Hi Welcome to the Eev Blog an Electronics Engineering Video blog of interest to anyone involved in electronics design. I'm your host Dave Jones Hi. It's quick oscilloscope tutorial time now. I'm going to take a look at one particular feature of the oscilloscope that a lot of people really don't have much idea about.

They find it a bit mysterious. In fact, a lot of people have never used it and well, quite frankly, they don't know what it does and it can be found as a dedicated knob on many higher end. in fact, most higher end older analog oscilloscopes, but especially on new digital. Scopes They will have this option that a lot of people go.

well. what's that? I'm not really sure they might twiddle it, but they don't really understand what it does. What is it? It's trigger hold off. Let's take a look at it.

First up, a bit of a background refresher on oscilloscope triggering. Now, this is going to be in reference to the oldfashioned analog oscilloscope. the old CRT cathode ray oscilloscope because the concept starts there and it it carries on over to digital Scopes which are effectively the same thing. The concepts the same, but it just, um, the background works better with the analog scope because that's where it comes from.

Now as you, we've got an analog CRT display here which draws a waveform on the display. Now the triggering system which I might have to do another blog on just just general triggering, but all we're interested in is the trigger hold off control this time. But the trigger point? what it does, is that the Uh. the beam inside the CRT sweeps across the display, which we'll look at down here.

but it sweeps across from left to right and it draws the waveform on the display. Now your vertical amplifier. the input channel to your oscilloscope is the one that causes the vertical or wide deflection like that. That, but the X uh Axis or the horizontal time base is based on a fixed Uh ramp.

Basically, it's based on a linear. it's an integrated saw tooth ramp like this. so it's a linear sweep. It's also called The Sweep.

Okay, it's the horizontal deflection sweep. or it's the voltage that's applied to the horizontal deflection plates which causes the beam to sweep across the display. And that's a Sawtooth waveform like this. So once this oscope triggers okay at this point, if you set your trigger level to say this y value here on the positive Edge like this, then the once that trigger is received, then it will start sweeping it.

This voltage will increase and the beam will sweep across a display like that, drawing your waveform. Now when it gets to the end, over here like this. Okay, it's got to somehow get the beam back to here. It's got a what's called a retrace so it retraces its step all the way back here and it does that very quickly.

Hence, the very quick ramp down like that. I've exaggerated the time period there and you don't want that to be displayed when it's retracing that beam back. Otherwise, you get a horizontal line on your display and that's no good. So what they do is they blank it like this.

when it's retracing, and then it arms, then the trigger circuitry arms again and it's ready for the next sweep. And it does that again and again and again for your heating signal. But what happens is there is actually an extra hold off time here. And that's what that analog hold off control on your analog oscilloscope does.

It actually increases this time. Uh, period. Up to a certain point, each oscilloscope is different. some will be calibrated, others will be like in actual time units.

So um, you'll be able to actually set an exact hold on time. Others will just be a just a simple analog control which allow you to increase that time. Now, the hold off time will be in addition to that retrace period. So when your Uh beam sweeps across like this and gets to the end of the waveform, instead of just immediately retracing back like that, what it does well, it still retraces back.

But instead of rearming straight away, rearming that trigger Cy ready for the next trace, it just holds off. And that's why it's called trigger hold off because it effectively disables your Uh trigger signal again for a fixed period of time. Now, of course, that hold off time must at least be as long as the retrace period to sweep that back and plus a little bit extra for it to settle before that trigger circuitry resets. But you can actually extend that.

But why would you want to do that? Isn't update rate important? Why would you want to sit there doing nothing, displaying nothing on your CRT screen, potentially missing um, information, missing data when you can just rearm it and go straight away? Well, there's a good reason to it, and there's some important benefits. That's why trigger hold off is an important concept that you should understand to really know your oscilloscope. So let's take a look at it. One more thing.

if you're talking about digital oscilloscopes, then it's effectively the same thing It really. it operates exactly the same, except there is no retrace cuz it doesn't need to physically fly back the beam back to the left side of the CRT like that. So really, there is no retrace period, but there might be a processing time and stuff like that before it can actually uh trigger. And of course, a a One of the benefits of a digital oscilloscope is that you get um, pre and postt Trigger so it will typically trigger trigger in the middle like this.

But really, the hold off concept is exactly the same. After after it's finished capturing the data, it will hold off triggering again for a certain predetermined amount of time. And the real benefit with digital Scopes is that they all have really, um, high resolution digital control for the hold of time. It allows you to set a precise value.

Now, let's take a look at an analog oscilloscope here now. Unfortunately, this isn't the best example cuz this is an analog digital comic scope. So it's a bit complicated on the display here. But what we're interested in is the dedicated control down here.

Now it says delay position, but that's only for the delayed time base. If I turn on, there we go. It's actually the hold off control in regular mode of operation. It's Hold Off now.

It's got a dedicated button, so you think it's a pretty darn important control. and it is. So let's take a look at the screen here when we operate the uh delay control. Now if you remember that sweep wave form we had before, okay, I've got it set to a very fast sweep speed so you can't see it, retrace or anything like that.

hold off is off, it's set to or or it's set to the minimum. Often they won't have an off, they'll just have minimum. Okay, but let's turn the time base down. Let's slow it down until we can physically see that dot sweep across the display like that.

Okay, now if I Now turn on this. Well, first of all, you will notice that as soon as it gets to the end bang, it retraces straight back like that and it it actually goes slightly further than the screen. That's why there is a slight actual delay there. But let's now turn off this.

hold off. now. You can't notice much at the moment. It looks pretty similar.

but if I really turn that up okay, adjust it all the way up. look bang. It waits 1, 2, 3 and then it starts again. You see that's probably the maximum for this control cuz it.

it's not really a fancy hold off control at all, but you can see it sweeps across. Then there's an extra delay before it rearms because I've got this in Auto sweep mode. Okay, which means it doesn't actually need a trigger input. It doesn't need a wave form to actually trigger it, but the concept is the same if you've got a waveform Fed into it.

And just to prove that's the case, let's actually get a real waveform on here. Okay, now we're actually going to switch it to normal mode here. So it's normal triggering. so if the trigger isn't there, there's no display at all.

There's no trigger, there's no sweep. Nothing happens, but if we allow it to trigger, we can't. Actually, this is an analog scope, so we can't actually physically see the trigger level here. Actually, we can.

If I turn the read out on there, you go see how the trigger goes above. Once it once that trigger point there goes above the waveform. the peak of the waveform. You don't get anything at all and as you can see, it's come down and down and down.

and because we're uh, triggering on the positive slope, then it starts to trigger and it on the positive edge of the waveform like that. But anyway, Let's uh, turn this hold off control so you can see. It does exactly the same thing here. Let me turn uh, that, read out down a bit.

It's a bit distracting and let's turn this hold off up and notice I'm not changing the time. Bas And bingo there it is. After it's finished, bang, it's got to wait. It's got that hold off time and it must wait before it gets the next trigger.

I Know what you're thinking. Whoopy! do what's the point of trigger? Hold off. That didn't show us anything, it just delayed The Sweep What point is that? Well, there is really no point for simple repetitive waveforms like that sine wave used. so let's take a look at another example where it's actually going to be of great value.

Now let's take the case of a digital signal here, but it doesn't have to be digital. It could easily be some complex analog signal or something like that, but this is a much easier example to work with and the one I'll show you on the display. Now let's say you've got a burst of data like this, separated by a long, uh, period of of nothing really. And then there's another burst of data and it might be the same burst.

Or it might be the data that you want to look at. Now if you don't have the hold off set at all. Okay, if you've just got the trigger set positive Edge Trigger set in the middle like that. Well, it could trigger off this point.

This point. This point. This point. this point, it doesn't really know where to trigger.

It's got no idea at all. the scope hasn't got a clue. this Edge over here or this one in the middle of the waveform is exactly the same as this one. Here, it it doesn't know.

So with no hold off at all what you get on your display, you've probably seen it before when you probe digital signals. it's just garbage. Really, you can see the positive in the negative level and you can see a couple of uh, traces in there. If you zoom in, you can see some edges and stuff, but it just jumps around and gives you a completely jumbled display.

And you don't want that. You want to actually trigger off a fixed p like that and you want to see the gaps in between here and you want to do all that sort of stuff. Well, that's what trigger hold off can do. If you increase that hold off time from the minimum or from zero, you switch it on.

And let's say the time period from here from here to here is say 100 microsc and you know it's 100 micros. You don't have to know, You can just in keep keep increasing the hold off time until you get the display you want. But let's say you did know that that dead time in there was 100 micros. Well, you might set your hold off time to just slightly less than that 90 microc, 95 microc.

And what that does to the display, Uh, what that does to the scope is that it the trigger will only trigger if there's 95 a minimum of 95 um, uh, micros of hold time of of dead time before that. Otherwise, it just ignores those trigger points. So what it's going to do is it's going to capture that first trigger point after that hold off time. and the result is Bingo! A magically stable display where you're actually able to see not only the individual uh packet down here, but the but the entire waveform display.

If you turn the horizontal out far enough and you get a stable display. Magic! Let's take a look at a real example where it does exactly that. Now what I've got on the analog scope here is this same, a very similar digital burst signal. It's actually a bit more complicated, but as you can see, um, if I adjust the time base here, then really, um, it's just.

you know, it's just Digital Data it's just. really digital. Uh, you know, garbage. Really? you can't see much at all.

You can see, it's transitioning and all that sort of stuff and everything's just fine. But really, you can't make out that there's actually uh, packets there so that's no of no use at all. So what what we want to do is we want to use the hold off control here. So let's turn the hold off control up.

I Don't know exactly what the time period is, but you can see the display kind of Shifting there and way. Bingo There it is. we eventually hit a hold off time. We go further than that.

Boom. it's gone again. So there will be that window there where it it just delayed enough that we could see the difference in those packets. There's the there's the individual packet there.

Magic. We've now stably triggered on packet of data and you can do the same thing on a complex analog waveform as well. Now there's actually one side effect of the hold off control that is a bit hard to get on the display, but I'll see if I can do it. Um, because when you when you add trigger hold off then it is.

Then the display is not retracing as often and as fast on an analog scope. so the intensity of your display is actually going to dim. So I've got no hold off or minimum at the moment moment. Now actually see if you can remember that Trace brightness and see if it dims when I turn up.

There we go. It actually dims when because there's not as many retraces on the display. Now let's feed the exact same signal into a modern digital oscilloscope like this: Ryle DS 1052 E Here, it's only a low cost scope, but it'll give you an idea. The even the high-end ones will have exactly the same functionality as you can see the same signal.

It doesn't know where a trigger from cuz we if we go into the trigger menu over here, we're only triggering off regular, uh, positive slope Edge Triggering It's an auto sweep and it's just it's not really doing anything at all. It's quite boring, so um, really, it's it's not doing much at all. Now if we change the time base here the horizontal, we can get to a point where we can see start to see that there is some sort of packet type information in there. You can see those dead periods, but you certainly can't trigger off it now.

I Know what you're thinking. There's this magic button up here. It's the auto button. can't you just hit that? And it auto scales everything and it should trigger.

Set up your trigger and the ho works well. Let's give it a try, shall we? Here we go. It's trying to figure itself out and Bingo. Okay, it's set it up.

It's triggered now. Like if we we could have gone like that. And let's let's hit it again. Change the time base boom right and it will auto scale like that.

but has it triggered off those packets? No, because it's not smart enough to do it, it just doesn't realize. But if we go into the trigger menu here and we go to setup, it will have as all modern digital Scopes do. they will have a hold off option now if you reset it, which is the default value when you're using the oscilloscope. Normally the hold Val in this case is 500 NS minimum.

Now let's increase that, shall we? Let's uh, select that and let's turn it up and I happen to know that that, uh, if we put in about 45 odd microsc that should allow us to trigger off this sucker because that's what that dead time is So well we're getting about. no, we're getting there. so let's let's go up. Oh Bingo There we go.

About 40 microsc and there. Bingo. Our trigger hold off has worked perfectly and we're actually triggering off that so you can. That live display like that is incredible.

Valuable so that you can actually, um, see if there's any glitches in there in real time and stuff like that. And of course you can, uh, stop it and then zoom into your data of course, and then you can analyze it. but that's just a way that you can get trigger in on a complex waveform using trigger hold off. It's brilliant and that's what it's for.

And just to show that, hold off again on the digital scope will the effect of it. Uh, I've just got some noise. I'm just measuring a noise signal here so you can see the updating on theplay gound Flash Flash Flash Flash flash because I've got the minimum hold time. Now let's change that hold off time to let's change it.

Say let's go massive. Let's go up to a second and you'll see it there. We go about a second. one spot on a second actually.

and bingo 1, 2, 3, 4, 5 Because it's not triggering, it's only going to trigger well in this case because it's very quick. Um, it's effectively once per second. Bang Bang bang like that. It's holding off the trigger for that one second.

Just For kicks, let's try that on a more upm Market digital scope. In this case, the brand new Agant Infinity Vision 2000 series. Now I've already set that hold off time so we're getting our packets. No problems at all.

We're seeing that data and it's just brilliant. Not a problem, but let's try that magic auto scale button shall we? that everyone thinks is so wonderful. Let's try it on this scope and see if it does anything here. We go.

There you go. it's it's hopeless. It doesn't know what to do either, and it's not surprising really. it just did the same as the Ry go.

So what we've got to do is we've got to go into. of course we've got to go into our trigger mode and the minimum hold off time there it is 40 nond and we've got to adjust that now. It's a bit touchy. this one.

it can jump around the place if you, uh, turn it too fast. So but if we get to 40 there we go. Bingo There it is. we've got our stable display using our hold Off control magic.

Now just to clarify that one bit further, what hold time will actually work is uh, actually the period of the entire repetitive cycle that you're trying to actually capture and that window in there is the value that will actually work. So I've set up the vertical cursors here I've set one right at the start of the packet which which I want to measure and let's go. let's move the other cursor here and as you can see it should, The hold off should work anywhere from about 40 microsc up to just before about 47 microc or there about. So any hold off time in that period should give you a nice stable triggered display like we're seeing here and this is actually a live display.

It's not actually that's actually captured. okay and that's actually live. So let's try that and see if it works works And here we go. Let's try and prove that the hold off time 37 micros.

So all all before 37 microc, it doesn't work. But after we should hit about 40 or Bango there we go. 39.6 is near enough cuz we didn't measure that absolutely precisely and it should work up to about 47 or thereabouts. Maybe with a bit of error, but it's still going.

No, it's still going. But there we go. It's starting to starting to jump around there there because we're getting a couple of other edges that's triggering off and then 50. It's lost the plot completely and after that, it's just not going to work at all.

And then of course, we're able to capture that and zoom into our hearts content. And we've got a nice, beautifully triggered, uh, complex waveform. So next time you're playing around with complex waveforms like this, be it digital or analog, just have a play around with the trigger hold off. There's no need to put up with that crap uh display which just goes all over the place.

Sure, we can, just, uh, stop that and capture the data. but it's good to have your live. uh, you know, if if you've got enough, if you've got a digital storage scope with enough memory, sure you can just capture it like that and everything's fine. But really, uh, you can't beat having that stable triggered display for live viewing.

so just play around with the hold off control next time you're using the scope. I Hope that was worthwhile. See you w.