WEBVTT 00:09.360 --> 00:11.560 Hi everyone, welcome to PCB Academy. 00:11.560 --> 00:12.520 My name is Avril. 00:12.800 --> 00:18.640 In today's video we are going to talk about what are modes in a coupled transmission line, some application 00:18.680 --> 00:23.120 of modes and waveform patterns of differential and common signals. 00:23.640 --> 00:29.160 Then we will quickly go through a couple of equations to estimate differential and common impedances, 00:29.160 --> 00:33.240 even an odd mode impedances along with their voltage components. 00:33.240 --> 00:34.560 So let's get started. 00:39.120 --> 00:44.160 We will start with very first question what are modes in coupled signals? 00:44.160 --> 00:50.280 To understand it better, we'll see three cases of differential pairs where we will change input of 00:50.320 --> 00:54.440 coupled line and try to understand the effect on signal received. 00:55.000 --> 00:58.280 Let's see the first case here we have two signal lines. 00:58.560 --> 01:00.590 Let's say signal one and signal two. 01:01.070 --> 01:07.730 Signal one has voltage level 0 to 1 traveling down the transmission line, and signal two stays at zero 01:07.730 --> 01:08.190 volt. 01:08.950 --> 01:13.910 Now, as soon as signal starts traveling, there will be crosstalk because of voltage change. 01:14.270 --> 01:18.510 And due to that, we'll see this type of response at t one time. 01:19.230 --> 01:27.030 At t two time it will be more steep and will see more voltage transfer between signals, and at T3 and 01:27.030 --> 01:29.550 T4 the crosstalk will increase. 01:29.750 --> 01:33.670 Now this is happening because of far end crosstalk and near end crosstalk. 01:33.710 --> 01:37.790 If you wanted to understand this concept, I have already created a video on this. 01:37.790 --> 01:39.790 You can click on I button and watch it. 01:40.870 --> 01:44.030 But at this stage, this point is not important. 01:44.350 --> 01:45.790 In case one. 01:45.790 --> 01:52.910 As we have excited on signal line and zero volt constant at second coupled transmission line, we have 01:52.910 --> 02:00.500 observed whatever the signal we have sent on SG one or line one, it is not equal to the signal received 02:00.500 --> 02:01.660 at the receiver side. 02:02.460 --> 02:08.260 In the next step, I am going to discuss two very special cases and those will define these modes. 02:08.820 --> 02:09.820 So let's see those. 02:14.700 --> 02:20.620 Case two is when we apply same voltages on both each coupled transmission line at the same time. 02:20.940 --> 02:25.460 Let's say it's signal one and signal two and voltage applied v1 and v2. 02:25.740 --> 02:28.940 So in this case V1 should be equal to v2. 02:30.140 --> 02:32.660 Since signals are traveling together. 02:32.660 --> 02:36.940 So there will be no change in voltage or current with respect to each other. 02:37.460 --> 02:43.980 If we probe at the both input and at the receiver side, we'll observe after t time delay receiver's 02:43.980 --> 02:46.540 voltage should be equal to v1 and v2. 02:47.340 --> 02:48.820 This is a special case. 02:49.060 --> 02:51.620 We'll talk about it more later in this video. 02:51.740 --> 02:53.220 Let's move to another case. 02:57.900 --> 03:05.570 Now let's talk about another case where we are sending 0 volt to 1V on first line and 0 volt to -1V 03:05.570 --> 03:08.210 on second line, and the condition is same. 03:08.210 --> 03:10.930 Line one and line two are charge coupled. 03:11.610 --> 03:18.530 So as signal propagate down the transmission line, both signals will generate noise on each other but 03:18.570 --> 03:19.810 opposite in polarity. 03:20.210 --> 03:26.810 So you can say the resultant noise at the receiver side will be negligible and the voltage pattern will 03:26.810 --> 03:27.930 be undistorted. 03:29.250 --> 03:30.970 So now what will be net voltage. 03:30.970 --> 03:33.810 We'll see at the other side of coupled transmission line. 03:34.330 --> 03:38.130 And that will be one volt minus -one volt. 03:38.450 --> 03:41.890 And net voltage will be two volt as you can see in the figure. 03:42.050 --> 03:50.290 So this is another special case that we are going to refer to define modes for these two special cases 03:50.290 --> 03:51.570 case two and case three. 03:51.570 --> 03:53.290 The voltage pattern will get at. 03:53.290 --> 03:55.170 The output will be undistorted. 03:55.410 --> 03:58.760 And we call these special state of signals. 03:58.960 --> 04:00.120 Mode of pairs. 04:00.840 --> 04:07.440 When a differential pair is excited into one of the two modes, either case two or case three, signal 04:07.440 --> 04:10.600 follows special properties and remains undistorted. 04:11.760 --> 04:18.040 So, as we have already seen, when we drive same voltage on both lines, we call it even mode, and 04:18.040 --> 04:23.080 when we drive opposite voltage on both transmission lines, we call it odd mode. 04:27.920 --> 04:34.200 As we have already discussed, odd and even mode are special case which propagate signals undistorted 04:34.200 --> 04:34.920 on the line. 04:35.480 --> 04:40.080 For example, as we have seen in case of differential signal and impedance. 04:40.080 --> 04:40.680 Tutorial. 04:41.040 --> 04:41.560 Impedance. 04:41.600 --> 04:48.080 A signal sees depends on coupling of adjacent trace and voltage pattern on the other line as well. 04:48.600 --> 04:52.920 Now, using odd and even mode terminologies, we can refer to these impedances. 04:54.440 --> 05:00.150 For example, we can refer a impedance of one line as OD mode impedance. 05:00.310 --> 05:02.510 If signal follows, od mode states. 05:03.230 --> 05:08.350 Similarly, if signal follows even mode states, we call impedance of one signal line. 05:08.430 --> 05:09.630 Even mode impedance. 05:10.630 --> 05:16.270 Now things will become more complicated when the geometry of each coupled transmission lines are not 05:16.310 --> 05:17.070 identical. 05:17.910 --> 05:19.750 So that will be our advanced case. 05:19.830 --> 05:22.110 We will talk about it in later videos. 05:22.750 --> 05:28.030 Now in the next step I am going to talk about how to estimate even and odd mode impedances. 05:33.190 --> 05:39.030 So as we have already talked about differential signals before, we have seen that differential impedance 05:39.030 --> 05:44.390 is series combination of impedances of each line with respect to return path. 05:45.070 --> 05:50.190 So as you can see in the diagram here z equivalent is equal to z differential. 05:50.910 --> 05:55.310 And where z differential is two times z odd. 05:56.150 --> 06:01.620 Z differential is Differential is differential impedance and z od is characteristic impedance of one 06:01.660 --> 06:01.980 line. 06:02.100 --> 06:09.500 When pair is driven in OD mode and from there we can define z od is half of z differential. 06:10.180 --> 06:13.660 Next we will see what is common and even mode impedance. 06:14.220 --> 06:18.740 So as we all know, the common signal is average voltage between signal lines. 06:19.860 --> 06:27.140 And when a common signal drives a coupled pair, we call it even mode state and the characteristic impedance 06:27.140 --> 06:28.060 of one line. 06:28.060 --> 06:32.700 When pair is even more driven, we call it even mode impedance. 06:32.700 --> 06:40.020 As you can see in the diagram here, z equivalent is z even parallel to z even where z even is characteristic 06:40.020 --> 06:41.500 impedance of one line. 06:42.180 --> 06:47.700 And when we further solve it, we'll find out Z even is two times of z common. 06:48.420 --> 06:55.180 So we can use this relation to find out the even impedance or the even mode impedance of common signals. 06:56.580 --> 06:58.290 Now let's take a quick example. 06:58.290 --> 07:01.330 If we have two 50 ohm edge coupled transmission line. 07:01.930 --> 07:07.970 In first case, if they are driven by event states then z event will be 50 ohm and z common will be 07:07.970 --> 07:08.930 25 ohm. 07:09.650 --> 07:16.290 But if they are driven by odd states, then z odd will be 50 ohm and z differential will be 100 ohm. 07:16.330 --> 07:16.810 All right. 07:16.850 --> 07:20.770 So it is that easy to find out even an odd mode impedances. 07:25.330 --> 07:29.090 Now at last we will see how to find out v common and v differential. 07:30.090 --> 07:36.370 So v common is average of voltages on each coupled transmission line and V differential is the differential 07:36.370 --> 07:37.090 voltages. 07:37.450 --> 07:43.570 So let's say we have a coupled transmission line which has V1 and V2 voltages on each line. 07:43.690 --> 07:50.610 Then V common will be one by two summation of v1 and v2 and v differential is v one minus v two. 07:51.850 --> 07:53.090 So that's it for this video. 07:53.090 --> 07:55.930 I hope you got the concept of even and odd modes. 07:55.970 --> 07:57.050 See you in the next video.