Signal reflections and Transmission lines - Ec-Projects

hello and welcome to easy projects so I have a project coming up where I will be using some fairly long signal wires to send data signal back and forth so I thought it would be interesting to take a look at transmission lines and signal reflections so we will not go into the math and the calculations of it but we will make a few demonstrations and hopefully we can get an idea about what transmission line is what a signal reflection is and practically how to get rid of it and then if you want to study it further you can go to look the math and try to do some calculations but we will not do it here so I think we should just start by a demonstration so I set up this microcontroller to generate a square wave just chuckling and I open as fast as possible the actual frequency of the signal is not so important it's more that it has a fast rising and falling edge to be able to see this phenomenon I would like to mention if you want to measure something like this you should use no ground lead or the probe just use the emotional connection on the probe because you'll get a much more representative reading of what's actually going on so if I were to hook up my ground lead and my hook you can see that already we have a big problem here because we get ringing on the signal which we don't really want this is not you saw before this is not the actual signal that's present on the pin but because we have this big loop and long leads here we get a problem kind of similar to what we are going to know you can also use a small spring clip like this it'll be fine Alton I saw the two component leads to the board here and I can just jam the probe in between and that gives the best rating for me so what do I think would happen if it's again a length of wire here we need one for the ground and one for the positive we just hook it up to these two pins and the other end is just opened that's nothing connected so you can see not much happen but we do get a little bit of wringing here let's continue at one more piece you can see we get a bit more ringing so I'll double the length of wire again and you can see we get more this ringing but now it actually doesn't look like a ringing anymore starts to get a bit square almost like a staircase that's getting smaller and smaller and if I connect up even more why you see the amplitude didn't get much worse but that's a longer distance between these oscillations so to understand this it helps to think of the electricity as a wave instead of just like a voltage you know this from anything else like water more sound if you could go inside a city and it's quiet at night and you clap your hands really hard you'll hear it goes from all over the place meaning you know that light has a speed so it takes a certain time for the light from the Sun to hit the earth and so on but everything has a like a speed and the speed of electricity is you could I reach the speed of light also but once it's travelling inside a wire it gets a slowed down a bit so it's not quite the speed of light but nevertheless it still has a speed so if you say you apply a voltage just normal DC voltage at the end of the wire it doesn't appear immediately at the other end of the wire if you just sort of say a 9-volt battery and you have a multi media at the other end of the wire in theory after some time you will mention just nine volts there and you don't think everything is fine but we have to know that this this voltage it starts here and it travels down the line and once it hits the other end we can measure it but what happens if we disconnect the battery again before the voltage reaches the far end well the thing that we have sent away already doesn't know that we disconnect the battery so this will just continue to travel down the line and you can now see that there would be a wave on the carpet just a voltage traveling down the line and at some point it would hit the other end but since this requires energy if there's a changing voltage there must be some kind of current energy in this so due to the conservation of energy it can't just they disappear go out into nothing and if the other end of the line is open there's only one thing left actually and that is for the signal to reflect off the other end of the cable and come back so we would see the signal come back and if this end is a open circuit also if you disconnected the battery it would in theory just bounce back and forth and of course there's losses in the cable and eventually it will die down so that is in fact what we see here that the longer we make the cable the longer time it takes for this kink here to appear and this is in fact the reflected signal that's coming back that makes this kink in the waveform and you can see that we have multiple reflections and one is a bit less but the first one so it kinda dissipates the energy so let me just change this to a normal wire that doesn't have all these junctions here because that will be a problem later this wire is a bit longer even so we will see longer time between the reflections so this is just a normal two conductor wire and used for many things like speakers or power cords whatever you can see just by hooking up this wire but nothing at the other end we get this really bad looking waveform here from the reflections so this year is actually not the worst part of the story this is just what we have at the transmitter end so what would happen at the you receiver end some of the other end of the cable I am touching this with my finger so it will not be exactly what is going on but it's very close and you can see it looks completely useless so this is a 3.3 volt microcontroller so the pulse that it sends out as an amplitude of 320 watts but you can see in the waveform where all the way up at 6 volts here and we had negative to about at the negative going Peaks here and our original waveform is completely lost here and you couldn't really like this one tiny little spot here in this line at around just 2 volts well you could maybe pick up the signal but it's completely unreliable and useless so this wire is 2.6 meters long and already here we couldn't send a six megahertz data signal through it but surely there's something we can do about this because if you know your normal Ethernet cable again 100 megabit connection can easily weigh up to 100 meters and this is just six megahertz over 2.6 meters so far we have used this cable with nothing connected at the other end and that would be a typical scenario for a receiver to have a very very high input impedance and just measuring the voltage across it's all wires like if you use a multimeter but what we can do is put a load across yeah just a resistor and this load we would call a termination resistor and so you might ask which resistor do we use and actually there's only one thing that determines that and that's the cable if you have played with TRS or solar panels or something like that you will know that to get the maximum amount of energy out of the system you will have a load that is equal to the source impedance in other words to get the most power out of your solar panel you will adjust the charge current to the battery according to the softs impedance of the panel and you can just make sure that you match your voltage and you measure the current and you just find the the peak where those two multiplied together gives me maximum power that would be a maximum power point tracking it turns out that the same analogy works or transmission line because we just want to take all the energy out of the line when it receives the other end so the next question is what's the impedance of this line and this impedance would also call the characteristic impedance the funny thing about that is if it's a completely uniform transmission line which never happens in the real world but theoretically if we have a completely uniform conductor the surroundings are completely uniform all the way along the cable it doesn't really matter how long this cable is it will have the same characteristic impedance and for the transmitter it will look like an infinitely long transmission line if it's terminated with the right load at the end here and we'll get no reflections because all the power will be dissipated in the load so we don't really know about this wire we could roughly calculate the characteristic impedance of this I don't actually know what it is just a random why I think I cut it off some lights or something but the manufacturing tolerances what wouldn't make it very reliable but you can buy cables like coax cables or coaxial cables that is manufactured to a specific characteristic impedance and most of the time you'll find this written on the cable like you 50 ohms for this one and you can get coax in different impedances 50 or 75 ohm is very common it doesn't mean that there's any resistance in the cable that it's not 50 ohm from one end to the other so this determines the termination note that you should put on the other end of the cable to get no reflections through it so you might have guessed it already we can very easily find the characteristic impedance of this wire by connecting a variable resistor here a potentiometer across it and then measuring the transmitter and see when we get no reflections on the signal we know that we found the right termination resistance which will be equal to the characteristic impedance of the cable so let's try that so I just saw a five kilowatt potentiometer to the end of the cable and this now has five kilowatts termination this is far too high and if we measure it at the transmitter we see that it had pretty much no effect on the signal but if we start turning this we'd see at some point it starts to get better Oh too much and right around here maybe you also noticed if we go too far we get the opposite effect before we looked at if the signal was open we get a positive returning reflection if the output was shorted together we would get a fully negative reflection coming back I don't want to short this really because it's just the output pin of the microcontroller and you can see that the amplitude gets much worse also and we will put unnecessary load on there so let's not do that but now we found the characteristic impedance of the line and we can actually just measured so what we get here is around about 125 ohm so that means if I want to send my signal down through this wire I will add a 125 ohm termination resistor at the receiver and then both ends of the signal should look perfectly fine this is not a good cable for any high frequency communication at all and you should really use a cable that's made for it like a cat5 Ethernet cable or something like that and for that cable you will also get the characteristic impedance specified by the manufacturer so you don't have to guess it so just for completion here we see at the transmitter end and for good measure we give it the receiving end you can see it's not as nice but it's pretty good so there is another thing that I would like to demonstrate also remember I said that this transmission line has to have a uniform characteristic impedance that means that roughly speaking that it has to be one cable without any connections and it cannot be damaged in any way and it should have the same shape like if I were to open this cable like this and change the impedance here you can see we get a reflection so this is at the far end of the cable and I want you to to notice the precision of this so let me put it in the center of the screen there so if we put this back let's paste it from I can turn normal and I go roughly to the middle of the cable and I open it just break the two wires apart you can see that the reflection happens sooner before it was here now it's yeah and now you can see I'm kind of permanently made this transmission by worse than it was and that's why again a coaxial cable is the preferred method because it has one conductor wrapped around the other and it has a fixed distance from the outside of the shield which is one connector to the core inside which is the second connector and this can be precisely manufactured and the outside forces that act on this doesn't really change that property but if you were to bend it really sharply so it gets a kink in the wire you hit it with a hammer or something so you changed the physical dimension of the cable it will be no good anymore and then we'll start to make reflections I'm not exactly sure how much a tiny little defect would have but it will make a difference of course it wouldn't be any good if you couldn't make connections between cables and send signals of long distances and in fact this can be done if you control the impedance of the connector so in a 50 ohm B and C for example they will try to match this impedance of the connector 250 ohm also so there's a lot more to this and I guess I could be rambling on but also I'm not an expert in this at all I just know the very basic things one common thing is that if you also want to get rid of these defects you can put a heat B termination at the source also a serious termination so if anything gets reflected back it will also be eaten up by this resistor at the source so I hope you like this video it was just a relatively quick and practical demonstration of why this matters showing that even for a relatively complex topic you don't really need to do any math if you just want to do a bit of groundwork so a project that I hope to be starting on soon will include an Arduino and some sensors and keypad and some sirens and maybe some flashing lights and maybe a smoke generator who knows I'm not sure this will be necessary to think about but it's always a good thing to think about it because the data rate in that project probably will be very low so but again the voltage spikes that you saw at the far end of the cable have no termination that could damage the equipment so that's one thing to think about also so saw that we could also increase the rise time a lot so that it's a lot more than the time it takes for the seconds to reach the end of the cable and then this effect will pretty much go away also but that will limit the data rate a lot so again thanks for watching and please subscribe to the channel if you want to see more and I hope to catch you soon for the next video see ya

Channel: EcProjects

Views: 6,280

Keywords: DIY, Electronics, reflection, transmission line, termination, 50 ohm, impedance, characteristic, HF, RF

Id: AJUGWGsMMfo

Length: 20min 15sec (1215 seconds)

Published: Mon Mar 04 2019