Hi. I wanted to show you a trap for young players about triggering on an oscilloscope and I found this while doing a repair video here which my, uh, patrons and subscribe star and forum supporters have already seen. I've done like a 35 minute video or a progress repair Anyway, I haven't completed it yet. I need some parts for it anyway.

They've already seen that video which is interesting but more to come on that perhaps. Hopefully I fix it. Anyway, I wanted to show you something interesting here. Now what I'm doing is, I'm just triggering off the 3.3 volt rail here.

and I'm just going to power up this scope. So what what I've got here is it's just on Channel Two here. My trigger levels just like you know, set to the middle of the Uh 3.3 volt waveform here. Okay, so what I'm going to do, I'm going to set it to a slow time base like 500 milliseconds.

Uh, per division. I've got my pro. Let's switch the product off. Okay, I'm going to trigger on the rising edge.

We expect to see a trigger point right in the middle there. So here we go. Uh hmm. Something's a bit strange.

What's going on here? There's our trigger point right in the middle there. Yet, here's our trigger edge over here. But I know what you're thinking. Oh Dave, there's some some.

There's obviously something in there. We have to zoom in and see, Um, what it's actually triggered on. Well, okay. I'm zooming in.

I'm zooming in. I'm zooming in. I'm zooming in. Um, there's nothing there.

Here's our trigger point, right? Our trigger points right up here at like, you know, a volt and a half or something. So it's got to rise above. Based on our trigger rules here. Our edge trigger.

Positive. Going Slope. It must. This signal must transition above our trigger level there before it will trigger.

But there is nothing there. There's absolutely nothing. Our positive going edges all the way over here. What's going on? So just for kicks, let's choose another scope here, signaling 1100.

Whatever Channel 2 Slope Rising Edge our trigger levels. You know, around about 2 volts something like that. So let's switch our thing off. single shot.

Let's switch it back on and capture where. Yeah, oh, that was just the decay of the uh. power supply. And let's switch the product on.

There it is. Uh, Bueller Beulah. Look at this. It's there's.

our trigger point right in the middle. Yet we're triggering over here. And there's nothing. I can zoom all the way in there.

and there's absolutely a waveform exceeding it's extended to the maximum. Fantastic. Anyway, there's nothing there. Why is it triggering here? And when our true rising edge is over here? Really got a trap for young players here, All right.

So what's actually going on here is that there's not necessarily a correlation between what is captured by the analog to digital converter and displayed on your screen and what's actually captured by the trigger circuitry. The analog trigger circuitry inside your oscilloscope. So maybe there's there really is something happening in in here in the trigger search circuitry of this scope because it's not going to trigger on nothing. Okay, it's going to Tr.

It's going to be triggering on something. but why can't we see it on the screen? Aha, we don't have enough detail. We don't have enough sample memory. So let's go into acquire here.

Memory Depth: 1.4 meg. That's pretty good, right? Please quit Stop. Please quit Stop Mode. Oh geez.

Anyhow, Really. Really, I can't change memory depth in stop mode. Let's go to 14 meg. Okay, let's repeat the exact same thing.

Exact same time base because the time base is going to matter. Single Shot Capture that. Let's zoom in. see what we get.

Aha, we start to see some funny business here, but it's still and there's a couple of glitches over here. Look, you can see these right? They're almost at the trigger level there, but not quite as still nothing there. We still can't see it. Well, let's and we're at 100 milliseconds per division before weren't we? Let's go down to say 10 milliseconds per division and let's do it again.

Single Shot Capture 14 Meg Memory Depth: Bingo. Look at this. There you go. Oh no, it's still not.

It's still not enough. Look, it's still not enough. There is still there's our, there's our trigger point. There's still there's our trigger level right there.

It's still not high enough. but it is still. Uh, there's a couple over here now. I hopefully you can see those.

They're very faint. Ah, maybe the lights off. So yeah, we can go into the horizontal here. Then we can zoom in on that there.

There certainly are some glitches in here which are well above that trigger level there, so you know it would trigger on that. But that's not the point. It actually triggered it. It triggered all the way back in here where there's nothing.

So hmm. we're still not seeing what's going on here. But anyway, where's all this funny business coming from? Well, if you're experienced in probing, you'll no doubt see where we've come. A gutsy here.

look at this big antenna earth loop here, right? This big ground plot Pro: Because we're being lazy in our probing, right? We're just probing a power supply We don't. you know, necessarily care that much about high frequency signal integrity. Big ground loop here. Okay, it's even worse because we've got this additional one coming over here.

But it's just a convenient point to put your ground probe perfectly fine. probing just for you know, troubleshooting around and stuff like that. And we've got big mains magnetic stuff in here. Yes, I've got my Chinese takeaway oopsy protection here.

Anyway, big magnetic components inside here. When you switch on, there's lots of Dvdt Okay, the change in, uh, magnetic field which then couples over to your grounding loop over here. and I've done videos on probing and and things like that so obviously that's being picked up. Okay, so that poor probing explains why we'd get stuff on there.

Still doesn't explain why it's triggered at that point when. uh, our trigger level is up here. Let's go back to our keysight scope here and do the same thing yet again. So let's single shot capture, but we'll take our time base out to I don't know, like a millisecond per division.

something like that. Bingo. Look at this. once again, very faint unless we zoom in, but ta-da look at that.

There you go. We've got huge amount of stuff in here, which uh, by the way, depending on how your scope uh implements the sign exon x uh in interpolation. if it does it like, updates it based on display data, you'll see there's been a nice smooth sign exon x thing here, but the uh keysight has been true and it's just telling us where it got, uh, those sample points there. So yeah.

anyway, so this is all switch on glitch that we got coupled via our probe down here. from all the magnetics, how it was switching on or whatever's happening over here, was coupling uh over to this big coil over here into the ground system and that was impacting. and this goes right back to like my old videos about the anistatic uh chair thing. we're way back if you remember that like a decade ago where I stood up from the chair and I could cause impulses on my Uh scope via coupling in via the ground system and things like that.

Anyway, fascinating old video that one. but you can see that's why it triggered at that point there, right? So it certainly did trigger at the right location and the keysight is showing us that true trigger. But unfortunately, because we're at such a slow time base now, we can't actually see that because the, uh, these pulses occur a couple of hundred milliseconds before. uh, what was happening over here.

So you know we might have to set it back to say 100 milliseconds or something like that. Let's go even down to say 50. Maybe we can capture that. and if we put, if we move you want to see more post trigger data, you can move your trigger point over to here, for example.

I don't like putting it right over here. I like to get a little bit of pre-trigger here, so on on this screen. So I I usually set it over to the Uh one graticule uh, one division over like this and we trigger again and on. Bingo once again.

like we, we got it. so we can actually go faster. Let's go for broke and try 10 milliseconds shall we? So let's yeah, look look at that. Okay, so let's take it over here and let's try that again.

nut. But check this out. Here's our trigger point. The key side, even at a fast time base, is still not showing genuinely what's going on in here.

It hasn't picked it up, so whether or not it picks it up or doesn't, it's kind of like a well. It's no. I'm not going to say random, but it's kind of. You know, you just don't know.

and if Murphy's not on your side that day, you'll you know you won't see anything there. So as I said, what's actually going on here is that the Uh trigger system inside the oscilloscope is a separate analog system to the analog to digital uh converter. and what's displayed on the screen and especially if you use like your external trigger input as well, that'll you know it's physically a different Uh channel. It's not taking that from the analog to digital converter, so there can be.

And this is the trap for young players. There can be trigger signals which are which your oscilloscope is genuinely seeing and genuinely triggering off, or your trigger circuitry is seeing, but your analog to digital converter is not. So how can we solve this problem? Well, as it turns out, oscilloscope manufacturers have thought of this and they implement trigger filtering. So if we go into our mode coupling menu here, there's actually two.

Most scopes will have these two options. They'll have noise rejection and high frequency rejection like this. Now if we turn on noise rejection, What this does is that it actually implements a hysteresis type action on the trigger so that it's more resilient to noise. All right.

So let's try that again. I've got noise rejection turned on and 100 milliseconds per division. Let's switch that on, see if it makes a difference. Nope.

Unfortunately, that hasn't done it. and once again, there's there's a little something doing down there, but obviously some sort of, uh, impulse is getting into the trigger system and the Adc is just not seeing that based on the sample rate and the memory depth and everything else, right. So let's go. High frequency rejection.

Let's try that. I think we might have a winner winner chicken dinner because on the key side here, the high frequency reject is around about 50 kilohertz, so it'll reject. It's an analog filter. It'll reject anything like above that.

and we're specifically probing like essentially low frequency stuff here. Um, I.e the ramp up of a power supply. So high frequency reject is the more correct thing to use here. so I can pretty much guarantee you this is going to work.

Where were we? 100 milliseconds per division? Bingo. Winner winner chicken dinner. There is our trigger level right there. There's some noise on that, but yep, it triggered at the exact point that we told it to.

Funny that. So there you go. Let that be a lesson to you. Um, these options exist for a reason.

Maybe I could find like a better, uh, example of where noise rejection works. I won't do that in this video, but high frequency reject is what we want here because we've got like, you know, even though yes, fourier and fast changing waveforms, very high frequency, blah blah blah. But in this particular case, we want to reject any any sort of high frequency stuff. In this case, for this scope above 50 kilohertz, it's probably going to be similar.

for other scopes, it's in the order of, you know, tens of kilohertz. Um, something like that. perhaps? Uh, you'll have to read the data sheet. Hopefully they'll tell you for your scope, but we want to eliminate all of that stuff into our trigger system.

And now with high frequency reject option on for our triggering, you can guarantee that this is going to work every time. and those little glitchy things are picked up by in this case, poor probing. but um, adequate probing for the task at hand. You know, just, uh, really.

just looking at. um, in this case, I'm just troubleshooting the Pcb. You'll have to, uh, supported someone seen the other video to know what I'm actually uh, doing here and you know it's like adequate probe for just you know, probing around. We don't necessarily care about signal integrity, just making sure signals are there, you know, and they're doing the right thing and stuff like that.

So yeah, but you can come a gutter when you try and trigger off something like that with nearby magnetic components which then couple into your ground system. Yes, we could get out our little high frequency attachment thing there, but then you've got to hold it on there. And hopefully I might actually do another. uh, interesting video which I follow follows on from this which shows about, you know, a neat little tip for when you're probing stuff like this and you don't have many hands and you don't have many places to hook your probe on.

Anyway, Okay, just to show you a real deep memory scope here, we've got this. uh, new Siglin 5000x. Uh, we've set it for uh 250 meg memory. Here we're at uh 100 milliseconds uh, per division.

So let's do exactly the same thing as before. Bingo. There's our there's our trigger point. Once again, like my trigger point is like smack in the middle here.

but if we zoom right in here, yes, we can actually see look, we have to go right in. Look how high frequency this is. 20 nanoseconds per division right? there it is. It's just above.

there you go. It just peaked above our trigger level here and that's why it triggered on that point and you can see that it's doing the on-screen sign Exon x interpolation. The waveform actually doesn't. You know it? Well, it's not necessarily looking like that.

it is just. um, interpolating that. where the actual you can go. Well, we can actually switch that off.

There we go. There, we go. we can switch. Ah, geez, that purple's not easy to see is it? Um, sorry about that, But uh, yeah, there you go.

It switches on like that and sign x on x, but you can see that with enough memory. We were able to capture that and if we go down to like, well, you know, two and a half meg of memory or something like that, we simply won't see that. 100 milliseconds per division and off single shot and then capture and bingo. Without that memory, we're just not going to see that data in there.

We're just not going to get it. Um, but of course it's gone into the trigger system. So this scope. Interestingly though, if we go into, uh, where's our trigger setup, if we go in here, we've only got noise rejection.

We don't have high frequency rejection, but let's turn noise rejection on here and see if that does the business. 100 milliseconds per division, single shot capture and nope. Look at that. Still doesn't do it.

And in the case of the Siglent 5000, we can just go into coupling here. It doesn't have a separate option you've got to actually go into the coupling for to get the Hf reject and we're good to go there. It is sweet and this actually might be an example where uh, you see The noise on the signal there, where your noise reject might actually come in because if you had like, maybe just some higher noise there, we don't actually know how much the hysteresis is, Whether it's like half a division, a division or whatever. We we just don't know unless you, uh, the manual doesn't actually tell you that unless you, uh, experiment with it.

I guess you could eventually find out. But uh, yeah. like if you had like a glitch over here and you're really critical about your trigger point and stuff like that, then you might want to turn on your noise reject as well. So there you go.

I hope you found that video useful and interesting. If you did, please give it a big thumbs up. As always and discuss down below. but we saw what the high frequency filter rejection option is capable of on our scope.

That has that. So anyway, I just thought that was a really cool example, which I, I didn't set up. I I simply, uh, stumbled across this while I was doing, um, troubleshooting a repair of a product. So yeah, it's fascinating.

Catch you next time you.