Hi. In this video, I'm going to show you how to simulate an oscilloscope probe or in this particular case, a times one oscilloscope probe. Because probing of your circuit can be a really important thing. Every time you probe, it doesn't matter how carefully you try and do it, you are effectively loading down your circuit.

Um, in a often complex way. Now I've done an entire video on this a long, long time ago. in a galaxy far far away. Uh, what is it? Mysteries of Times One oscilloscope probes revealed.

So I'll link that in up here and down below if you want to watch it. And that's where I tell you about. Of course we've got a probe like this one which has a switchable times 1 times 10 switch on it and in times one position spoiler alert. It has a very low bandwidth.

In this particular case, about six and a half or almost all switchable probes. This one has six and a half megahertz. So your bang hundred megahertz, 200 meg scope. If you switch this to times one, you only got a bandwidth of six and a half megahertz.

The reason I'm doing this is because well, sometimes it can be important and we might see this in a future video perhaps. So circuit loading probe loading can be a big thing and system setup and measurement and stuff like that. I won't say any more at the moment. Um, because I haven't done my simulation yet.

So what I want to do is simulate this peepee. 510 Siegeland Oscilloscope Probe Switchable. You know, a real jelly bean. 100 megahertz Switchable.

times 1 times 10 probe. Now we're going to be using Lt Spice here because not only is it a really good simulator, it's not the easiest to use, but it's really good and it's free from Lt. There's lots of support and community out there for it. In this particular case, we're using Lt Spice because it has transmission line stuff.

If you go in and place a component, it's actually got a transmission line like this. But this is an ideal lossless transmission line. This thing, as you've seen in the video is not lossless. In fact, it is deliberately lossy.

It's deliberately designed to be lossy. And thankfully they do have another one called a lossy transmission line. So this is what we're going to use. And when you actually go and place your lossy transmission line like this, it doesn't just work.

And when you open up the parameter window for this, it's not obvious like how this works at all. It's just not going to do anything on its own. In fact, if you put it in there and try and simulate it and go, all your parameter values are 0 and everything else. It's just going to ruin your day.

Now there is actually a handy resource for this sort of stuff if you go over. There's actually a an Lt Wiki and it's got a specific like a usage information on the lossy transmission line here and then it's got the Spice model parameters. the spice directive as it's a chord which we'll use, but here's all the parameters that have to go into it. You can't just set it up in that model.

In Lt Spice, it has to be included as what's called a Spice directive. So you've got the resistance in Ohms per unit length because this is a lossy coax cable. It's not just a regular coax cable, it's lossy. It is going to have Ohms per unit length because if you get your multimeter out, Go and do it Right now.

If you haven't seen that video, you'll be surprised. Put your probe in the center here, put it on the end. put it in times one and you don't measure zero. Ohms, you measure a couple of hundred Ohms and lossy coaxes are a very cool thing.

Invented uh by John Kobe at Uh. Tektronix. I don't know when it was a long time ago. Um, but yeah.

hats off. Brilliant stuff. A lossy transmission line. Lossy coax.

That's how we get the really high frequency. Really superb performance in modern passive probes. Anyway, we've got in inductance in Henry's per unit length because, well, every wire's got uh, inductance in it and uh g, that's conductance in Siemens. Um in one on which is one on Ohms.

Your multimeter might have a conductance mode, but its value is going to be so high it doesn't matter. We're just going to leave that out today. And of course, capacitance per unit length. Then as I said uh, there's another parameter length.

You can specify the unit length and then there's all sorts of uh, sort of really specific stuff which we're not going to touch here. There's various flags you can set for really more complicated simulations and stuff like that Newton Raphson method for time step control and impulse response to keep your impulse response errors low and interpolation when your Cod Quadratic fails and stuff like that right. This is only if you're on like the bleeding edge of simulation would you have to touch this stuff. But all we have to do is set the Rlc and a length and they show you how to do that here.

And this is what's called the Spice Directive so we can actually if you actually copy that over and we go into Lsd Spice, this is where you have to start setting it up. but as I said, you can't just enter it in here. it's not going uh to work. So the first step is to right, click on that and then set up.

The value is the name of uh, your transmission line because you want it to be something descriptive. So I put times one probe here and then we have to go up here and insert a Spice directive and you can even have like a regular comment like down here oscilloscope import or this Spice directive and we can actually paste that in and then that will go and you put that anywhere on your schematic like this and that just sets up the directive to the Spice engine. This one's labeled my lossy uh transmission line here, but I've done another one. I've named it times one probe here and the Ltra thing case uh, doesn't matter here, that's uh, the command to the uh spice simulator to actually, uh, do the lossy uh transmission line that's the label you have to put in and then you simply put in all the parameters that you want and the resistance 210 ohms, the capacitance 83, puff, uh, the inductance 208 and the length is 1.2 meters.

In this particular case, or its unit lengths, it doesn't have to be meters, so I'll go into how I calculated uh, these values in a minute. but let's actually go over. We've got our source here, I've just set that up as a sine wave at a one volt amplitude and then I've got a 50 ohm source impedance here. and then it just goes into our transmission line model that we've got here.

and then we've got our oscilloscope input. So we're modeling our the input uh to the oscilloscope because we we want to get a ton. We're not just simulating the probe itself, if you did that, then you wouldn't have the oscilloscope input. but I want to model the whole effectively the system uh response including the oscilloscope input And then this, 68 Ohm and 22.5 puffy is actually the compensation network.

I just realized that this one's actually up in the probe here, but some of them are down in the base here as I've explained in that video. So oh, technically, I've got that back to front, but that's just designed to taper the response off. So anyway, but yeah, I should actually put that up in the other end. Anyway, I'm just going to leave it there for now.

Um, I can't believe I just made that error. Anyway, I don't plan this stuff before I hit record. So how did I get these values of Uh 210 Ohms? Well, I simply measured it so it's back to front so all the electrons are going to fall out. It has to be so that my eyes look in the right direction for the screen capture.

What do we got? 329 Ohms. There you go. So I need to change that to 329 Ohms: Where did we get the 83 picofarads from? Or 83 Puff as we call it? Well, it's in the data sheet. here.

It's um, says in times one mode 85 puff to 120 puff. So it's within that range. so let's just call that an even hundred puff. And then if you divide that by 1.2 meters because that's the length of this actual Uh probe.

So we get a one meter unit length. Um, then we get a value of 83 puffs. So how do we calculate the inductance of 208 Nano Henries here? Well, we already know the cable capacitance of 83 puff here. Um, so let's assume a 50 Ohm Coax, for example, the characteristic impedance is equal to square root of, uh, the L square root of Lc.

So simply rearrange that formula to get L on one side and that works out to 208. Nana Henry's cool. Actually, I'm just going to leave this uh compensation network here for now at the oscilloscope end of the probe instead of up here. We might rearrange it later if we've got time Now, let's go simulate this thing.

and well, is it going to work. Edit: Simulation command like this. So we want an Ac analysis like this, We'll just run that linear number log rubbish number of points. I'll just take a thousand.

Whatever. That's plenty to get like a a sweep start frequency one hertz stop frequency 20 meg Because I know the bandwidth. This is going to be about six and a half meg and you can see here how it's added this spice directive here so that just you know it's this gooey interface is just a nice way of using spice. but spice is basically a command line thing that sort of runs in the background.

So and all of your simulation engines, they use the spice as the backbone so it's just up to the Gui interface just to make it nice. So let's actually run that simulation, shall we? Analysis Failed Matrix is singular. Now you might actually see this pop up a lot in simulations and not just on Lt Spice. It might give you a similar error in another simulation engine.

And what this means. The matrix is singular. It's got nothing to do with Kyanu. It has to do with the fact that it thinks there's another part of your circuit which is floating.

Basically, there's no like Dc bias connection between them. So you'll you'll get this sort of error if you're doing a transformer simulation or something like that. So just assume this was a transformer. You have a coil here like this.

You have another coil on the other side like this and there's no Dc bias to get to the other side does. so. the simulation just chucks are wobbly and it just can't handle that. It doesn't like it, so we can actually get around that by adding another ground point on this side over here because our ground point for our circuit is on this side.

but obviously the transmission line model. it doesn't like having the other side effect. It thinks the other side is floating and it gives you that error. So if we fix that, we'll find that that should now run Tada.

And it does. Check out our response over here. Let me reformat this. Let's get rid of the phase plot there because that just start confuses things and we'll add a cursor 3 db there because that's our bandwidth point.

Our minus 3 db point and it's four and a half. Well, four and a half megahertz there. So there you go. The simulation is now working, but of course, you may not want to simulate with a ground reference point on both sides of the circuit here, because you can come again, so that can ruin your day.

If you're doing larger simulations, it might work for just something like this. That's fine. So how do we get around that? So if we delete that ground point, we can actually whack in a resistor here. Flip that around and we'll put in a high value resistor.

Make it one gig. Ohm, something like that, and that will give a Dc bias path, and it won't affect the simulation at all because it's such a high value. and we'll find that now when we've got that Dc bias path, this other side is no longer deemed to be floating in terms of the simulation and we can run that. and Boom! We get exactly the same result.

It's just it's it's run again. It just updated then. and Bob's your uncle, so that's a handy tip. If you ever see that error message, you know that something's floating.

You're going to have to add a Dc bias or another ground reference in there to handle it. Now, how do we know this? Our compensation value here of 22.5 puff. And once again, go back to the spec sheet here and it actually tells you the compensation range is between 10 and 35 puffs. So we'll split that down in the middle and you get 22.5 And normally they have a series resistor in there normally 68 Ohms.

but it's not going to make a huge difference like we can lower that to you know one Ohm and you'll find that this won't really run that again. And yeah, it's like it's barely changed it. like you'll you won't even notice it. So it's just a basically just a compensation network.

Uh, to ensure that the Uh response doesn't go higgledy-piggledy at high frequency, it just rolls off and matches it properly. That's why there's actually the adjustment screw on here or on here. It's effectively the same at either end as some manufacturers have. At one end, some have it the other, it's like meh, but that is designed to compensate to match the input capacitance of your scope and basically give you like a total system response, a flat response.

And that's why it's important to compensate your pros. But in terms of like the bandwidth and stuff like that, it doesn't have a huge impact. So let's actually change that to 15 puff and we'll run that again and we should see that will change a little bit. There you go, it's 4.96 now, so not a huge difference there.

So what I'm going to do now, is move this compensation network over to the input side to match this specific scope probe that we actually have here. Okay, so I'm just going to delete that in there. and then we'll add a resistor in here like this and this will be our uh, 68 Ohm jobby. And then we'll add a cap well across there like that, this stuff up here.

There we go. So that's our variable capacitor compensation network, but even it's not actually inside the transmission line because the transmission line starts. The actual coax. The lossy transmission line starts after.

There's a physical resistor and a physical cap in there for the compensation network. So there you go. 68 Ohm 15 puff. I've taken this out of circuit so I can leave the components in there.

They're just going to do absolutely nothing and we can run that again. And what do we get? There you go. minus 3 db of 4.77 meg. So we're not quite getting the six and a half megahertz that we expect out of this thing.

So here's where we come to the practical difference between the compensation network in here in this signal probe. This is another signal probe, but it actually has the compensation network in there. If you can see the hole, this is a 350 megahertz bandwidth one, so this is higher bandwidth. They'll typically have them um, at the end on the higher bandwidth once.

The thing about this is that we can measure the resistance from here to the tip and that 329 ohms we put in there. Um, this one actually measures uh, 220 or something. But the problem is when you've got your resistor your compensation network in here, it's a series resistance, so you can't Unless you chop your coax, you can't measure your coax. It's going to include this resistor value up here.

So what if I go 329 minus 68. that's 261.. So I'm going to change that. Okay, so here's where we could have Kamagatsa if we didn't realize that if you didn't know that there's a series resistor in this top compensation network here.

So let's run that again. Okay, we're getting a higher bandwidth, will we get our six and a half megahertz? 5.5 Oh, come on, hang on. I just realized that the sheet that actually comes with the probe does not have the Times One bandwidth on it. So sigilant Datasheet Probe Series: The Pp510 Probe: Hundred Megahertz.

There you go. Oh, it's six megahertz. I thought it was uh, 6.5 I read that somewhere else. Um, so we're actually after a figure of six megahertz so that you know there's going to be some wiggle room in there.

But in theory, we can actually calculate the precise value of the input resistor in the compensation network because we can physically measure the resistance of this probe. And then or we And we know the other parameters of it. And then we can just back calculate. We can just iterate the value of that 68 ohm resistor to actually find what value it is to give us the six megahertz bandwidth.

and interestingly, the compensation range down here. This is actually different. It's uh, 10 to 30 whereas this sheet says 10 to 35. So ah, let's just go down to say 12 Ohms Like that? we'll have to change that to 318 in that case and we can run that again.

But hey, what are we gonna get? Come on, 6.04 We got one. There you go. We've just determined that it's likely that this Ppe 5510 pro likely has a compensation uh, resistor in here of 12 ohms and a lossy coax of roughly 318 ohms Like that, because that gives us our response bandwidth that we're after. It gives the nominal data sheet value.

So there you go. Pretty cool, huh? And of course, you often don't need to go to this sort of level of simulation. You can just like simulate your just using the tip capacitance or the system capacitance as it's called because it takes into account the probe and everything else. You can just usually just lump it as a tip capacitance um, element.

So you don't have to go to this detail. but sometimes you might Or you might for example, want to, uh, like model the uh, the actual clip. Lee, you're in inductive clip lead like this or you might want to, have another. You might want to model like a transformer coupling into your ground lead like this.

If you've got like 50 Hertz mains or something or some other magnetic thing nearby that is coupling in the ground. uh, lead, You can like use this sort of simulation to actually see what's going on here. We could actually delete the oscilloscope input like this and we can run that again and then we can go. well.

Then we can probe that like that as you can see that's going to be a higher bandwidth -3 db. There are 7.8 megahertz if we don't include the oscilloscope input here. So just the probe itself has a bandwidth of 7.8 meg. but it's lower when you include the oscilloscope input with its oh, it's gone off there with its um 15 nominal 15.

I think it's a 15 puff. Input on the signal might have check could be 18 or something like that, but you get the idea. This is how you simulate a oscilloscope probe or a the complete system oscilloscope probe with the ground reference on this side. So there you go.

I hope you found that useful and interesting. If you did, please give the video a big thumbs up. And because that feeds the algorithm and as always you can discuss and down below catch you next time.