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in this video we're going to address terminations you know when and why you need them you've seen me use little through terminators like this one here or end line Terminator like this one or even a 50 ohm termination built into the input of a scope so the short answers of why you need terminators are why you'd want to use them is that terminations help to minimize signal reflections and the distortion that can occur along the transmission path or the transmission line at all you'd use them typically when the propagation delay down the transmission path whether it's a piece of coax or it's a length of trace on a circuit board when that trans propagation delay becomes a significant portion of the rise time or wave length of an RF signal typically greater than ten or twenty percent or sometimes less than that so it'll be frequency dependent when you need them it is why you don't typically see terminators used for DC signals but as the signal frequencies go up and the rise times go down the use of transmission lines and proper terminations becomes more and more important in order to take a look at why we need to take a look at some longer examples longer answers and some examples so let's go do that okay so let's consider a simple example of sending a digital signal or a rising edge down from a signal source down to a load now the propagation down that line is not going to happen instantaneously there is a speed associated with it which is really a function of the dielectric constant now this line that we're sending this down could be a trace on a circuit board it could be a coaxial line there's always a return path down through ground so what happens is when the signal starts rising up okay where we got a whole tidge source here going through some source impedance because nothing's got a perfectly zero source impedance and then as soon as the voltage starts rising up at the input of this line a couple things are happening we're going to start inducing a current going through that line and start raising the voltage on that line but it doesn't happen limitless leave you to think about this line you can think about it as having you know distributed inductance through that line so as the current is going through it's got to basically go through that inductor and also there's a distributed capacitance across that line okay now from the capacitance from the signal line itself to ground or maybe from the center conductor of the coax to the shield so what's really happening is as we send this you know voltage transient through this line we're basically sending current in that's going through this inductor and charging the distributed capacitance as you go along and because the line ideally is fairly consistent in its physical dimensions the distributed inductance is fairly constant across the length of the line and the distributed capacitance is fairly constant along the line so what that means is we're going to have a voltage and current wave front okay a change in voltage is going to cause a change in current that's going to equal to some constant value as we go through this line and that constant value is what we call z0 or the impedance of the line so if we have a 50 ohm transmission line the instantaneous change in voltage over change in current is going to have a 50 ohm relationship that's what we mean by a 50 ohm transmission line if it's a 75 ohm line then that voltage and current wave front as it's going down the line we'll have that impedance so what's interesting is what happens when we get to the end of the line so if we've got this wave front coming down here that has got this relationship of say 50 ohms when we get to the end of the line here if the load doesn't present that same impedance what will happen is there would be a step voltage change so if we've got say this voltage and current wave front that's coming down here with a 50 ohm relationship and then this is a high impedance load while the sudden the voltage is going to jump up higher well that voltage jumps up higher here that's going to now induce a current flowing in the other direction and that's going to propagate in the other direction with that same voltage and current relationship so at any point on the line we have these two voltages the incident voltage coming in and the reflected voltage coming back adding up to create a voltage at different points on the line so as this signal is propagating down as reflection is coming back you'll get some distortion in the middle of the line now so in the case of say a digital signal you might be you know sending a signal out here that is going out to maybe multiple loads connected up so they might all see some distorted version of that waveform so let's go take a look at that on the scope to see what we're talking about so what we've got set up here is a signal source generating a square wave in this case at ten megahertz that's coming out through this piece of coax here i've got two joined up into a t because i'm going to look at the what's going on in the middle of the line here in a moment and then the other end of that is going up into the scope and right now i've got the scope terminated in 250 ohms which is what that generator expects and i can see a really nice clean version of that square wave on the scope screen now let's go take a look at what's going on in the middle of the line i connect that up here as well turn on channel two i can see I've got a nice clean signal in the middle of the line as well and if we look carefully if I put a cursor up here we can actually see you know there is a propagation delay between the end of the line and the point that we're observing here let's zoom in on that take a better look so looking at this a little bit more closely on the scope we can see up on top here channel one is the signal that we're seeing at the very end of the 50 ohm transmission line terminated into 50 ohms and then on channel 2 down below here we're looking at the signal on a midway through the line is about five or six feet of coax on either side of this T and that's terminated into a high impedance so that we're just tapping off this line with a high impedance it's not going to affect things too much now we can see there's about an eight point four nanosecond or so delay from this point all the way down back through to the end now the signal not only at the end of the line but also in the middle of the line looks really nice and clean and that's because we're doing the proper thing with term nation's our signal source has got a 50-ohm source impedance we're going into a 50 ohm transmission line and when terminating in 250 ohms so when we launch that signal in we're getting essentially a 50% voltage divider between the source impedance from my signal generator and the 50 ohm line okay and then the voltage and current going down the line has this 50 ohm relationship it hits a 50 ohm load gets properly terminated there is no signal reflected back and everything is nice and clean and that's what we have on the scope so let's take a look at what happens if we don't properly terminate the end of the line with the characteristic impedance of the line let's say we make it go to a high impedance like 1 megohm now I see two things one is that the signal level at the output or the end of the line is doubled in amplitude talk about why that is I also see some distortion occurring at our tap point in the middle of the line so why are the both of those things happening again if we take a look at our quick little model here we're using a signal source that has a output impedance of 50 ohms so when we initially launched that signal into the transmission line the transmission line looks like a 50 ohm resistor so we're going to get that voltage divider between the 50 ohm source impedance and the line itself and that's going to propagate all the way down the line and then when we get to the end of the line instead of seeing 50 ohms it sees a high impedance so the voltage jumps up and then starts going back the other way and then those will add up to our tap point so that's actually what's going on here as we launch the signal into the line eventually we're going to see it show up at our tap point right here and it's going to show up at half the amplitude because that's the when the signal is just going down the line on its way towards the end of the line when the signal reaches the end of the line here we're going to get a reflection coming back because we've got a high higher impedance here than we have on the transmission line that reflected energy coming back goes all the way back through I wanted to add when it passes by our tap point again it adds up with the voltage that was there and creates our full lamp and that goes all the way back then the same thing happens on the falling edge now of course this could cause some real problems if we had a logic device sitting here trying to measure whether we were at a 1 or 0 that distortion could certainly affect that and maybe create noise and shatter so that's certainly one thing that can happen with not properly terminating a digital line or a digital bus and oftentimes with digital signals the source impedance may not match the line impedance so what would happen in that case that reflected signal coming back wouldn't see 50 ohms looking into the source and another reflection would happen and would go back this way and that process would continue back and forth until those reflections die out and what that would look like you know in the from a digital signal standpoint looks like ringing if you've ever seen ringing on a signal that looks like that often times that ringing is due to these reflections bouncing back and forth between the source and the load so we took a look at some of the distortion that can happen on digital signals along a transmission line if they're not properly terminated you can also have problems with RF signals you know think of an RF signal you know as a sine wave at a higher frequency reflections from Mis terminated lines can also reflect back so you essentially can have a wave traveling in this direction another way of travelling in that direction with that what will happen is depending on your location along the line the sum of the incident wave and reflected wave will either add constructively or destructively so what that means is that you can have a signal amplitude that will vary depending on where you're looking at it along the transmission line or if the frequency is varying you can get an amplitude that will vary as a function of frequency the result of the summing of the incident wave and the reflected wave sets up something called a standing wave and if you've ever been maybe at the ocean and you've seen a wave slam into a bulkhead and then the wave gets reflected back or maybe you're looking over the side of a boat you see a wave at the side of the boat and go back oftentimes you'll see those two waves overlapping and it results in the the surface of the water bouncing back in for kind of like this but really not a wave traveling in one direction or the other so that's the same property of standing waves so before going to the scope here's a quick way of looking at the reflection okay got just a sheet of plastic here as I move this across and kind of see you know my regular signal if you look carefully hopefully it picks up in the video we see a reflection that we're getting from the plastic as we can see as we move this back and forth change our position on the line we can actually see we get situations where that reflection will add up in phase with the existing signal making it essentially twice as big or larger or if we move across we might get to a situation where the reflection will add destructively or simply try to cancel out the signal going on so depending on your physical location along the line you can get this constructive or destructive addition of these two waveforms now it's not clear from this picture that you wind up setting up a standing wave so let's take a look at that on the scope and we can see how the resulting sum will be a wave that is standing and just bouncing back and forth essentially in place okay so I'm cheating here a little bit since I don't have a way of separating out the incident wave from the reflected wave I'm actually just applying two signals at basically the same frequency here so we could see if these two signals are essentially in phase with each other if we take the sum of them add them together we can see I get a signal that is you know basically in phase with them and add it up if I move these two signals so it looks like one is the incident wave and was the reflected wave you'll be able to see how the sum is staying essentially in place with respect to the other two signals even though one is moving in this direction one is moving in that direction the some is standing in place it's just changing an amplitude so let's set things in motion and we'll watch I'm going to simulate the incident wave moving in this direction the reflected wave moving in that direction and watch that the some of them will change in amplitude the won't change in position horizontally see how that that sum is kind of staying in one direction let's move it at the other way you can kind of see the we get a standing wave essentially are the Sun between them even though the incident wave and reflected waves are moving in opposite directions so that's what we mean by getting essentially a standing wave a standing wave pattern on the coax itself if you're not properly terminated because the reflected wave and the incident wave will add up but whether because of the same frequency as they add up they create this wave that stands in place but just changes in amplitude so if you observe the signal at different points within the line you're going to see a different amplitude so let's take a look at that another way okay so I'm now back to the situation where I've got a sine wave going in through a piece of coax I'm tapping halfway through that coax on channel 2 down here and then the coax is finally running off to the channel 1 and getting terminated into 50 ohms if I go over and change the frequency of my input signal we can see that the amplitude of the inputs the signal at the termination and the amplitude and the middle of the line are basically the same as I bring the frequency up or down there's no problem so a properly terminated RF transmission line is going to have a nice consistent RF amplitude across the line of course there'll be losses across the line but you're not going to have any reflected energy that's going to affect the ability for that signal to have the same amplitude as we go down the line now if we improperly terminate the transmission line we're going to get that reflection we'll get those signals adding up so let's go to the extreme case by making the output the termination at the output a high impedance so now we're going to get a large signal reflection coming back and now as I change frequency we're going to effectively be changing our position along the line because relative to the phase of the signal we're going to be looking at a different position within the line with respect to the phase of that signal so as I bring the signal frequency up you're going to see two things you're going to see the output amplitude change and you're also going to see the amplitude at the middle of the line change so as I bring the signal up notice how the that the voltage at the right at the top point of the line here is almost gone away at this frequency here which is 33 megahertz I've almost completely lost the signal at that point in time if I keep going okay so now I'm at 45 megahertz my signals back again if we keep going again right now my signal is actually growing even more I'm at 68 mega Hertz there and now my signal is dropping back off again so right about right about it 100 megahertz I've almost lost the signal again so depending on you know the frequency and essentially our position along the line with respect to phase of the signal is going to we're going to see that effective change in amplitude at different points within the line and that's the result of that standing wave now you noticed as I wrap the frequency up we reached a point where the signal basically went away and that's actually that's a pretty interesting point okay at that point the length of that stub that we've got between our tap point and the open circuit termination is equal to a quarter wavelength of the signal frequency now it could be an odd quarter wavelength but since we started at a low frequency and ramped up that this is now a quarter wavelength long now why does a quarter wavelength long that's open circuited at the end cause a short circuit to appear to make the signal go away if you think about a sine wave if we've got a transmission line as a quarter wavelength long that's ninety degrees so if we have a signal that goes out 90 degrees gets reflected back when it comes back it's 180 degrees out of phase now 180 degrees out of phase is like inverting the signal so if you add a signal and it's inverse it cancels out so a quarter wavelength transmission line has got a pretty unique property that if left open circuited at the frequency where it's a quarter wavelength long that will look like a short circuit even though it's an open circuit going in so this is actually an interesting way of measuring the length of a piece of coax or transmission line along as you know the velocity factor associated with that line so here's a in this particular case we we know we found that first null of the power going away right at 33 megahertz now we could run a pretty simple calculation say at 33 megahertz the wavelength in free space is 300 divided by 33 megahertz gives me 9.0 9 meters now in this coax this rg-58 coax has got a 66% velocity factor so the wavelength in the coax is essentially equal to the free space wavelength multiplied by the velocity factor or 6 meters now for a quarter wavelength line we just have to divide that by 4 so 6 meters divided by 4 gives me 1 and 1/2 meters that's the length of the coax converted to feet it's about 4.9 feet so and that is indeed the length of the hunk of coax here so using that property we're able to measure the length of that coax by just changing the frequency and looking for the lowest frequency that causes a null and we know that's the quarter wavelength frequency or the quarter wavelength of that line so anyway I hope you enjoyed this video learned a little something about terminations and why we use them and the effects that you can have if you don't use them properly thanks again for watching