oscilloscope in order to get more channels. I've seen an article where there were 4 inputs calling it a quad tracer. In that article, the channels were not isolated, in this apparatus it is. The second thing seems to be the isolation amplifier part (https://www.google.com/search?q=isolation+aplifier+switch) but I haven't done any tests on that.Quote from: EPAIII on October 13, 2022, 03:49:35 amThe phase locking is in reality not necessarily there, I just showed some easily recognizable signals.Quote from: EPAIII on October 13, 2022, 03:49:35 amIndeed it is meant to work with different input signals (different waveforms, different amplitudes, different frequencies, and different phases).As described the apparatus has a switch 'Trig-out' where you can select one of the input signals, the BNC-out connector shows the corresponding original signal. However, that doesn't trigger the yellow combined signal.More on that later, but my question is: if you only have the yellow signal how can you achieve a stable signal on a (digital) oscilloscope. I can set a trigger on the overall signal that shows the yellow signal more or less triggered but not the different signals in the respective regions. It all looks like it is triggered because it is a screenshot, in reality, there is no locking in the regions even if I use the same frequencies.Good questions on how the yellow signal is generated. I'm still exploring don't know yet.Quote from: EPAIII on October 13, 2022, 03:49:35 amAs described I have the separate signals available, but I would like to work only with the yellow signal. I have seen video's on how to trigger an oscilloscope if you have two separate signals with different frequencies. That indeed is not easy. This case is different, I have one signal with three regions, and just putting an edge trigger with a trigger level in one of the regions does not give a stable image.Quote from: EPAIII on October 13, 2022, 03:49:35 amI'll try to give answers to your posts.

author: RAPo
author_rank: Super Contributor
timestamp: October 13, 2022, 07:36:00 am
content: Quote from: EPAIII on October 13, 2022, 04:10:29 amI'm not looking for a trigger to produce signals. I'm looking for guidelines howto set up a trigger on the yellow combined signal that gives a stable image on the digital oscilloscope.For E.g. is it better to use an edge/pulse or window trigger? Where should the trigger level(s) be set. Just some guidelines to get in the ball park.Quote from: EPAIII on October 13, 2022, 04:10:29 amSee the earlier post, just assume you have only the yellow combined signal with three different regions where each region has a different signal. What is the best way to trigger such a yellow signal.Quote from: EPAIII on October 13, 2022, 04:10:29 amHow to setup a trigger of the yellow signal on a digital oscilloscope, not assuming any of the underlying signals, such that the display is as stable as possible and you still see some information on the three signal bands.Quote from: EPAIII on October 13, 2022, 04:10:29 amIt looks like that because it is a screenshot and the signals have roughly the same frequency and because I just by trial and error got a nice picture. The yellow signal e.g. can jump up/down when setting an edge trigger with a level between the 3 regions.

author: RAPo
author_rank: Super Contributor
timestamp: October 13, 2022, 07:39:50 am
content: Quote from: jonpaul on October 13, 2022, 04:59:28 amI haven't found any details on this apparatus. In the attached image you'll see the front end, the inner parts and another experiment.

author: RAPo
author_rank: Super Contributor
timestamp: October 13, 2022, 07:48:24 am
content: The quad tracer I mentioned has the following schematic.

author: RAPo
author_rank: Super Contributor
timestamp: October 13, 2022, 05:52:09 pm
content: I think I am getting to answer my own question.Step 1: arrange signal heights so there is a space to set an edge trigger with a level, not in the signal regions. This stabilizes the overall image.Step 2:The apparatus has a pot, with it you can arrange a chop_freq (between 6.5kHz and 23.8kHz).Try (the pot is very sensitive) to get the chop frequency such that it divides the frequency of one of the input signals.The chop_frequency is not a constantStep 3: Put an holdoff slightly larger than 1/chop_freq.Seehttps://www.flickr.com/photos/renseposthumus/52425634949/in/photostream/

author: nctnico
author_rank: Super Contributor
timestamp: October 13, 2022, 05:59:10 pm
content: Such a scope channel extender won't work for a dso because a dso can't display several traces simultaneously like a cro can (at least optically)

author: RAPo
author_rank: Super Contributor
timestamp: October 13, 2022, 06:04:48 pm
content: Quote from: nctnico on October 13, 2022, 05:59:10 pmDid you see the video with the Rigol mso104? The crux is that the apparatus converts the 3 input signal into *one* output signal. So only one trace that embodies the input signals in its upper/middle/lower part.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2022, 01:33:04 pm
content: Is this supposed to be like "chop" mode on a CRT scope? It's usually only used for low frequency (whatever that may mean for a particular model). The scope just switches as fast as possible between inputs and puts the beam on the screen!It triggers on whatever you want before the chop circuit AFAIK.

author: RAPo
author_rank: Super Contributor
timestamp: October 14, 2022, 02:02:05 pm
content: Quote from: Alex Eisenhut on October 14, 2022, 01:33:04 pmYes it works in a similar way as far as I can tell.Quote from: Alex Eisenhut on October 14, 2022, 01:33:04 pmNo. If I look at the triggered output BNC, it shows (depending on the switch settings) the one-off original signals, the composite signal (the yellow trace) is not triggered even if I use the trig-out signal as an external trigger.

author: tom66
author_rank: Super Contributor
timestamp: October 14, 2022, 03:41:19 pm
content: The only way you could get a circuit like this to work is to have the scope sweeping fast enough that in the modes that use intensity grading the traces overlap. Generally that occurs once the timebase is much below one whole display frame (assuming the screen refreshes at 60Hz and 12 div horizontally, that starts to happen below 1ms/div)However note that such modes give relatively poor performance for measurement functions etc and single shot modes will show one of the 3 different traces.Really this does not work well with digital scopes due to how digital phosphor modes work, they behave only superficially like an analog scope does.You still need to find some way to trigger the scope to the tracer's pulse output and you would not be able to trigger on the actual waves themselves, a quick think about how digital oscilloscopes manage pre and post trigger times will make that clear pretty quickly.What you want to buy is an analog capture card of some kind, they often capture 16+ channels at one at low frequencies. You cannot do what you want for any reasonable real world application with a DSO. Sorry.

author: RAPo
author_rank: Super Contributor
timestamp: October 14, 2022, 04:09:20 pm
content: On an analogue scope, it is indeed easier.

author: RAPo
author_rank: Super Contributor
timestamp: October 14, 2022, 04:22:26 pm
content: Quote from: tom66 on October 14, 2022, 03:41:19 pmYes I discovered that just a moment ago (are you telepathicQuote from: tom66 on October 14, 2022, 03:41:19 pmNot necessarily because there is only one trace. If the frequencies are near each other of an exact multiple of a specific frequency, it gets easier.Quote from: tom66 on October 14, 2022, 03:41:19 pmindeed but trying this out on a digital scope triggered (pun intended) a nice dicovery/learning process.Quote from: tom66 on October 14, 2022, 03:41:19 pmOn an analogue scope using an external trigger tied the trigout-BNC of the apparatus triggers on the selected input signal. Of course the other signals start to roll.Quote from: tom66 on October 14, 2022, 03:41:19 pmSorry is not needed, many thanks from my side is. I have no real application for this apparatus, it was an impulse buy from the local craigslist thinking: that looks like an interesting device, lets try it.For 65 euro's one can have a lot of fun.

author: tautech
author_rank: Super Contributor
timestamp: October 14, 2022, 08:59:59 pm
content: One normally starts an exercise like this by learning about the many and various trigger types a DSO has over a CRO and how to use them.Another useful trigger tool is Holdoff however one must first decide where on a complex waveform one wants/needs to trigger and then apply the trigger type and Holdoff if required to get the result needed.

author: RAPo
author_rank: Super Contributor
timestamp: October 15, 2022, 07:11:53 am
content: Quote from: tautech on October 14, 2022, 08:59:59 pmLife throws things sometimes in a different order.I already had some scopes then this apparatus came and then one wants to find out how the heck it works.Quote from: tautech on October 14, 2022, 08:59:59 pmindeed:Quote from: RAPo on October 13, 2022, 05:52:09 pm

author: StillTrying
author_rank: Super Contributor
timestamp: October 16, 2022, 01:32:12 pm
content: If you want to trigger separately on 3 or 4 channels there's no way to do that, there's no analogue ALT like option. All the sampling and displaying timing is locked to the one trigger channel so you can't stabilize different frequency inputs.You'd have to look a single shots?Triggering on the yellow conbined is not much use because its switching frequency isn't related to the analoge input frequency(s).You might be able to make 1 DSO channel look a bit like 3 inputs by using persistance and dots mode?https://www.batronix.com/files/Rigol/Oszilloskope/MSO5000/MSO5000_UserGuide_EN-V2.0.pdf

author: RAPo
author_rank: Super Contributor
timestamp: October 16, 2022, 01:45:18 pm
content: Quote from: StillTrying on October 16, 2022, 01:32:12 pmI'm trying to trigger on 1 channel (the yellow one), not 3 channelsQuote from: StillTrying on October 16, 2022, 01:32:12 pmThat goes for a *digital* scope, the analogue scope can (within bounds) trig on the yellow signal and one of the 3 input signals, see images.Quote from: StillTrying on October 16, 2022, 01:32:12 pmyes, will try that.

author: tooki
author_rank: Super Contributor
timestamp: October 16, 2022, 02:26:15 pm
content: Quote from: RAPo on October 13, 2022, 07:48:24 amCan you link to the original magazine in question instead of a poorly-cropped low-resolution screenshot in which nothing can possibly be read? I can make out that it’s Popular Electronics, so it’s almost certainly available for us on worldradioarchive.

author: RAPo
author_rank: Super Contributor
timestamp: October 16, 2022, 02:31:49 pm
content: Quote from: tooki on October 16, 2022, 02:26:15 pmHere it is:https://worldradiohistory.com/Archive-Poptronics/90s/92/PE-1991-07.pdf

author: StillTrying
author_rank: Super Contributor
timestamp: October 16, 2022, 02:45:13 pm
content: "I'm trying to trigger on 1 channel (the yellow one),"It should be possible to get some sort of temporary stable trigger on that yellow because Signal 3 always shoots up to Signal 1 giving a long +ve edge.Perhaps move the trigger level away from the middle Signal 2 level, trigger on AC coupled, or add "Noise Rejection" to the trigger, can't think of much else.

author: RAPo
author_rank: Super Contributor
timestamp: October 16, 2022, 02:51:40 pm
content: Quote from: StillTrying on October 16, 2022, 02:45:13 pmI arranged the things such that there is no overshoot from the signals in the 3 regions. What you see are the chop lines.

author: tooki
author_rank: Super Contributor
timestamp: October 16, 2022, 02:57:09 pm
content: Quote from: RAPo on October 13, 2022, 07:39:50 amIt’s on page 63 of the catalog here:https://www.yumpu.com/en/document/view/5663030/electrical-machines-elwe-lehrsysteme. (Use the login fromhttp://bugmenot.com/view/yumpu.comif you want to download the PDF.)The manufacturer went out of business and was bought up years ago by a company called LD Didactic, which is still around. They may be able to supply the manual. (They have adownloadablearchive of manuals, but requires registration to download.)

author: tooki
author_rank: Super Contributor
timestamp: October 16, 2022, 02:59:40 pm
content: Quote from: RAPo on October 16, 2022, 02:31:49 pm404 not found.You sure about that URL?

author: RAPo
author_rank: Super Contributor
timestamp: October 16, 2022, 03:03:32 pm
content: I'm sure about:https://worldradiohistory.com/Archive-Poptronics/90s/91/PE-1991-07.pdf(downloaded at my side).

author: RAPo
author_rank: Super Contributor
timestamp: October 16, 2022, 03:05:51 pm
content: Quote from: tooki on October 16, 2022, 02:57:09 pmow wow, many thanks. My apparatus has the signal out channels at the back, the one you linked to has them on the right front side.

author: tautech
author_rank: Super Contributor
timestamp: October 16, 2022, 06:24:31 pm
content: Quote from: StillTrying on October 16, 2022, 02:45:13 pm100%, this is the only edge to target but it must be done with Holdoff to prevent retriggering until the next same edge so a Holdoff of ~550us should work just fine.Other smaller edges can be targeted in exactly the same way albeit with a more defined trigger level.

author: tooki
author_rank: Super Contributor
timestamp: October 16, 2022, 08:08:30 pm
content: Quote from: RAPo on October 16, 2022, 03:03:32 pmAh! The first time around, you wrote “…/92/…” instead of “…/91/…”Quote from: RAPo on October 16, 2022, 03:05:51 pmStill, it should work the same. Maybe they can get you the manual.

author: nctnico
author_rank: Super Contributor
timestamp: October 16, 2022, 09:16:53 pm
content: Quote from: StillTrying on October 16, 2022, 01:32:12 pmThat is not entirely true. Some DSOs (some models from GW Instek for example) offer alternate triggering. In that mode the DSO triggers on each channel seperately. Now this won't give you an image where the signals are correlated (as in sampled the same time) but it does allow to put stable signals on screen which have no synchronisation between eachother. This can be handy every now and then.

author: StillTrying
author_rank: Super Contributor
timestamp: October 16, 2022, 11:33:00 pm
content: Quote from: nctnico on October 16, 2022, 09:16:53 pmI know,my CML+ does ALT with 2 separate triggers and timebases.https://www.eevblog.com/forum/testgear/siglent-cml-firmware/msg1177621/#msg1177621But not thehttps://www.batronix.com/files/Rigol/Oszilloskope/MSO5000/MSO5000_UserGuide_EN-V2.0.pdf