WEBVTT 00:00.480 --> 00:05.280 Controlling the impedance of your PCB traces is so important for maintaining signal integrity. 00:05.520 --> 00:07.800 So today I will talk about trace impedance. 00:07.920 --> 00:13.360 Why the impedance of a PCB trace is so important, and how to bridge ideal and real world devices. 00:13.560 --> 00:18.280 Along with demo on cadence orcad and piece for lossless transmission lines. 00:18.360 --> 00:19.520 So let's get started. 00:19.640 --> 00:21.840 What is impedance of a PCB trace? 00:22.320 --> 00:27.520 Electrically impedance is relation between current through a device and voltage across it. 00:27.800 --> 00:34.080 As you can see on a given diagram, its impedance will be voltage divided by current through this device. 00:34.600 --> 00:36.480 There are two types of devices. 00:36.520 --> 00:38.520 Ideal devices and real devices. 00:38.840 --> 00:44.720 Ideal devices are those on which we can perform simulation, mathematical modeling, circuit theory, 00:44.720 --> 00:51.320 applications, etc. and real devices can be interconnects, traces on the board, leads in a package, 00:51.320 --> 00:53.320 and decoupling capacitors, etc.. 00:54.360 --> 00:59.200 Now here we can recall two very popular statements about real and ideal devices. 00:59.280 --> 01:00.640 Maybe you have heard these before. 01:01.000 --> 01:05.920 First one is we can't measure impedance of an ideal device, but we can calculate. 01:06.360 --> 01:11.720 And second one is we can't calculate the impedance of real device but we can measure it. 01:11.840 --> 01:15.360 Then what is the solution for this ideal and real world crisis. 01:16.440 --> 01:24.200 Our goal is to model real devices using ideal circuit elements, so its simulated impedance can be approximately 01:24.200 --> 01:26.680 equal to actual measured impedance values. 01:27.160 --> 01:28.120 I will show you how. 01:28.520 --> 01:33.920 As you can see on the left side of your screen, we have a physical or real world ceramic capacitor. 01:34.200 --> 01:37.080 And at the right side of the screen we have ideal capacitor. 01:37.400 --> 01:40.200 And in both cases the value is one nanofarad. 01:41.200 --> 01:41.800 What do you think? 01:41.800 --> 01:42.960 Which one is correct? 01:43.800 --> 01:50.000 Obviously in ideal world we will get exact value, but in real world we will get few parasitics along 01:50.000 --> 01:51.080 with capacitance. 01:51.600 --> 01:58.350 As you can see for this case we will have capacitor which will be 0.67 Nanofarad. 01:59.150 --> 02:04.110 Measured resistance will be 0.5Ω and inductance will be 1.78 nano. 02:04.110 --> 02:04.550 Henry. 02:05.030 --> 02:10.270 Now, if we move this information to ideal world, we'll get a equivalent circuit of a capacitor. 02:11.230 --> 02:17.390 And if we'll simulate this, we'll get impedance versus frequency response, which will be equal to 02:17.390 --> 02:20.390 the measured value of impedance at particular frequency. 02:21.470 --> 02:26.630 Now I'll show you how we can model a physical ceramic capacitor in ideal world. 02:27.070 --> 02:29.510 For that I'm going to open capture sis. 02:29.790 --> 02:36.430 And here you can see I've made a very simple circuit which has voltage source, source resistor and 02:36.470 --> 02:40.350 equivalent circuit of ceramic capacitor with its parasitics. 02:40.790 --> 02:44.670 Now if we run this simulation, we'll get these kind of waveforms. 02:45.190 --> 02:51.870 One is the voltage across the equivalent capacitor which will be voltage between this point and ground 02:53.070 --> 02:55.350 and current through the equivalent capacitor. 02:56.710 --> 03:01.270 Now just recall the very first formula we have discussed at the beginning of this video, which was 03:01.830 --> 03:04.790 impedance is equal to voltage upon current. 03:05.310 --> 03:06.710 I did the same thing here. 03:07.070 --> 03:11.670 So this is the plot of impedance versus frequency response. 03:12.190 --> 03:14.550 So on this x axis we have frequency. 03:14.550 --> 03:16.670 And on the y axis we have impedance. 03:17.390 --> 03:23.430 So as you can see on this impedance versus frequency response at the very lowest point of this plot 03:23.430 --> 03:25.630 this capacitor will act like a resistor. 03:26.510 --> 03:30.670 And at the left side of your screen it will act like a capacitor. 03:31.270 --> 03:31.510 Why? 03:31.670 --> 03:35.870 Because when we are increasing the frequency, its impedance is going low. 03:36.910 --> 03:40.710 And similarly, at the right side of the plot, it will act like a inductor. 03:41.510 --> 03:44.990 Because when we increase the frequency, its impedance will increase. 03:46.310 --> 03:49.350 I also plot for log of impedance. 03:50.310 --> 03:55.620 So maybe you are familiar with these kind of plot on the data sheet of ceramic capacitors. 03:58.300 --> 04:04.620 So now these values will be very much similar to the actual measured values, because we have imported 04:04.620 --> 04:07.460 the physical capacitor on ideal world. 04:08.420 --> 04:15.060 Now I will show you the plot of which is recorded on VNA which is impedance versus frequency response. 04:15.260 --> 04:21.020 And you will find it very similar with the simulated plot for a ceramic capacitor. 04:24.060 --> 04:32.180 So till now we have discussed very general properties of impedance and how we can bridge ideal and real 04:32.180 --> 04:32.620 world. 04:32.860 --> 04:37.820 From now on we will talk about impedance of a PCB trace and transmission lines. 04:38.660 --> 04:42.460 Another topic is concept of instantaneous and characteristic impedance. 04:42.940 --> 04:46.260 We will start with drawing a first order model of a transmission line. 04:46.540 --> 04:51.380 And if you do not know about the transmission line, you can simply click over the I button. 04:51.420 --> 04:55.900 I have already made a made a detailed video on this topic on this first order model. 04:56.020 --> 05:00.980 As signal travels through the transmission line, it will see some impedance depends on the value of 05:00.980 --> 05:05.620 loop inductance and its coupling capacitor between signal and its return path. 05:05.620 --> 05:11.300 That impedance is instantaneous impedance of a transmission line, and we represent it with z naught. 05:12.020 --> 05:17.500 Z naught is equal to square root of inductance per unit length, divided by capacitance per unit length. 05:18.260 --> 05:24.540 On the same first order model, if impedance is same at every instance means z naught, one should be 05:24.540 --> 05:26.740 equal to z naught two and z naught three. 05:26.780 --> 05:29.220 We call it uniform transmission line. 05:29.220 --> 05:34.420 On this given transmission line model, its instantaneous impedance characterizes the line. 05:34.420 --> 05:37.180 That's why we call it characteristic impedance means. 05:37.420 --> 05:42.420 For a uniform transmission line, instantaneous impedance will be equal to characteristic impedance. 05:42.900 --> 05:44.340 Now I have a question for you. 05:44.700 --> 05:48.580 If line is non-uniform what will be its characteristic impedance type? 05:48.580 --> 05:50.460 Your answer in comment section below. 05:50.460 --> 05:51.770 Low till now. 05:51.770 --> 05:57.690 After these many discussions, we got to know the importance of uniform transmission line and why impedance 05:57.690 --> 05:59.930 should be same throughout the interconnects. 06:01.090 --> 06:02.530 Now I will show you a quick demo. 06:02.850 --> 06:07.690 What will happen if impedance is not same throughout the transmission line or track? 06:08.250 --> 06:10.170 For that, let's open capture Rs. 06:10.530 --> 06:13.730 And here I have made a very simple Spice model. 06:14.130 --> 06:17.970 As you can see, I have added a voltage source for step response. 06:18.370 --> 06:25.290 Then it has a source resistor of 200 ohm, and I have added three transmission line of 50 ohm, 80 ohm 06:25.290 --> 06:26.490 and 50 ohm again. 06:26.690 --> 06:29.570 So that means there is a impedance discontinuity. 06:29.570 --> 06:32.690 And I have terminated those transmission line with a 50 ohm resistor. 06:34.050 --> 06:36.490 Time delay for each transmission line is same. 06:37.290 --> 06:42.570 Now before going for the simulation, I want you to expect what will be the results of simulation. 06:43.050 --> 06:46.170 So we will start with this point here. 06:46.170 --> 06:50.170 Signal is travelling and it will see a first transmission line of 50 ohm. 06:51.170 --> 06:58.410 Then there will simply apply a voltage divider which will be 50 divided by 200 plus 50. 06:59.090 --> 07:03.970 So if you calculate that, you will get 0.2V because the step response is for one volt. 07:05.530 --> 07:12.890 Now again if will if signal move further after 170 picosecond it will see another transmission line 07:12.890 --> 07:14.130 which has 80 ohm. 07:14.170 --> 07:22.770 Then again will put voltage divider and will get 80 divided by 200 plus 80 which will be 0.285V. 07:24.570 --> 07:29.650 So that means after 170 picosecond it will move to 0.285V. 07:30.330 --> 07:31.850 Then again it will see a 50 ohm. 07:31.850 --> 07:35.530 That means it will return back to 0.2 volt. 07:36.450 --> 07:38.690 And after that we have a termination resistor. 07:38.770 --> 07:41.410 Then it will be remain constant on 0.2V. 07:41.530 --> 07:43.210 So this is what we are expecting. 07:43.210 --> 07:44.290 Let's see the waveform. 07:45.410 --> 07:54.680 So after running the simulation as expected for 170 picosecond, it is there at 200 Millivolt. 07:54.920 --> 08:03.160 Right after that it is going to 0.28V or 280 millivolt because we have a 80 ohm transmission line. 08:03.320 --> 08:05.520 Then again it's come back to 50 ohm. 08:05.520 --> 08:13.320 And here we are getting some glitch or some further reflection which is because of termination resistor. 08:13.360 --> 08:13.600 All right. 08:13.640 --> 08:16.360 And then it is stabilized continuously. 08:16.840 --> 08:20.440 Now here I am just sending a single step response. 08:20.680 --> 08:23.480 Let's suppose we are sending continuous pulse here. 08:23.840 --> 08:27.160 Then this kind of reflection will be happening on each step. 08:27.800 --> 08:29.880 All right so I'm going to attach this project. 08:29.880 --> 08:34.160 You can download it from description I want you to just spend some time with it and, you know change 08:34.160 --> 08:37.240 values and play with the reflection concept. 08:37.320 --> 08:43.160 So from here you got to know how these impedance continuity is so important. 08:44.440 --> 08:48.400 Let's move to four factors that affects the impedance of a PCB trace. 08:48.720 --> 08:50.200 First one is trace width. 08:50.240 --> 08:51.560 Second is copper thickness. 08:51.920 --> 08:53.360 Third one is dielectric thickness. 08:53.440 --> 08:56.400 And last is dielectric constant. 08:56.440 --> 08:59.000 Trace width is width of a copper foil. 08:59.200 --> 09:05.000 And as we increase the trace width, its capacitance per unit length will increase and its inductance 09:05.000 --> 09:06.640 per unit length will decrease. 09:07.240 --> 09:10.040 And overall, its characteristic impedance will decrease. 09:10.040 --> 09:15.640 Because characteristic impedance is square root of inductance per unit length divided by capacitance 09:15.640 --> 09:16.400 final length. 09:17.000 --> 09:23.640 As you can see on table 1.1, I have increased the trace width from 5 to 10 mil and its characteristic 09:23.640 --> 09:33.160 impedance reduced from 81 to 63, and inductance per unit length decreased from 11.6 to 9.038. 09:33.560 --> 09:38.000 And similarly, capacitance unit length increased from 1.7 to 2.26. 09:38.480 --> 09:41.040 But there will be no change on propagation delay. 09:41.520 --> 09:47.430 You can ask why there is no change in propagation delay because it is a function of length of a transmission 09:47.430 --> 09:53.230 line and speed of signal, it is independent of other physical properties or parameters. 09:54.550 --> 09:56.950 Copper thickness is thickness of a PCB trace. 09:57.430 --> 10:02.150 We can also say it will increase plating thickness or copper weight. 10:02.550 --> 10:05.110 The copper thickness will increase of a PCB trace. 10:06.310 --> 10:09.750 Let's see how it is related to the characteristic impedance z naught. 10:10.030 --> 10:16.790 If will increase the copper weight, plating thickness or copper thickness, capacitance per unit length 10:16.790 --> 10:22.750 will increase and inductance per unit length will decrease and overall you will see the impedance characteristic 10:22.750 --> 10:24.230 impedance will decrease. 10:24.470 --> 10:26.390 Let's see from table 1.2. 10:26.910 --> 10:33.430 I have increased the copper plating thickness from 1oz to 2 ounce and its characteristic impedance reduced, 10:33.990 --> 10:38.390 inductance reduced and its capacitance increased. 10:39.630 --> 10:45.910 There will be no change on propagation delay and similar results are found on table number 1.3. 10:46.550 --> 10:51.950 When I have changed the copper weight from 0.5oz to 1.5oz. 10:53.390 --> 10:58.670 Dielectric thickness is the thickness of insulating material between signal and its return path. 10:59.230 --> 11:04.430 If we increase the dielectric thickness, capacitance per unit length will decrease and inductance per 11:04.430 --> 11:05.670 unit length will increase. 11:06.190 --> 11:09.750 But you will see the overall characteristic impedance of a signal line. 11:09.990 --> 11:11.070 It will increase. 11:11.350 --> 11:18.430 As you can see on table 1.4, I have increased dielectric thickness from 10 to 20mm and its characteristic 11:18.430 --> 11:21.990 impedance increased from 54 to 79. 11:23.270 --> 11:25.630 Its inductance per unit length increased. 11:25.870 --> 11:28.070 Capacitance per unit length decreased. 11:28.310 --> 11:30.950 But there will be no change on propagation delay. 11:31.710 --> 11:32.950 Dielectric constant is. 11:32.950 --> 11:39.910 Ratio of electric permittivity of material and electric permittivity of vacuum for a fixed frequency. 11:39.950 --> 11:45.420 If we increase the value of dielectric constant, its capacitance per unit length will increase, but 11:45.420 --> 11:51.220 there will be no change on inductance per unit length, and overall characteristic impedance will decrease. 11:51.820 --> 11:55.620 Now here you can ask why I haven't included any table here. 11:56.100 --> 12:03.420 Because for variable frequency we can't predict the impedance behavior for any dielectric value. 12:04.060 --> 12:08.260 It can increase the impedance for a particular frequency. 12:08.740 --> 12:13.300 And similarly the same dielectric material can decrease the impedance for different frequencies. 12:14.420 --> 12:19.340 So these are the four factors to look at to ensure signal integrity in your designs. 12:20.180 --> 12:25.500 I hope you found this video useful and now have a better understanding of what impedance is in terms 12:25.500 --> 12:29.220 of PCB trace, and why it is important to maintain good signal integrity. 12:29.940 --> 12:32.700 Make sure to check out links provided in the description below. 12:32.820 --> 12:38.140 If you would like to learn more about impedance and in an upcoming video, I will talk about single 12:38.140 --> 12:39.700 ended and differential impedance.