Hi. In a recent video, I looked at a few problems with the new RIE Goldie's 1054 Z Oscilloscope and also the DS 2006 cap I Discovered a problem with the AC trigger coupling and well as it turns out, this is actually not a new problem in the DS 2000. It was reported back in July 2013 and so apparently Rygel haven't fixed it and it seems new to them again. Anyway, they responded very quickly.

they said yes, they're looking into it, they're working on it and all that sort of stuff. but a lot of people seem to have an issue with exactly what AC trigger coupling on an oscilloscope is and so many people confused it with input AC input coupling the usual AC DC input coupling that you're used to. In fact, a lot of people have been using the source codes for you said I have no idea what AC trigger coupling does and so I just leave it on DC all the time I don't know what the problem is and this bug in the Royal Silla scope only occurs with the AC trigger coupling. So I thought I'd do a very quick video explaining what AC trigger coupling is and exactly why you would want to use it and even Reigle seemed a bit and I perplexed about this.

They said, well, why does pretty much anyone want to use AC trigger coupling when you've got a DC input signal? Well, anyway, let's take a look at the overview and we'll go straight to the bench. Alright, what I've drawn here is an old school analog CRT oscilloscopes just because it looks nicer. but digital scopes work pretty much exactly the same way. So most people are familiar with what's called our AC input coupling or just input coupling input Channel coupling.

When you select that menu for your channel 1 or channel 2 input, it says do you want AC or DC coupling and I'm pretty sure everyone understands that well. DC coupling just lets all of your DC signal all of your signal straight through whereas I see if you switch it to AC coupling here, then it removes the DC component ie. so only frequencies from like a you know, a couple of tens of Hertz upwards are displayed. So if you've got an input signal here that is offset by a DC amount in this case, 1 volts.

if you turn on AC input coupling, then it simply removes the DC component like that. Pretty easy. I Think everyone understands that. So that's basically how your inputs work.

We've got an input amplifier here which you can select AC or DC coupling. Do you wish to remove the DC component and then you can select? well. Do you want to display channel 1 or channel 2? We won't go into how they're multiplexed and all that sort of stuff, but we can select which channel we want to display and then this amplifier goes and drives our vertical Dickel plates here and moves our waveform up and down in response to the input. Pretty easy.

and you've got that duplicated for Channel 1 and Channel 2. But what a lot of people don't realize is that oscilloscopes both analog and digital have AC trigger coupling as well. So this is import coupling and this is trigger coupling and I've drawn the trigger circuit in red here. Now what the trigger circuit is you've probably seen like on a traditional to channel oscilloscope, digital or analog.

It'll typically have a third input here called external trigger input and it just allows you to use both channels with and then an external trigger signal to get a stable waveforms. There's a selection switch either on the front panel of the scope or in the soft menu for the trigger menu that allows you to select do I want My trigger signal come from channel one, from Channel 2 or from external. Pretty simple, but then you'll notice that the trigger circuit has exactly the same AC DC coupling selection as we had up here in the input. Here it is here on DC trigger coupling mode, which on most digital scopes is the default, whereas on analog oscilloscopes Ac is actually the default.

We'll talk about that in a minute. so it's exactly the same thing. So it takes the signal before it's coupled here. Okay, before it's coupled here on the inputs and then you can have AC or DC selection.

Then we've got the trigger circuit and of course the trigger circuit and horizontal sweep drives our waveform across. or it drives. see in here you if it was a digital scope, you'd have a digital analog converter and all that sort of jazz, but makes no difference. So that's the basic input architecture.

both the analog channels and the triggering channel on either a digital or an analog scope. But I will say that modern digital scopes like the Roy Goldy S 10, 54 Z they do all of their triggering in the digital domain. So it's not like that they actually have a signal kicked off here and then a physical AC coupling cap. Here they they actually take it.

This goes into the analog to digital converter and then they take that into the trigger signal and then they do it in software. It's all done in the digital domain, but hey, it's an oscilloscope. It is supposed to work like a real oscilloscope, ie. an analog oscilloscope or any oscilloscope people are used to.

So if a digital oscilloscope has AC DC trigger coupling, it should do exactly the same thing as it does on an old school analog scope, regardless of whether or not it's done in the digital domain or whether it's actually tapped off a signal here and a real cap in a real relay there to select the signal and done in the analog domain. Anyway, Enough of that. Let's go to the bench. Now if we have a look at both at old school analog oscilloscope like this: Tektronix triple to 5 and this Reigle D is 10 54, 0 Silla scope which does as I said, all the triggering in the digital domain.

but hey, essentially it is and does and is supposed to work exactly the same way. So if we take a look at our analog oscilloscope you, you'll notice that we have our trigger coupling down here. Not to be confused with your input coupling over here AC and DC entirely different things. Now you'll see that's got AC and DC down the bottom.

We won't concern ourselves with the low frequency and high frequency reject. they're they're just our variations of the AC couple in there. So what we've got is the AC coupling. trigger coupling is there by default, it's up the top.

There's a reason sup the top is because on analog oscilloscopes you much by-default want to use AC trigger coupling and I'll show you why in a minute. Now on our analog oscilloscope here we have the mode control as well and we've got it on normal mode. Okay, I'll explain that in second and we've got our level control which adjusts our trigger point which starts our sweep all the way over here. so you'll notice that is the start of a trigger point whereas on a digital scope, it's in the middle so we can adjust our level control here and the trigger point.

Once it gets up to there boom, it's gone because it's no longer triggered, our trigger lead has gone off, and we can take it all the way down. Bingo! It's gone right off the bottom of the waveform or it's very intermittent. It doesn't know how to trigger on that's right on the edge. So input coupling is obvious.

If this is, this is our ground level reference here. If we switch it to DC coupling and I change our input signal, I can raise up the DC offset there. And as I said, if you switch it over to AC coupling, it will simply remove that average DC level and shift the waveform down easy. Now, I'll show you how a C trigger coupling works.

Okay, we've got it set to AC Over here, our input signal is DC but as I said it, the trigger takes the signal from it, basically directly from the input connector. so it doesn't matter the trigger circuit. It doesn't care whether this is AC or DC like this, but we've got AC coupling here. You'll notice that the start on the waveform, the trigger point is basically in the middle of that waveform.

Okay, and if we adjust the DC offset on the input look, that trigger level remains exactly where the trigger position remains exactly the same, regardless of where of the DC value of that waveform. And why is it doing that well? Because we're using a C trigger coupling, so it's taking that input signal and removing that DC component. so as far as the trigger set circuit over here is concerned, it is got a basically a signal with no DC level. So the trigger level you set here okay is not affected by that DC input because it's AC coupled.

but if we take that same waveform and switch it over to DC trigger coupling over here. okay, if we adjust our DC offset, you'll notice that the that the start value is actually changing there and when it gets to a point it just loses it like that. So we're exactly the same as before. except we're losing our trigger and our trigger point is starting differently because we're DC coupled and we're changing the level of our DC input.

So that's the advantage of using AC trigger coupling on your oscilloscope. over. Here is that it doesn't matter what DC level your input waveform is at or what type of input waveform, it's going to be easier and simpler to trigger off it. Okay, let me demonstrate this very dramatically.

where AC trigger coupling is useful in a practical scenario, when you're probing a circuit. now over here. I've got it on our normal trigger mode I've got it on AC coupling. Okay, so we're going to see the advantage of it.

Input is DC coupled. Okay, so we'll be able to see DC offset. What I'm going to do is I'm going to probe for different signals I've got four function generators set up here with different DC offsets, but just assume that we're probing a circuit. We don't know what's going on.

We want to find out what happens. Okay, out. look our waveforms nice and triggered here. Okay, we set our trigger level where we want.

but because we've got AC triggering. Watch. What happens if I change? Look to a different signal with a different DC offset. It still reliably triggers off that.

No problems at all. It's a different waveform. It could be a different frequency or whatever. Let me try a third signal that we're probing.

Look at a higher frequency. It's a different DC offset. We're still automatically triggering on that. Fantastic.

And if I choose another signal even higher. DC offset again. Once again, it's still triggering. But look, if we switch this down to DC mode were useless.

There's that third signal look at. briefly. Showed up there for a second. there's our second signal.

briefly. showed up for a second and we're back to our first ere. It can't even trigger off that we have to tweak it again for that. There you go.

There's a dramatic example of a tactical use for AC input coupling when your probing and unknown circuit. It just makes it easier to trigger from. And this is precisely why analog oscilloscopes like this always have the AC coupling. or mostly always have the AC coupling right up the top as if it's sort of the de facto default trigger coupling method because it just gives you an easier trigger point to work from and removes the coupling here.

And it makes no difference about the coupling on the input here. And for those who think the input coupling makes a difference as we saw on the diagram, it doesn't, we'll switch it to AC coupling. There's our signal there. Okay, it's gone off the screen here.

Okay, that's no good. So we might want to change the position back to here. But anyway, let's now switch over to our second signal. Bingo! We're triggered is remove the DC but it's triggered for us.

Let's go to our third signal here. Bingo, it's triggered. Let's go over to here. it's triggered.

But if you go to DC trigger coupling down here, it's useless. There's our full signal. Our third signal doesn't even show up our second and back to our first. Absolutely useless.

So the takeaway from that is, if you use DC trigger coupling down here, then every time you probe a new signal on your circuit, an unknown signal and it might have a different DC offset. and you're using DC input coupling like that because you'll want to see the DC offset your probe in your circuit, you want to know if there's a DC offset there. so you're using DC mode. Typically, you might do, and you've got to manually go in and tweak the level control each and every time if that DC input varies, whereas if you've got AC coupling, you don't have to do that.

You can set typically set one zero trigger point there and it'll trigger on a good lot of input signals whereas DC coupling may not do that. But I Hear people complaining? Well, digital scopes are different. They're not analog. Well, let's try it.

Here we go. We're in our trigger menu. Okay, this is trigger not input coupling. We go down to settings.

this is our AC our trigger coupling. We select AC Okay, once again, different wave forms with different DC offsets. look which, they're just fine. I switch over to another waveform.

Bingo. We're automatically triggered. look at that magic and then we go over to another waveform with a different DC offset. Bingo, we're still triggered.

I Mean granted, this is going to change if our trigger level is slightly different, but you stand more chance of getting a signal First, go by using AC trigger coupling. So let's select that to DC here and see what happens. No, our waveforms. No, it's not triggered properly.

Let's go back to our waveform over here. No, it can't trigger on that and it can trigger on that one here because on DC trigger coupling, you have to muck around every time you probe a new signal and set you trigger level like this. And that is the difference because with these new digital oscilloscopes, people are so used to seeing this trigger line and this trigger level They've just got into a different mindset of using the scope. and every time they trigger eyes, every time they probe a new unknown signal, they go oh well.

I've got to set my trigger level down here. Well, if you had a C trigger coupling turned on here, you might not have to do that. It is advantageous. Even on these modern digital scopes, it works the same way.

Has the same advantage. Now, just as a little aside, I'll show you something that I don't particularly like about the Rye Gold digital scopes. This is on the DSR 1000, ZD series and also on the DSR 2000 series, but curiously, not on the original DS 1052 II Now you'll notice that the trigger knob here has pushed to 0. on the Rygaard, also on the 2000 series, but on the Agilent X-series and also on the Tektronix MDO 3000 series, it's got push for 50% instead of 0.

And this brings us to one of the big advantages of modern digital scopes in that look, right? We've got an untreated waveform here. We've just probed it. It's brand-new we had no idea what the DC offset was, whatever, and our trigger level happens to be right up here. Well, to get it triggered.

of course, we can move our trigger level down, but we don't have to do that on a modern digital scope regardless of where the waveform is, as long as it's within side the capture area of the ADC Look, it's got trigger push for 50% so we can just push that and the scope figures out where the waveform is and automatically puts that trigger level smack in the center. Brilliant! So that's sort of like a one button solution to not using AC trigger coupling. So instead of having to always just muck around with this trigger level control, you can just go bang. Give it to me, Thank You very much.

Works the same on the Agilent and on the tech. but if you want to do the same thing on the right, well here's our level control here. It's around about there. Yeah, we can tweak it and bring it down to trigger our unknown signal, but because it's set the level to zero, if we push it like that, it drops down to zero.

and if you don't have your AC input coupling on, it's fine. If you're actually using input coupling of AC then you know if you've got your unknown signal, you'll actually trigger from it like like this. No problems at all, but it doesn't. It's not intelligent enough to do to work out a DC to do that, and it's got nothing in the menu to set it to 50 percent.

There's no option. Don't like it. It's the same on the 1000 and the 2000 series. I Much prefer the 50% level.

and here's where it's another step back from the venerable Diaz 1052 II Yes, it has the reset to zero level down here, but it's got a dedicated 50 percent button, so yeah, we can dicker if our unknown signal. We can dig around with our level control, put it in the middle and trigger or we can just go bang hit 50% Thank you very much and some people were confused. Why? If you go into the trigger menu and you go into the coupling and you set it to AC you no longer get that trigger level control there because it doesn't make sense relative to your input channel. so we don't get that bar anymore.

even though it tells us our trigger level is down here so it's still displaying it, but it just doesn't make sense to print the level on the screw because we're because the trigger level over here is AC coupled from our displayed signal, so no longer makes sense to actually put it in there relative to the graticule and the waveform. So I hope that clears that up. So there you go. I hope you found that interesting and why I see trigger coupling even on modern digital scopes, is still an advantage when probing unknown circuits and it's a shame that people don't use it more often.

Has seen so many people on the forum and on the comments for the previous videos say well I've never used AC coupling. it doesn't matter and lo I don't care if this thing has a bug in the AC coupling menu although Rui Golde S2000 I don't care I never use it anyway. Well, you're probably doing yourself a bit of disservice. It's not a bad feature.

I see trigger coupling Have a play around with it next time there you go anyway if you want to discuss it. Links down below to the Eevblog Forum: A Leave comments on Youtube, do whatever and if you liked the video, please give it a big thumbs up. Catch you next time you you.