EEVblog #685 - What Is Oscilloscope AC Trigger Coupling? - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: November 20, 2014, 12:09:29 am
content: In this tutorial Dave describes what AC trigger coupling is on an oscilloscope and why it can be useful. Not only on old analog CRT oscilloscopes, but modern digital scopes as well.How and why is it different to AC channel input coupling?Also, use of the 50% trigger control, and how the venerable DS1052E is still more usable than the new DS1054Z.Previous video on Rigol DS1054Z and DS2000 Jitter Problem

author: eV1Te
author_rank: Regular Contributor
timestamp: November 20, 2014, 12:33:59 am
content: Great video, as alwaysI already knew about the AC triggering but I found the explanation and also the comparison of the Agilent to the Rigol very interesting.Maybe you could do a oscilloscope shootout like you did with millimeters a while back, comparing the responsiveness of the buttons for each brand and some of the functions, like the 50 % trigger level?The only thing I am not satisfied with on my DS1000z is the responsiveness of the interface, easily worth a few $100 extra just for that if I had known before hand.

author: nixfu
author_rank: Unknown
timestamp: November 20, 2014, 02:14:32 am
content: Ok now I understand. I guess I just too it for granted on my analogs because it is just the way things were by default.At least it's mearly a nuscience that it does not work correctly at the moment(you have to play with the trigger level and DC triggering), but at least I know what I am missing as far as functionality.I am also more glad I kept an analog scope around as well. I still prefer analog for doing stuff like signal tracing and unknown circuit, and watching signals with its "real-time" unprocessed waveform display.There is a place and time for both analog and digital scopes, especially if your doing random repair, diags, etc.

author: EEVblog
author_rank: Unknown
timestamp: November 20, 2014, 02:26:02 am
content: Quote from: eV1Te on November 20, 2014, 12:33:59 amI think responsiveness is ok for the price.I show this in my review video supporters have seen, even with everything turned on, it's still not bad.

author: rs20
author_rank: Super Contributor
timestamp: November 20, 2014, 02:27:53 am
content: Dave, you briefly pointed out that the digital scopes perform triggering on the digital data stream, rather than the pre-input-coupling signal at the input. However, I disagree with your claim that the scope should behave identically nevertheless. It's more nuanced than that.In particular, the digital stream already has AC input coupling applied (if selected), so if you select DC (or what could be more accurately termed "whatever input coupling is" or "what you see on the screen") trigger coupling and AC input coupling, the Rigol oscilloscope would trigger just fine on those four waveforms you presented to the scopes. So far, this is exactly right and good because:Seeing the trigger level visually on the screen is a very good thing, and only meaningful for DC trigger coupling (as you rightly point out). I can't stress this enough, a DSO that defaulted to AC trigger coupling and therefore wouldn't display the trigger level on-screen would be an awful step backwards. I'm all for consistency, but this is why digital scopesreally shouldbehave differently to analog scopes.It makes little sense to use true DC triggering with AC input coupling, so it seems fine that AC input coupling "auto-enables" "AC trigger coupling" in this sense. Implementing true DC triggering with an AC-coupled digital waveform would require some hacky measurement of the DC offset to compensate the digital triggeringcircuitryFPGA gates, or expensive analog trigger circuitry that would be prohibitively expensive at this price point.The problem is, you often want to see the DC component, yet trigger off the AC-coupled signal. You did mention this, but not really in the context of how this is theonlytime you want AC trigger coupling in the Rigol design.I expect the other big DSO players (Agilent, Tek) have made the same design decisions for the same reasons? Nevertheless, I think it's reasonable that Rigol's AC input triggering implicitly overrules the selected trigger coupling. I'm writing this to preempt the "but I use AC input coupling with DC-heavy signals all the time and always had success with the default 'DC' trigger coupling, this just doesn't mesh with what you've drawn on the whiteboard..." confusion that peoplemightbe thinking.I agree that Rigol seemed to express some doubt that AC triggering was necessary, and it's important to recognise that it'slessnecessary with scopes that draw the trigger signal off after input coupling. It's more of a niche thing than with analog scopes. Only with that context in place can you fully emphasise that in real life (in particular, mixed voltage digital logic and analog signals living on different biases), it'sstillabsolutely necessary. Fortunately, we've got news from Rigol that they're fixing the bugs, so everything's just perfect now! Apart from the lack of a 50% auto trigger level feature!EDIT: Btw, sorry for double-posting across forums, Dave, but I do think this is worthy of wider discussion, being the pedantic nutter that I am!

author: EEVblog
author_rank: Unknown
timestamp: November 20, 2014, 02:37:58 am
content: Quote from: rs20 on November 20, 2014, 02:27:53 amOf course those 4 signals would have triggered fine with AC input coupling, you missed the entire point of the argument. It's for the situation when youwant DC couplingon the input, not AC input coupling.QuoteI did more than just mention it, it was the entire example point in my video!

author: Bud
author_rank: Super Contributor
timestamp: November 20, 2014, 02:42:47 am
content: This is what the 50% control thing looks like on a Tek 2467B, was occasionally wondering what it does, now know, thank you.

author: rs20
author_rank: Super Contributor
timestamp: November 20, 2014, 02:56:02 am
content: Quote from: EEVblog on November 20, 2014, 02:37:58 amI'm sorry, you're right about this, my message was far to long and rambly. I should have focussed on my main point that expecting analog and digital scopes to behave identically is not right. May I just ask what your opinion is on this point:Quote from: rs20 on November 20, 2014, 02:27:53 amI think it's important to highlight that "DC trigger coupling" is much more versatile in the Rigol scope than in the analog scopes (in that choosing AC input coupling always automagically forces faux-AC-trigger coupling), hence why it's the default. There's a really tricky compromise with these digital scopes: either choose DC trigger coupling and get to see you trigger level drawn on-screen, or choose AC trigger coupling and get to not worry about manipulating your trigger level continuously. With analog scopes, you never get to see the trigger level, so it's a non-issue.

author: EEVblog
author_rank: Unknown
timestamp: November 20, 2014, 03:42:24 am
content: Quote from: rs20 on November 20, 2014, 02:56:02 amOf course you do, it's the start level of the trace on the screen!

author: rs20
author_rank: Super Contributor
timestamp: November 20, 2014, 03:56:50 am
content: Quote from: EEVblog on November 20, 2014, 03:42:24 amI... I... but... "not for digital waveforms with such fast rise times that the scope doesn't show the rising edge"... no, that's just getting way off-track and pedantic...sigh... *walks off with tail between legs*...I feel so sure I had a point worth sharing, but evidently I don't. You win this round!

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 20, 2014, 05:34:48 am
content: Interesting. I had never noticed the AC coupling trigger menu item before the "jitter bug" (Sounds like a 1930s dance craze) video. On that video, I became aware of it, and the use of it is now obvious.But playing with it a bit on my DS1052E, I believe I've found a subtle bug in this area of that venerable scope.I set up a sine wave at 300mV p-p, with a DC offset of 250mV. So the entire waveform is always at a positive voltage, never crossing 0V. Using DC coupling on the display, and AC trigger coupling, it couples beautifully at a trigger level of 0mV, and I can play with the trigger level and watch the waveform go back and forth a bit just as you showed in your video.But the bug is this: If I press the DS1052E's dedicated "50%" button that you showed, it sets the trigger level to 250 mV! That would be the correct 50% level if I were using DC trigger coupling -- it's the midpoint between the top and bottom of the waveform on the DC coupled display. But I'm using AC trigger coupling, so a value of 250 mV is outside the range of the waveform, and it fails to trigger.When the trigger is AC coupled, I think the "50%" button ought to select the midpoint of the AC coupled waveform, which would usually be pretty close to 0 (not necessarily exactly zero -- consider a square wave with a very high or very low duty cycle). It's useless to select the midpoint of the DC coupled waveform in this scenario, but that's what the scope does.This isn't a serious bug, because it's easy enough to work around the issue by setting the trigger manually or pushing the trigger level knob to quickly set it to 0. I don't hold out much hope that Rigol would fix such a minor problem for a product so near its end of life.FWIW, I have firmware 00.04.01.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: November 20, 2014, 05:52:33 am
content: OK, May I get back to the basics?I get it, setting the AC Trigger coupling allows me to quickly find a signal on the screen. I get it, that using the AC Probe coupling eliminates the DC part of a signal so I may focus on the actual waveform.I think I understand that to switch from AC Probe coupling to DC Probe coupling allows me to find any DC components of a signal riding on the overall signal.However, where or when would I use the DC trigger coupling? Why not just keep the trigger coupling on AC all the time? Also with DSO type scopes, does this even matter if I use the AUTO function to find the signal in the first place?-Please excuse my ignorance in electronics- I started just to repair a stereo receiver, and got hooked on the colorful objects and all those cool trashcans! Oscar (the Grouch) would have a field day running around on circuit boards.

author: rs20
author_rank: Super Contributor
timestamp: November 20, 2014, 06:24:46 am
content: Quote from: TheNewLab on November 20, 2014, 05:52:33 amEDIT: What Dave says below; single-shot is a leading usecase for true DC trigger coupling.AUTO is terrible because it messes with your horizontal timebase and vertical scaling, takes a second or two, and requires you to dedicate a hand to pressing the auto button. As Dave points out, this is completely awful if you're probing around different points on a circuit rapidly. In the same situation, AC trigger coupling works wonderfully (and/or AC input coupling with digital scopes).On a digital scope, you might also want to use DC trigger coupling because it shows the trigger level visually on the screen as a horizontal line as you adjust it (this display doesn't appear with AC trigger coupling because it makes no sense). Seems silly, and in a sense it really is, but the average digital scope user demands that they see the trigger level visually on the screen as it's adjusted, so that's why DC trigger coupling is the default on digital scopes. Combine that with the fact that digital scopes effectively enable AC trigger coupling when AC input coupling is selected (even if the trigger coupling is left on "DC"), you find that "DC" trigger coupling is OK-ish most of the time, hence why most people didn't even know AC trigger coupling existed. But of course, Dave demonstrates in this video why you might want to switch explicitly to AC trigger coupling, and maybe even leave it permanently on AC trigger coupling (I'm strongly considering this, I think I can handle an invisible trigger level) even on a digital scope.

author: EEVblog
author_rank: Unknown
timestamp: November 20, 2014, 06:38:24 am
content: Quote from: TheNewLab on November 20, 2014, 05:52:33 amYou need DC trigger coupling in order to trigger off the exact point you want if you have a DC or very low frequency input signal.e.g. AC coupling would be the wrong thing to use if you had a slow changing single shot waveform, like say the start-up ramp of a power supply you were trying to capture.DC trigger coupling is an absolutely essential feature.

author: tautech
author_rank: Super Contributor
timestamp: November 20, 2014, 06:48:16 am
content: Quote from: AG6QR on November 20, 2014, 05:34:48 amYes you would expect the 50% button to set the trigger to ~mid-point of a waveform. AC or DC.Fair enough that a Channel Position button would set the trace to mid-screen (0) but what good is that for a trigger level setting.Yes call it a Bug.

author: peufeu
author_rank: Contributor
timestamp: November 20, 2014, 10:23:38 am
content: Quote from: rs20 on November 20, 2014, 03:56:50 amActually it does too, because a good analog scope has a big reel of cable inside as a delay line, so it can show you what happened just before the trigger point...

author: megajocke
author_rank: Regular Contributor
timestamp: November 20, 2014, 01:21:57 pm
content: Input coupling selection is usually not done in the position shown in Dave's diagram. It is usually the very first thing the signal passes through, even before it goes to the attenuator and input amplifier, and this goes for both digital and analog scopes. The trigger pickoff point is therefore almost always after the input coupling selection.For that reason, when using AC input coupling, the trigger is also looking at the AC coupled signal even when in DC trigger coupling mode. If AC trigger coupling mode is selected, the signal will be high pass filtered twice before reaching the triggering circuitry/code.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 20, 2014, 02:06:38 pm
content: Quote from: megajocke on November 20, 2014, 01:21:57 pmI made the same point in a post I composed last night, but I decided not to bother posting in the end.Yes, the trigger pick-off point is after the input stage - it's usually just before the beam switching (and another take-off point is usually provided just after the beam switch for ALT triggering - which lets you view 2 traces at once that are not related to each other in the time domain - occasionally very useful).I figured that Dave drew the diagram that way for the sake of getting the message across - I'm very aware from my job that sometimes in education, it's necessary to tell little white lies - although TBH it probably wouldn't have made much difference to have drawn it correctly.One thing I am sure about: lots of engineers, at every level of education and experience, are often confused by AC and DC matters. The ability to think about each separately is an important skill - a very basic application of the superposition theorem - and time spent thinking about it is time well spent. I see that as a positive outcome from the discovery of this issueAnother thing I see evidence of almost daily: many people - again, at every level - are confused by oscilloscope triggering. Ever since peak-to-peak trigger setting became the norm (early '80s?), you hardly need to think about triggering when viewing simple waveforms. When teaching, I make a big deal about being able to see the start of the sweep - I line it up on the left-hand edge of the graticule (it was against the LH edge of the CRT on the video - possibly slightly off to the left at times) and make people adjust the trigger level (and other settings) while watching it. Surprisingly few seem to realise that it's just a comparator feeding a D-type flip-flop!I had a baptism of fire though. My first 'scope had a faulty trigger IC that was an unobtainium custom type. Nothing that a new PCB with 5 or 6 ICs couldn't fix though! I learnt some hard lessons doing thatThat 'scope has analogue storage, and is great for capturing PSU start-up behaviour and similar things. That's when single-shot with DC trig coupling is essential (as was said earlier).QuoteDave, will this review be released generally one day? I'm awaiting your detailed opinion of this one with interest.

author: (In)Sanity
author_rank: Regular Contributor
timestamp: November 20, 2014, 02:34:06 pm
content: It's funny, my Tek 2465BDM has DC trigger coupling on by default. It also has a 50% auto trigger button. Not bad for an analog scope. I have a DS2000A series as well.Rigol should allow the user to select what they want the "Zero" button to do.Rigol should address the external trigger being DC only on the DS2000 series and either fix it or remove the menu options for AC.Rigol should ......Overall they are good scopes "for the money". Rigol needs to do everything in their power to get people like me to stop using that term.Just my two cents,Jeff

author: open loop
author_rank: Regular Contributor
timestamp: November 20, 2014, 10:37:18 pm
content: Thanks for the video this has explained/cleared up things for me, I have now a much better understanding on the subject!I was that person you mimicked "Well I have never used AC coupling ... don't care about the bug..."will play around with it when I get a chance.Still would go and buy the Rigol 1000Z as I cant afford the Agilent MSO-x 3000 scope, what are my needs? Intensity graded display, serial decoding and high re-fresh rate.

author: ornea
author_rank: Unknown
timestamp: November 21, 2014, 01:15:07 am
content: I too was beginning to wonder why you would ever use DC coupled triggering after watching your discussion on AC coupled triggering. Thanks for clearing this up.

author: Salas
author_rank: Frequent Contributor
timestamp: November 21, 2014, 02:12:25 am
content: FORCE: in Normal and single trigger modes, press this key to generate a triggersignal forcefully.Says the DS2000 manual. Alright, can you mention practical example cases?

author: rs20
author_rank: Super Contributor
timestamp: November 21, 2014, 02:21:51 am
content: Quote from: Salas on November 21, 2014, 02:12:25 amHere's one: in "Normal" mode, the oscilloscope will just go dead if the trigger level is not intersecting the waveform. If you want to just get a rough snapshot of where the signal is sitting, what it looks like roughly, you can just whack the "Force" button and you can see what's going on.Another regards single-shot mode. If you're tracking some interesting event at a very slow time-base (100ms/div or whatever), then you might want to trigger when you see some particular interesting thing happen. It might be something optical, not electrical, or maybe it's just hard to set the trigger up just right and you just want to eyeball it. It's the most sophisticated triggering system in the world: your eyes & brain! But the "jitter" of the human triggering system is pretty terrible.

author: Salas
author_rank: Frequent Contributor
timestamp: November 21, 2014, 04:33:36 am
content: In single mode I can make an off-trigger waveform appear by pressing force. In run mode it just keeps running horizontally.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 21, 2014, 04:47:09 am
content: When the triggering is done digitally on some new scopes, what I would like to see is a smart trigger mode based on autocorrelation.

author: Odysseus
author_rank: Regular Contributor
timestamp: November 22, 2014, 05:03:59 am
content: As explained by Dave, the default on my analog scope is AC trigger coupling. However, suppose you are observing a square wave of widely varying duty cycle. At the extremes, if left on AC trigger coupling, you will loose stable triggering since the DC component of the signal is so close to the edge of the waveform. Here, it would be beneficial to just fix a DC trigger level at 50% that won't change with the signal.

author: rf-loop
author_rank: Super Contributor
timestamp: November 22, 2014, 09:07:40 am
content: Trigger coupling AC and duty cycle of signal - problems? No.In analog scopes it is highly related to scope quality.This digital oscilloscope have true digitized side trigger engine.Here 3V p-p pulse. Pulse cycle perod set for 100 000 000 ns (100ms) and pulse width set for 25ns and set for rise time 10ns and for falling time in this case same 10ns.Input channel DC coupled.Trigger mode Normal and Rising edge.First Trigger DC coupled.Next Trigger AC coupled. (and of course in AC coupling can also adjust trigger level)Persistence was on for collect jitter sum. (generator itself jitter/drift + oscilloscope delay jitter/drift + trigger jitter)(This "square wave" Duty cycle = ~0.000025%)Visible is collected total jitter peak to peak over ~30s time and it looks stay around inside 1ns.Around same with AC and DC trigger coupling. Trigger AC coupling do not make any difficulty for rock solid trig.Total peak to peak time jitter over 100ms time period is 1ns (0.01 ppm) independent of Trigger coupling AC or DC.Of course time when take picture, current last trace is in random place inside this jitter range. And (repeat) all timing jitter is really not trig jitter alone. (it is sum of jitters and drifts during 30s period in generator and also oscilloscope delay counting during 100ms delay time + trig jitter)(more tests about this oscilloscope basic triger perfomance can find later in other place - not relevant in this thread)

author: rs20
author_rank: Super Contributor
timestamp: November 22, 2014, 09:34:30 am
content: Quote from: rf-loop on November 22, 2014, 09:07:40 amYou're confusing the issue by using an AC-coupled triggerwith a non-zero trigger level(don't think I didn't see the "L 1.42V" on your screenshot). If you put a 99.999% duty cycle signal in, instead of a 0.001% duty cycle signal in, your scope would fail to trigger on it until you fiddled with the level. Not so with DC-coupled trigger.This is the point people previously posting in this thread are making. In your example, if you set DC coupling with level set to 0V,and leave the settings like that, the scope will trigger on 0.00001% and 99.99999% duty cycles flawlessly. However, if you set AC coupling with the level set to 0V, it'll only work from 2% to 98% (OK, I completely made these numbers up, but you get the idea). Sure, you can cheat and "fix" the problem by manipulating the trigger level, but that's really annoying to mess around with. Hence, DC trigger coupling is more reliable and easier.Quote from: rf-loop on November 22, 2014, 09:07:40 amI imagine the range of duty cycles supported by the scope when set on AC triggering, level = 0 will vary from scope to scope, but there isn't a scope on the market where DC triggering isn't a simpler and more reliable solution to the problem.

author: rf-loop
author_rank: Super Contributor
timestamp: November 22, 2014, 11:24:43 am
content: This was not just test for this named case.This was only picked up example from testings what I have made for this scope trigger system for find its goods and bads and that AC trigger can easy adjust also and it do not have any issue for trig. Álso in this case point where is trigger event detected is far out from sampling buffer in this time when it capture signal after this delay and it show that delay counting do not add any bad jitter.(Small "bug"as there can see in pictures there is one guite small problem related to how it show trigger level. (in picture there can see only this triggeer level mark in right side. Level number is qquite ok but level mark is not. When adjust level it also show trigger level dashed line over display. Related to signal image trigger level indicator is in wrong vertical position. Yes number is right. But trig mark distance for AC "zero" is wrong. It may be confusing because need adjust visual trigger line nearly signal top or even over it for trig (if want trig is around 50% p-p). AC zero is very near this signal bottom. But it display trig marker related to DC zero. Trigger level number is related to this AC "zero" and it is right. Trigger level mark and marker line when adjust is not right level. This is not right when there is selected trigger coupling AC and signal coupling DC.Small thing but "not so nice".)

author: rs20
author_rank: Super Contributor
timestamp: November 22, 2014, 12:11:59 pm
content: Quote from: rf-loop on November 22, 2014, 11:24:43 amThis is exactly why most (?) scopes don't display the AC trigger level visually on screen at all. In theory you could have the oscilloscope measure the average DC value and actually calculate & display the trigger level on-screen, but I think this doesn't work either because AC-coupling only couples frequencies over a certain value, so it's not even that simple.In other words, the bug is that it even tries to show you. That triangle should just be hidden for AC trigger coupling.

author: ciasteczkowypotwor
author_rank: Contributor
timestamp: November 22, 2014, 06:02:54 pm
content: This is not a good schematic block diagram of an oscilloscope triggering, analog or digital.

author: artag
author_rank: Super Contributor
timestamp: November 23, 2014, 05:03:43 pm
content: But what about the case where there is more than one zero-crossing per cycle ? Imagine something like a heartbeat with two pulses, one bigger than the other, and zero-crossings between both.This is similar to the problem with extreme duty cycles, except that there will be a large region where AC trigger will jitter between the two zero crossings. You really want to trigger on a crossing that only occurs on the bigger peak.You can get around that with DC triggering (but at the cost of being unable to reject a DC offset) or with trigger holdoff (at the cost of being sensitive to frequency). I'd prefer to be able to see the DC-blocked trigger waveform with the actual trigger level overlaid on it.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2014, 05:41:10 pm
content: The only thing AC trigger coupling does is remove the DC component of the trigger signal. If you had a waveform with multiple levels which could be triggered on, then setting the trigger level when AC trigger coupled triggering used works the same was as when DC coupled triggering is used. Set it high enough to pick up only the largest pulse edge. This assumes however that the frequencies of interest are not filtered out as AC trigger coupling has a low pass cutoff just like AC input coupling.The thing I find interesting about how AC trigger coupling fails on the Rigol oscilloscopes is that low frequency reject coupling is just AC trigger coupling with a higher cutoff frequency. How can one work but not the other?

author: Smoking
author_rank: Contributor
timestamp: November 24, 2014, 12:00:56 pm
content: Some people have requested a video of the trigger Siglent SDS2000.Here is a youtube video showing there is no Jitter problem :

author: Teneyes
author_rank: Frequent Contributor
timestamp: December 13, 2014, 12:34:32 am
content: If one wished to Auto search the input for a signal which mode of these 4 would one desire most?1. DC input coupling. With. DC. Trigger coupling2. DC input coupling. With. AC. Trigger coupling3. AC input coupling. With. DC. Trigger coupling4. AC input coupling. With. AC. Trigger couplingWhat does your DSO. Do?

author: rs20
author_rank: Super Contributor
timestamp: December 13, 2014, 12:46:03 am
content: Quote from: Teneyes on December 13, 2014, 12:34:32 amAre you referring to the "Auto" button that automatically sets up the scope's parameters based on the signal present at the scope's inputs? DC input coupling and AC input coupling are among the settings reset by the "Auto" button, so I claim your question makes no sense. The state of those settings before you press the "Auto" button has no impact on what happens whatsoever, they just get wiped out. We could speculate as to what the internal implementation of the "Auto" button actually does while it's going through its Auto routine, but I don't think that's particularly useful. In short, please clarify your question.However, I will also mention that contrary to the video, mostDSOs have absolutely no difference in behavior between your #3 and #4 cases, since the trigger signal is branched offafterthe input coupling; if the signal is already AC coupled at the input, AC coupling it again at the trigger does virtually nothing. Hence why most DSOs default to "DC trigger coupling", which could be more accurately named as "triggered off the signal you see on the screen".

author: Teneyes
author_rank: Frequent Contributor
timestamp: December 13, 2014, 04:06:16 am
content: Quote from: rs20 on December 13, 2014, 12:46:03 amTo explain:In a Rigol Beta FW a surprise is that the Auto button is somewhat configurable.See New Menu. pix belowQuote from: rs20 on December 13, 2014, 12:46:03 amYes , I agree #3 & #4 are the same .For the Rigol DS2000 the trigger level line is shown in AC input w/DC Trigger modeBut the trigger level line isNOTshown in AC input w/AC Trigger modesee pic. for DC Trigger, a way to indicate Trigger Coupling Mode

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2014, 01:20:04 pm
content: Quote from: rs20 on December 13, 2014, 12:46:03 amThere is more to it because if they were triggering off of the literal signal seen on the screen with DC trigger coupling, then changing the vertical position might be expected to change the trigger position; oscilloscopes where this happens support AC triggering or something more sophisticated to avoid this behavior which otherwise looks rather peculiar.Modern DSOs use digital triggering so the vertical position adjustment can easily take place after the point where the trigger is processed however DSOs which support offset adjustments have this problem; the DC offset will change the trigger point unless AC coupled triggering is used.Some oscilloscopes combine the vertical position and offset adjustments into one control since the later can do things that the former cannot. Rigol apparently lacks an offset control despite their misleading documentation implying that they do have one since adjusting the vertical position has no effect on the trigger point when DC coupled triggering is used.

author: rs20
author_rank: Super Contributor
timestamp: December 14, 2014, 08:20:11 am
content: Quote from: David Hess on December 13, 2014, 01:20:04 pmThis is hard to word well, so bear with me. Isn't it possible that the software just looks at the vertical position control, commands the offset circuitry accordingly (this must happen surely, or else you'd need more than 8 bits of ADC to get 8 bits of output), and then it calculates which digital value corresponds to the chosen trigger level and sends that to the digital trigger circuitry? That way, a change in vertical position will change the both the digital trigger level, and the values coming in off the ADC in exactly the same way, so that it all cancels out and you end up triggering at the same points in time.FWIW, I was just playing with my DS2202 and moving the vertical position causes the trigger level indicator to follow on-screen, staying on the same voltage-at-the-input, as you would hope. However, if you thusly move it far enough off-screen, it clamps and you can see the trigger level voltage (also shown numerically on screen) changing as it reaches values outside the range of the ADC. Specifically, at 20mV/div, and with the vertical control centered (such that the 8 vertical divs on screen cover the +/- 80mV range), the trigger may be set anywhere from -100 mV to +100mV, as if the ADC range actually covers 10 divs. With the vertical on-screen range extending from -120mV to 40mV, the available trigger settings are -140mV to 60mV. Always an extra div available at the top and bottom of the range. So that seems to confirm that the trigger level selection is converted by the software into a corresponding digital (or at least, post attenuation+offset in the analog domain) value, and used accordingly. This seems like ideal behaviour to me, modulo the limitations in the range of available trigger level voltages inherent in doing triggering post-ADC.BTW, you're right that my wording was perhaps deceiving -- all I meant by "what you see on the screen" is just referring to the idea that the trigger point can be rendered meaningfully as a horizontal line on-screen w.r.t. to the trace.

author: Bud
author_rank: Super Contributor
timestamp: December 14, 2014, 07:39:54 pm
content: Quote from: ciasteczkowypotwor on November 22, 2014, 06:02:54 pmIn which case can you explain how you can have a AC Trigger (beside external) on a DC coupled channel.

author: rs20
author_rank: Super Contributor
timestamp: December 14, 2014, 10:59:22 pm
content: Quote from: Bud on December 14, 2014, 07:39:54 pmHehe, good catch. But the answer basically is, "in that 'trigger amp and LF/HF filter' section". I do agree with you, in that a more conceptually useful model would have the AC coupling cap switch after the trigger selection switch; this would represent how the scopebehaves. However, in many digital scopes with digital trigger, it's well-and-truly digital by this stage, so it's just another option on that DSP block (basically it's a certain kind of digital highpass filter, after all).

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2014, 11:47:58 pm
content: Quote from: rs20 on December 14, 2014, 08:20:11 amThis is certainly possible and even likely.QuoteWhen DSOs used analog triggers it was always possible to do the trigger pickoff before any offset. With digital triggers that is not possible so if the position control acts as an offset at the input to the digitizer which is a desirable feature, then the trigger level will have to be made to track to prevent it from changing with position changes.QuoteI only brought it up because I have run into this before on oscilloscopes. Where it gets complicated is if the DSO has separate position and offset controls. The former should not affect the trigger level but the later might. None of my current DSOs have both and I do not remember how the ones I have used which did handled it. I think they compensated for both so the trigger level did not change.On my older high performance analog oscilloscopes, the vertical position controls either act by generating an offset or they also have dedicated offset controls. In both cases they affect the trigger level but by default on these oscilloscopes, the trigger is either AC coupled or uses peak-to-peak triggering so any DC offset if removed. If DC trigger coupling is used, then the trigger level becomes a fixed point on the display.

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2014, 12:37:32 am
content: Quote from: rs20 on December 14, 2014, 10:59:22 pmThe way I like to think of it is AC trigger coupling is same same as LF reject coupling but with a lower cutoff frequency; LF reject couplingisa type of AC coupling. At least in all of the oscilloscopes I have used, AC trigger coupling extends below 60 Hz so it may be used with AC line sources while LF reject coupling rejects AC line sources.