Learning About External Triggering - Page 1
Forum section: electronics / beginners

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 05, 2023, 08:39:03 am
content: Hi, I'm trying to wrap my head around external triggering. My scope (Tektronix 2431L) is a 2 channel + 2 external trigger affair.I read the manual, but it just says that you can trigger from the two external trigger inputs. So that wasn't of much help.There's overall not much info about this topic.I readhttps://www.eevblog.com/forum/beginners/learning-how-to-use-external-trigger-and-sweep-function/and watchedBut it's still not clear to me. The video makes it seem like you've actually got a 3 channel scope (In my case a 4 channel) because plugging in the channel 1 probe into the external trigger generates something on the screen. In that case it is just a flat line for some reason.But then when he plugs in a 3rd probe to channel 1, the trigger flat line disappears and we see the CH1 and CH2 outputs again but its obviously triggering on the 3rd probe.1.) How do you select the trigger level or type or how do you go about selecting the trigger point for the external trigger? When it was a flat line, what does that even mean? But if it's a flat line, how do you know how its triggering? How do you select the trigger point?2.) Why did the trigger trace appear and disappear?

author: pdenisowski
author_rank: Super Contributor
timestamp: October 05, 2023, 11:08:10 am
content: That's an extremely confusing video.Normally the trigger in signal is connected to a dedicated trigger in (BNC) connector and the trigger signal isnotdisplayed on the oscilloscope.I discuss external triggering in this video. It describes what this is and how it's configured for the MXO4, but frankly, all modern oscilloscopes configure external triggers more or less the same way.https://youtu.be/cG3CP8vCNyY?t=1118I can't speak to the Tek 2431L, but it should have a switch to select external as the trigger type and (ideally) some way to set the trigger level to an appropriate value based on the expected input trigger signal. Note that external triggering is an "edge" type trigger - the trigger occurs when the trigger in signal crosses a voltage threshold.

author: CaptDon
author_rank: Super Contributor
timestamp: October 05, 2023, 01:30:16 pm
content: All of the level selection, slope + or -, and H.F. reject all work the same for an external input as they do when triggering from CH1, CH2 or line. You may have an ability to see the external triggering input on screen so that you can adjust the trigger points and see the effect. No, you don't really have a 3 or 4 channel scope. Only the ability to view the triggering waveform. One use for such an external trigger input would be to trigger from a clock input of a counter and then use your CH1 vertical input to look at the output stages of the counter. You could have CH1 look at the CLK/2 stage and CH2 to look at the CLK/4 stage while your trigger is actually coming from the CLK input. It wouldn't provide a steady display but would prove a point. Some of the TEK scopes did have a weird way to display the 'phantom' channels but it wasn't very useful.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2023, 12:26:18 am
content: The 2400 series DSOs were a little unusual in having two separate external trigger inputs, and actually have a third external trigger input if you count the optional word recognizer. I suspect they had two external triggers because they have separate triggers for the main and delayed timebases. Tektronix mainframe oscilloscopes of that era also have separate external triggers for each timebase with dual timebases having two external triggers, and 2400 series of oscilloscopes as well as the 2200 series 4-channel oscilloscopes duplicate some other features of those mainframe oscilloscopes.The 2200 and 2400 4-channel analog oscilloscopes do not need external triggers because they have 4 channels.As far as operation, the external triggers have all of the same trigger modes as the vertical inputs, at least in this case, but a more limited dynamic range. They allow triggering off of a third signal without dedicating one of the two vertical inputs to it, which in some cases is as good as having three channels.

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 06, 2023, 12:31:26 pm
content: Thanks guys. Additional pieces to the puzzle. I also found various descriptions of the trigger system strewn throughout the manual that I didn’t catch from the contents page or the index. Lots of new info. Lots to learn, lots to go through!

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 12, 2023, 01:49:57 pm
content: Since my last post, I have been reading and re-reading the oscilloscope manuals every day. I’ve read through dozens of Google hits for dual time base scopes, dual trigger scopes, internal and external triggering, you name it. But I’m still unclear on this whole thing. I’m sure it’s something simple that I don’t understand or maybe a few things, but I just don’t get it.- How often are external triggers used?- What are their use cases?- According to the manual, the scope originally came with 2 probes but there’s 4 connectors in total with the two ext connectors. Do the external triggers have the same characteristics as ch1 & 2? i.e. should the probes that plug into these ext connectors be of the same frequency spec as for ch1 & 2? This gets back to Q2, do external trigger ports support the full bandwidth of the scope?

author: donlisms
author_rank: Frequent Contributor
timestamp: October 12, 2023, 07:04:25 pm
content: Just to make sure we're in sync, because it's not yet clear where you are on learning curve, I'd like to lay some foundation. Pardon me if you know all this; at least it will define terminology and such.A "trigger" or "trigger event" is essentially the electrical representation of a moment in time. That's all -- just the instant; it is a pulse whose leading edge means "now."There is a trigger circuit in the scope whose job is to recognize the conditions in a waveform that identify that moment in time. The conditions it uses are (1) the signal level, and (2) the direction, i.e. the positive or negative slope. The circuit won't work if the slope is too close to zero, i.e., the level is not changing fast enough to be unambiguous.It's surprising how often that simple algorithm works to produce a useful trigger. The most common will be when the level is zero, and the slope is upward. These are the conditions at the start of a sine or square wave, for example. The circuit recognizes the conditions and says "Now."The input signal to the trigger circuit might be a version of a channel (vertical) input signal after it's been scaled to fit nicely on the display, which coincidentally puts it at nice level for the trigger circuit. It might also be an external signal of some kind, usually already at an appropriate level, or perhaps some level divided by 10, depending on switch settings. Details in your scope's manual. Memorize and swallow.The output of the trigger circuit is always that pulse.The pulse goes to the sweep circuit, which has been waiting patiently for such an event, and it starts the horizontal sweep on the display, at the rate you have chosen. It will ignore events that occur during the sweep, and during the reset period that's necessary after the sweep. Then it starts looking for another trigger event pulse.So...Suppose I want to see an amplitude sweep over a frequency range. My signal generator can sweep the frequency from low to high as I have set, at a rate I have set. Every time it does, it generates a pulse on its trigger output jack. So I have a trigger signal I can send to the oscilloscope that means "now", to start the horizontal sweep. It's just a pulse, but the trigger circuit still looks for the conditions in the input signal as usual, and generates an output pulse in its usual way.The "normal" signal output from the signal generator goes through my circuit under test, and to the oscilloscope input channel. Its amplitude, at each moment during the sweep, will become visible. I want the sweep time of the oscilloscope display to roughly match the sweep time of the signal generator. I want the signal sweep and the oscilloscope sweep to start at the same time... "now." The signal generator defines "now".One place an external trigger might come from, then, is the output of some other piece of test gear. If you had a well-equipped lab back in the day, your oscilloscope would not be the only instrument that had a "trigger" jack on it. Some will be outputs, some will be inputs. They talk to each other. They agree on "now," after one of the defines "now." Time is important in these situations; triggers are a way of working with it deliberately. The interconnections are made with BNC cables. It would be normal that an output trigger pulse is then received by another instrument as a signal with easily-recognized characteristics, and then converted into an internal trigger pulse. Maybe the trigger output has a level control that make it fit the next input better.For example, I could use a pulse generator and its various parameters to generate a trigger, use it to start the signal generator sweep, to start the oscilloscope sweep, and to start some other process somewhere, all synchronized.If you don't have test gear like that, and/or you don't have investigations you need to do like that, you probably won't care too much about external triggers.So... dual?If the oscilloscope has two sweeps, the point is that we are then able to look at the signal, or signals, at two different sweep speeds. Each of those sweeps needs a starting point. The first connection between their timing is the delay setting. This is a timer that is started by the trigger event, and whose duration is typically some fraction of the horizontal sweep time, measured in divisions. So... trigger happens ("now!"), sweep starts, delay timer starts, and after, say 4.55 divisions of horizontal sweep, the timer says "now!".In the simple case, the timer's output event switches things to the second sweep speed (always faster) and the trace finishes at that higher speed. Mix and match. Different scopes have different modes for dual sweep displays, though, so there are other options, e.g. "Do the A sweep (the slower one), and after the delay, do another complete sweep at the B speed." This is a matter of switching back and forth between them, as ALT does. It's also usually possible to just turn off the A sweep, and only look at the fast B sweep detail that was initiated by A.In a more complex case, we might not immediately start displaying the B sweep after the timer expires. Instead, we use it to enable a search for another trigger event. This second event conditions can be different than the first. Suppose you have a complex waveform where you can recognize the start of it by some strong positive pulse, and then some stuff you ignore for a fixed time, and then there is a really interesting little fast detail that begins with a small negative edge. The A trigger is set with a level and slope to recognize the first positive pulse. The delay is set to skip over the bunch of unteresting bits. Then the B trigger is set to recognize that small negative edge, and start the fast B sweep.That was all analog, but there might be cases where this would be handy in a digital circuit, as well. Not great, but workable for finding certain kinds of issues.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2023, 08:13:22 pm
content: An excellent description.Another important feature of the trigger on analog CROs is "auto" trigger, which generates a trigger pulse if no otherwise defined trigger events happen during a certain period, preset in the instrument.On analog CROs, if there be no trigger the line disappears, so "auto" can be selected so you always see something on the CRT screen.

author: donlisms
author_rank: Frequent Contributor
timestamp: October 12, 2023, 08:56:17 pm
content: If you think about the trigger circuit, it's clear it will have to be able to recognize the conditions if any input signal. So that's the ballpark of performance.The fact that some scopes can display the signal going to the input of the trigger circuit is a bonus. It can hel figure out why you're not getting a trigger event. But since it's there... in some scopes, you can sorta use it as a more normal signal display. Probably with limitations.And if a 4 channel scope uses one of them for a trigger input, it is because all of that existing circuitry is a handy way to condition the signal for the trigger circuit. And it saves space inside and out.

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 13, 2023, 08:51:39 am
content: Thanks donlisms. Although it's still not all 100% clear yet, I have realised that I think the problem might have been me confusing myself and over complicating things. I was trying to find functionality where none existed.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2023, 12:21:40 pm
content: Quote from: PixieDust on October 12, 2023, 01:49:57 pmUsing an external trigger is an exception rather than the rule, but when you need an external trigger, there is no substitute. Most often I use it to free up a vertical input to display a different signal, effectively adding a third or fifth channel.QuoteMost often an external trigger is used to make up for lack of another channel when it is not necessary to view the signal which is being triggered from. A good example of this is the "line" trigger available on most oscilloscopes which triggers from the power line cycle, although in this case the more general purpose external trigger input is not used.An external trigger is also used with accessories like word recognizers where a trigger condition is qualified based on several different signals, like a digital address or data.QuoteThe external trigger inputs usually have the same characteristics as the vertical channel inputs, so the same probes can be used. They usually but not always have the same bandwidth, but there is no requirement that trigger bandwidth match the vertical channel bandwidth. Some specialized oscilloscopes have a countdown or sync external trigger which allows phase locking to a much faster signal to extend the trigger bandwidth.

author: Zenith
author_rank: Frequent Contributor
timestamp: October 13, 2023, 01:22:16 pm
content: It's much the same as having a dual trace scope where you can display channel 1 only but trigger from channel 2, except with the external trigger you can display both channels and trigger from the external trigger. Channels 1 & 2 might be weak or noisy signals and the external trigger a related clean, strong signal. The external trigger channel usually only has crude attenuation, normal and divide by ten. It may not have the same properties as the vertical channels. How useful it is depends on what you are doing. Some people might use it every day, others might never use it all the time they have a scope. I'd say it's a feature worth being familiar with and having in mind, because sometimes it's extremely useful.Some scopes have a trigger view to show the trigger waveform. Usually it shows the trigger waveform while a button is pressed. Some scopes allow the trigger waveform to be displayed permanently, but you usually have limited means to shift the trigger waveform display. Then with scopes with delay sweep, you can trigger the B sweep from its own external trigger input and the trigger view may display that as well. All potentially useful features, but it's something of a stretch to call these 4 channel scopes.You can get to grips with it by using one of those new-fangled dual channel DDS function generators to experiment with it and see what it can do, say displaying a square wave on channel 1 and triggering off a triangular wave via the external trigger.

author: pdenisowski
author_rank: Super Contributor
timestamp: October 13, 2023, 02:15:28 pm
content: Quote from: David Hess on October 13, 2023, 12:21:40 pmAnd in fact, there are a lot of cases where youdon'twant the trigger in to have the same bandwidth as a vertical channel. External trigger in is normally a simple edge type trigger, and limiting the bandwidth (filtering) of the trigger signal* can improve the stability of a (noisy) external trigger.*and/or using some kind of hysteresis

author: macboy
author_rank: Super Contributor
timestamp: October 13, 2023, 02:59:27 pm
content: On a digital scope, usually the digitizer is constantly capturing the signal. When a trigger occurs, the system can display any or all of the signal captured prior to the trigger. Very often, the triggering event is shown in the middle of the display, so we get some pre-trigger and some post-trigger signal being displayed.An analog oscilloscope begins the horizontal scan when the trigger occurs. If triggering on the same signal being displayed, then the trigger event always appears at the far left side of the screen. Most scopes, especially higher bandwidth ones, will have a delay line (a piece of coax cable, coiled up inside the scope) which the displayed signal must propagate through before being displayed. This can provide maybe a couple nanoseconds of delay so that we can see a couple nanoseconds of the signal prior to the triggering event. So on the fastest timebases, you may just see the entire rising edge which caused the trigger. On slow timebases, this has little practical effect, and the triggering event is still far left on the screen. You can't see any pre-trigger signal.Using an external trigger can allow you to trigger on a separate signal which comes prior to the signal transition that you are interested in seeing. Often, a signal generator or pulse generator will have a separate "sync" or trigger output which leads the output signal by some amount. This allows the scope to start scanning/displaying before the event of interest occurs, so that it is actually displayed.A useful trigger source could be some other signal separate from - but related to - the signal(s) of interest. For example, SPI consists of a data line, a clock line, and a chip select. If I want to see an SPI transaction from the start, I could trigger on the chip select going active. Triggering on the clock signal may give me screen full of bits in the middle of a transaction instead of the start of it. Similarly, for an I2S digital audio signal, I could trigger on the word clock (LR clock). Then I could display the data and the bit clock on the two channels. To view a composite video signal, you could trigger on the horizontal sync, or to see a specific line of the video, trigger on the Vertical sync + some fixed delay. That delayed timebase is available on better analog scopes.

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 04:19:16 pm
content: Quote from: pdenisowski on October 13, 2023, 02:15:28 pmOn a typical analog CRO, the trigger source can be selected from "internal" (one of the vertical channels), "external" (one or more external panel connectors), or "line" (50 or 60 Hz power input).After that selection, the trigger coupling can be selected (typically) from DC, AC, LF reject (ignore hum), and HF reject (ignore high-frequency noise), along with slope (rising or falling) and level (after coupling).Sometimes, there were TV-specific modes to trigger on vertical or horizontal sync.

author: PixieDust
author_rank: Frequent Contributor
timestamp: October 16, 2023, 06:32:16 am
content: Quote from: Zenith on October 13, 2023, 01:22:16 pmThat's what I'm trying to do, to at least be aware of capabilities so I know where to look if the need arises.Quote from: macboy on October 13, 2023, 02:59:27 pmI've played around with SPI in the past. But it seems I haven't played around with it enough for that example to ring any bells. Either way, thanks, I'll come back to this example in the future, maybe it'll click one day.I saw a YouTube video about the video signal triggering example also, but it didn't make much sense to me either.

author: pdenisowski
author_rank: Super Contributor
timestamp: October 19, 2023, 10:36:46 am
content: Quote from: PixieDust on October 16, 2023, 06:32:16 amTriggering on analog "video" signals (NTSC / PAL / etc.) isn't terribly common anymore and unless you're working with analog video signals, you will probably never use it.I cover video triggers briefly in one of my videos:https://youtu.be/rg6CECPGbAQ?t=527

author: EPAIII
author_rank: Super Contributor
timestamp: October 20, 2023, 08:14:40 am
content: External triggering means simply that the signal for the trigger is coming into the scope on an input jack that is marked "Ext" or "External" of the equivalent in Chinese or Japanese or German or whatever language the labels on the scope use.And Internal triggering means that the signal for the trigger comes from an input jack that is marked "Ch 1" or "Ch 2" or "Ch 3" or "Ch 4" etc.That's it. It is simply where the trigger signal is passing through the outside enclosure of the scope. Nothing more and nothing less. By having an "External Trigger" input jack, a scope with only two input channels can display two different signals from a circuit while the trigger signal comes from a third point. In working on video circuits with two channel scopes, I made frequent use of this capability to see the time relationship of two signals which were either difficult to use as a trigger or which had to be viewed in relation to that third signal.Now, all you need to do is learn all the rest about triggering. AC, DC, Hi Pass, Lo Pass, TV H, TV V, and in today's world of scopes, about a hundred forms of filters and types of digital signatures that can be used to trigger the trigger.You mention a Tektronix scope. Tektronix usually has a pretty good manual. I suggest that it is a good place to start.

author: EPAIII
author_rank: Super Contributor
timestamp: October 20, 2023, 09:05:43 am
content: Some hopefully direct answers to your questions:Why did you see a flat line with only the trigger input?Because only the two channel inputs are displayed on the screen. At least that is the normal thing."... because plugging in the channel 1 probe into the external trigger generates something on the screen."This does not make sense. There is no reason to connect the probe of a Channel input to an External Trigger input. That is two inputs connected together. Do you mean connecting the Ch 1 probe to the same circuit point as the Trigger probe is connected to? If so, see below.How do you select the trigger level or type?That's a good question and the answer is it varies. An external trigger is usually used when two other signals need to be viewed and they do not provide the needed trigger point. Very often you use those, visible waveforms to set the triggering. After all, it is those waveforms that you are actually interested in seeing, not the trigger. In other cases I have had specific instructions oh how to set up the trigger in the manual for the piece of equipment I was working on. Another thing could be to connect both the trigger probe AND one of the channel probes to the same point so you CAN SEE the trigger signal. And then, when the trigger is working properly, move the channel probe to the signal that needs to be displayed. I am sure there are other situations and you just need to tackle them as they arise."How often are external triggers used?"This is a hard question to answer. The best answer is they are used when you can not get the trigger you need from one of the Channel inputs. How often that happens depends a lot on what you are doing."What are their use cases?"Again, it is hard to say. And again, it depends a lot on what you are doing. Some may use them for one thing and others for another. This reminds me of the time when a 'smart' manager told me to list what I did in performing my job and how each task was done. I told him that I would retire before I could finish that task because everything I did was different and required different procedures. I had to make it up as I went along. He did not like that so I submitted a three part procedure: 1. Observe problem. 2. Analyze problem. 3. Fix problem. He still didn't like it, but had to concede that anything more detailed was just not possible in a reasonable amount of time. My apologies for comparing you to a manager.Do the external triggers have the same characteristics as ch1 & 2? i.e. should the probes that plug into these ext connectors be of the same frequency spec as for ch1 & 2?The characteristics can and will depend on the individual scope. This is something that I never worried about because, when I was working, scope probes came from Tek or HP and cost a lot. So you were lucky to have two 10X probes for a two channel scope. I often had to make improvised probes from ordinary coax (RG-58 or RG-59)or use 1X probes for the External trigger signals. They usually worked OK. I would advise not worrying about the specs of a trigger probe until you encounter a situation where that is shown to be important. Frankly, I never did in over 55 working years in electronics. But YMMV."This gets back to Q2, do external trigger ports support the full bandwidth of the scope?"Again, this can and will vary with the individual scope."I saw a YouTube video about the video signal triggering example also, but it didn't make much sense to me either."The composit video signal, NTSC, PAL, of SECAM, is a very complicated construction. It contains timing signals for three different rates (horizontal, vertical, and color sub-carrier) as well as the luminance and chromance components. Oh, and then there were two fields in the NTSC, at least. And some devices worked on individual lines in the vertical interval (closed captions, vertical interval test signals, network clocks, and others). Extracting a trigger from that cacophony of signals can not be done with just a simple filter. If you don't need to work with such signals, I would not worry about it. But many analog scopes and most modern digital scopes did/do have the necessary circuitry for at least extracting the horizontal and vertical triggers. Those trigger circuits were great time savers for those of us who did need to work with them. Specialized scopes, called waveform monitors, were able to extract all of those different triggers in order to display any part of a composit video signal.