WEBVTT 00:09.320 --> 00:11.560 Hi everyone, welcome to PCB Academy. 00:11.560 --> 00:12.520 My name is Avril. 00:12.680 --> 00:18.680 Today I am going to talk about what is reflection in a transmission line and how voltage level fluctuates 00:18.680 --> 00:20.720 when there is impedance discontinuity. 00:20.960 --> 00:26.120 Then I will talk about some intentional and unintentional causes of reflections in a design, along 00:26.120 --> 00:30.240 with couple of simulations in security Aurora 17.4 and Topology Explorer. 00:30.440 --> 00:31.720 So let's get started. 00:36.200 --> 00:40.720 We will start with very first question what is reflection in a transmission line. 00:41.000 --> 00:45.160 So reflection is distortion on signal line due to impedance discontinuity. 00:45.360 --> 00:49.080 And it may look like ringing but it is completely different from that. 00:49.160 --> 00:51.040 We will understand that throughout the video. 00:51.520 --> 00:58.840 Now due to reflections AC problem voltage can go low or high at any instances and cause false degree. 00:59.720 --> 01:02.840 Now let's talk about what I mean by impedance discontinuity. 01:02.840 --> 01:02.970 duty. 01:03.530 --> 01:08.890 So impedance discontinuity means the change in instantaneous impedance signals is down the line. 01:09.490 --> 01:11.730 Now it can be intentional or unintentional. 01:11.730 --> 01:18.770 For example added corner on the transmission line, added vias, branches or stubs, connectors or packages, 01:18.770 --> 01:20.930 etc. as you can see on the diagram. 01:21.570 --> 01:27.250 Now let's try to understand reflection in terms of magnitude of signals sent and reflected. 01:27.610 --> 01:29.970 So till now we got the idea of reflection. 01:29.970 --> 01:34.930 And as a signal propagate down the transmission line it sees instantaneous impedance. 01:36.210 --> 01:42.330 And if instantaneous impedance is uniform that means impedance one is equal to impedance two. 01:42.370 --> 01:43.690 Equal to impedance three. 01:44.090 --> 01:46.210 Then we call it characteristic impedance. 01:47.450 --> 01:53.730 But if instantaneous impedance ever change for whatever reasons, some of the signals will reflect back 01:53.730 --> 01:59.290 in opposite direction and some part of the signal will continue, but with different amplitude. 01:59.690 --> 02:05.290 So here all the instances where impedance change we call them impedance discontinuity. 02:05.290 --> 02:05.380 Continuity. 02:10.260 --> 02:14.260 In the next step, I am going to talk about how to estimate reflection voltage. 02:14.780 --> 02:21.500 So imagine on a PCB we have impedance discontinuity at one instant and their impedances are z one and 02:21.500 --> 02:22.060 z two. 02:22.900 --> 02:27.260 Now we are sending a signal on this transmission line of voltage V incident. 02:27.540 --> 02:31.580 But due to impedance discontinuity some signal voltage will reflect. 02:32.020 --> 02:36.180 We call it v reflected and some signal will continue to transmit. 02:36.220 --> 02:37.620 We call it V transmitted. 02:38.100 --> 02:39.700 As you can see on your screen. 02:40.140 --> 02:46.860 So the relationship between the magnitude of incident and reflected signal will be v reflected divided 02:46.860 --> 02:52.140 by v incident is equal to z two minus z one upon z two plus z one. 02:52.820 --> 02:57.340 We also represent it with row and we call this ratio reflection coefficient. 02:58.300 --> 03:05.540 So from this equation we got to know the amount of reflection is directly proportional to delta z or 03:05.580 --> 03:06.860 change in impedance. 03:07.260 --> 03:10.430 Now let's see let's see a quick example of how to apply this equation. 03:10.430 --> 03:13.310 To calculate we reflected and we transmitted. 03:13.710 --> 03:21.670 So as you can see in this example Z1 is equal to 50 ohm, z2 is equal to 75 ohm and v incident is one 03:21.670 --> 03:22.070 volt. 03:22.670 --> 03:26.190 Now we'll apply the formula v reflected divided by v incident. 03:26.310 --> 03:31.790 It will be 75 -50 divided by 75 plus 50 which will be 0.2. 03:32.710 --> 03:37.750 So if we multiply it by 100 we'll get reflection coefficient is 20%. 03:38.150 --> 03:43.830 That means the amount of reflected voltage is 20% of V incident which is one volt. 03:44.190 --> 03:47.670 And if we do that we'll get 0.2V is v reflected. 03:48.750 --> 03:52.510 Similarly v transmitted will be v incident minus v reflected. 03:52.990 --> 03:55.150 So it will be 0.8V. 03:55.670 --> 03:59.590 So this is how you can apply this equation to calculate reflection voltage. 04:00.510 --> 04:02.870 Now as we discussed reflection coefficient. 04:02.990 --> 04:09.350 Similarly we have a term transmission coefficient which is t is equal to v transmitted divided by v 04:09.350 --> 04:13.160 incident is equal to two times z to upon z one plus z two. 04:14.360 --> 04:18.520 Now how we got the equations of transmission coefficient and reflection coefficient. 04:18.720 --> 04:21.840 We have applied Ohm's law on these two equations. 04:22.480 --> 04:28.520 First one is I incident minus I reflected is equal to I transmitted v incident minus v reflected is 04:28.520 --> 04:29.800 equal to v transmitted. 04:29.960 --> 04:33.000 And here we have to apply v is equal to I into z. 04:33.600 --> 04:40.480 For z one and z two and we will get transmission coefficient and reflection coefficient equation from 04:40.480 --> 04:40.800 there. 04:46.280 --> 04:50.400 Now we can go ahead and see the simulation on Aurora 17.4. 04:50.720 --> 04:55.960 So in Aurora 17.4 I have simulated statics two development board. 04:56.320 --> 04:59.120 And this is the reflection post layout analysis. 04:59.320 --> 05:05.560 And here you can see in the simulation table I have simulated for all the data buses which has 64 bits. 05:06.320 --> 05:11.120 And from focus data you can select for which parameter you wanted to see the results. 05:11.480 --> 05:13.650 So I have selected propagation delay here. 05:14.690 --> 05:18.810 And by double clicking over propagation delay you can sort this table. 05:19.290 --> 05:25.010 So as you can see parallel data bus 56 has maximum propagation delay. 05:25.570 --> 05:27.890 So what I have done I have just selected that. 05:28.970 --> 05:36.930 So as you can see this highlighted part of the track has maximum propagation delay and impedance discontinuity 05:37.090 --> 05:43.370 due to these change in impedances inside the simulation plot. 05:43.410 --> 05:48.090 You can see what you are sending from U7 which is our FPGA controller. 05:48.290 --> 05:52.010 And what you are receiving at XU2 which is a Dimm module connector. 05:52.330 --> 05:59.090 So if you select this waveform you can see this waveform has lot of false triggering. 05:59.090 --> 06:04.170 It is not even crossing vine for few nanoseconds. 06:05.010 --> 06:10.250 So this is happening due to reflection or impedance discontinuity on the line. 06:11.290 --> 06:15.700 And similarly you can check for parallel data 57 5859. 06:15.900 --> 06:22.740 All this set of propagation delays and it will highlight all the tracks which has impedance discontinuity. 06:23.300 --> 06:27.140 So you can go back to layout, process and change all these layouts. 06:28.380 --> 06:34.020 So if you want me to create a separate video on how I have done that post layout, reflection analysis, 06:34.020 --> 06:37.020 step by step process, just let me know in the comment section. 06:37.660 --> 06:40.220 Now we are good to go for bounce diagram. 06:44.900 --> 06:50.220 So bounce diagram is representing the whole chain of reflection on a transmission line due to impedance 06:50.220 --> 06:54.460 discontinuities even because of source resistors. 06:54.780 --> 06:59.700 So for predicting reflection on any transmission line we need to know following parameters. 07:00.460 --> 07:04.020 First is time delay of transmission line or length. 07:04.060 --> 07:08.420 Second is impedance of each region of signal propagation. 07:08.780 --> 07:10.540 And third one is V incident. 07:11.220 --> 07:17.020 If we know these three parameters we can easily predict reflection through bounce diagram. 07:17.860 --> 07:20.780 Now I'm just going to give you a quick demo of bounce diagram. 07:21.260 --> 07:22.220 So let's see that. 07:22.540 --> 07:29.060 So let's consider an example where source driving an open termination transmission line and we have 07:29.220 --> 07:31.060 v source is equal to one volt. 07:31.380 --> 07:37.380 Source impedance is equal to ten ohm which will be impedance discontinuity for 50 ohm transmission line. 07:37.860 --> 07:40.460 And time delay is given one nanoseconds. 07:41.420 --> 07:46.660 So in case time delay is not given you have to find out what is the length of the transmission line, 07:46.660 --> 07:48.900 and you can calculate the time delay from that. 07:49.860 --> 07:53.540 So in the very first step we are going to calculate v incident. 07:54.100 --> 07:58.700 So we have v source which is connected to ten ohm source resistance. 07:59.220 --> 08:01.460 And we have a 50 ohm transmission line. 08:01.500 --> 08:04.580 Now we have to find the voltage between 10 ohm and 50 ohm. 08:04.580 --> 08:07.820 And that will be V incident of 50 ohm transmission line. 08:08.580 --> 08:14.500 So we'll apply simple voltage divider V incident is equal to V's multiplied by 50 divided by 50 plus 08:14.540 --> 08:14.900 ten. 08:15.140 --> 08:18.580 And when you will calculate it you will get 0.84 V. 08:18.580 --> 08:19.030 four volt. 08:20.030 --> 08:22.990 Now we are good to go to create a bounce diagram. 08:23.830 --> 08:28.710 So in a bounce diagram we'll have source 50 ohm transmission line and resistor. 08:29.070 --> 08:32.950 But remember that this source has ten ohm source resistors. 08:33.070 --> 08:35.630 That means we incident is not one volt. 08:35.630 --> 08:39.950 It is 0.84V that we have calculated in step one. 08:40.870 --> 08:44.910 Now step two is what will receive at the receiver side. 08:46.070 --> 08:53.710 So incident voltage 0.84V will reflect back because it is a open termination at the receiver end. 08:54.230 --> 09:03.190 And when you probe it you will find at the receiver side will get 0.84V plus 0.84V, which will be 1.68V 09:03.190 --> 09:04.270 at the receiver side. 09:04.910 --> 09:09.230 So it will put that in a waveform you will get after two nanoseconds. 09:09.230 --> 09:15.190 So one nanoseconds it will be zero because at that time signal will be travelling from transmitter to 09:15.230 --> 09:15.710 receiver. 09:15.990 --> 09:22.040 After one nanosecond it will reach there and will get the voltage of .68 volt. 09:23.120 --> 09:24.720 Now let's talk about step three. 09:25.760 --> 09:31.600 So because of open termination at the receiver end .84 volt will reflect. 09:32.040 --> 09:34.440 And now this will be the incident voltage. 09:35.280 --> 09:39.680 We'll apply the simple formula of V reflected upon V incident. 09:39.680 --> 09:45.840 So that will be reflection coefficient is equal to z two minus z one upon z one plus z two. 09:46.440 --> 09:49.280 So ten -50 divided by ten plus 50. 09:49.480 --> 09:53.080 And we'll get -0.67V. 09:53.440 --> 09:55.640 So this will be reflection coefficient. 09:55.640 --> 09:58.200 And from that we have to find out reflected voltage. 09:58.560 --> 10:04.480 So we'll get it by multiplying it by v incident which will be 0.84in this case. 10:05.040 --> 10:09.680 So we'll get v reflected is equal to -0.56V. 10:10.800 --> 10:19.760 Now this v reflected will be again reflected back to receiver side where at the other end we have 1.68V. 10:20.240 --> 10:29.690 So again it is a open termination resistor when it will reach at the receiver side will get 1.68, -0.56, 10:29.690 --> 10:31.250 -0.56. 10:31.690 --> 10:39.530 If will summarise, it will get 1.68, minus two times 0.56 and will get 0.56V. 10:40.730 --> 10:48.690 And if you see that in the waveform for one nanosecond, 1.68V will maintain because at that time signal 10:48.690 --> 10:51.450 will be travelling to the transmitter side. 10:51.490 --> 10:52.170 All right. 10:52.170 --> 10:54.050 And then it will be reflected back. 10:54.890 --> 11:00.810 And this time we'll get 0.56V at the receiver when we'll measure it. 11:01.610 --> 11:05.730 So the receiver voltage will back to 0.56V. 11:06.170 --> 11:07.810 Now let's move to step five. 11:08.090 --> 11:17.130 So at step four we have seen that -0.56V will incident and -0.56V will reflect from receiver end. 11:17.850 --> 11:18.490 All right. 11:18.490 --> 11:22.490 It will take one nanosecond further to reach at step five. 11:23.210 --> 11:25.020 So again due to open Point. 11:25.060 --> 11:25.780 Termination. 11:25.940 --> 11:28.300 -0.56V will reflect. 11:28.500 --> 11:31.700 And again we'll going to calculate reflection coefficient. 11:31.860 --> 11:39.380 We'll use the same formula where we'll have v reflected upon v incident minus ten -50 upon ten plus 11:39.380 --> 11:39.860 50. 11:40.260 --> 11:43.140 So it will be -0.67. 11:43.580 --> 11:47.100 Now we have to multiply it with v incident to get the v reflected. 11:47.540 --> 11:49.900 So v incident is -0.56. 11:49.900 --> 11:52.460 So we'll get v reflected 0.37. 11:53.220 --> 11:57.780 So now incident voltage for step six will be 0.37. 11:58.860 --> 12:01.220 And we have open termination at the other side. 12:01.380 --> 12:06.300 So it will multiply by two and add it to whatever the voltage there at step four. 12:06.820 --> 12:12.300 So it will be 0.56 plus two times 0.37 which will be 1.3V. 12:13.580 --> 12:18.740 So that means now the waveform voltage will go to 1.3V. 12:19.820 --> 12:27.660 So after n number of reflection like we have discussed from point one to step five, our waveform will 12:27.660 --> 12:32.910 be will be stable at one volt because one volt was the exact incident voltage. 12:33.910 --> 12:36.670 All right, so I hope you got the idea of bounce diagram. 12:36.750 --> 12:39.190 Let me know in the comment section in case of any doubt. 12:43.990 --> 12:49.390 So at last I am going to summarize this video with list of discontinuities that can cause reflection 12:49.390 --> 12:49.990 problem. 12:50.350 --> 12:54.190 And the first one will be end of transmission line should be terminated. 12:54.350 --> 12:59.590 So if you are not using proper termination there will be reflection on the line and which will cause 12:59.630 --> 13:00.590 AC problems. 13:01.350 --> 13:02.990 Second is a package lead. 13:03.150 --> 13:05.750 Third one is an input capacitors pad. 13:05.910 --> 13:09.830 So if you are using any capacitors on the line make sure they are paired. 13:09.870 --> 13:13.910 Should be very small or you should use a small package of capacitor. 13:15.110 --> 13:17.150 Fourth is vase on the signal line. 13:17.270 --> 13:19.390 Fifth one is a connector stub. 13:19.430 --> 13:22.710 Branches and test pads can cause reflection issues. 13:23.710 --> 13:27.670 And last but not the least, crossover another transmission line. 13:28.150 --> 13:30.030 So that's it from reflection side. 13:30.070 --> 13:31.190 See you in the next video.