WEBVTT NOTE This file was exported by MacCaption version 7.0.06 to comply with the WebVTT specification dated March 27, 2017. 00:00:00.667 --> 00:00:04.922 align:center line:-1 position:50% size:66% Bandwidth is one of the key banner specifications of oscilloscope probes. 00:00:04.922 --> 00:00:08.926 align:center line:-1 position:50% size:61% Understanding just a few concepts can keep you from making common mistakes 00:00:08.926 --> 00:00:12.346 align:center line:-1 position:50% size:36% that can cause inaccurate measurements and errors. 00:00:18.518 --> 00:00:22.814 align:center line:-1 position:50% size:48% Hi, everyone! I'm Erin, and welcome to the new series Probing Pitfalls. 00:00:22.814 --> 00:00:27.903 align:center line:-1 position:50% size:58% In this series, you'll learn all about common probing pitfalls and how to avoid them. 00:00:27.903 --> 00:00:31.073 align:center line:-1 position:50% size:58% This will give you the confidence that the signal you see on the scope screen 00:00:31.073 --> 00:00:35.702 align:center line:-1 position:50% size:57% is the best representation of what's coming out of your device under test, or your DUT. 00:00:35.702 --> 00:00:38.789 align:center line:-1 position:50% size:42% In this episode, you'll learn about critical bandwidth pitfalls, 00:00:38.789 --> 00:00:41.333 align:center line:-1 position:50% size:28% like not knowing about the 3 dB point, 00:00:41.333 --> 00:00:44.711 align:center line:-1 position:50% size:43% forgetting to consider bandwidth with respect to rise time, 00:00:44.711 --> 00:00:49.424 align:center line:-1 position:50% size:62% not understanding harmonics, or not calculating the entire system bandwidth. 00:00:49.424 --> 00:00:56.014 align:center line:-1 position:50% size:65% Many engineers believe using a 100 MHz probe to analyze a 100 MHz signal is the perfect setup, 00:00:56.014 --> 00:00:58.475 align:center line:-1 position:50% size:34% but there are a few things they don't realize. 00:00:58.475 --> 00:01:00.811 align:center line:-1 position:50% size:35% The first is the 3 dB point. 00:01:00.811 --> 00:01:04.856 align:center line:-1 position:50% size:64% The easiest way to understand probe bandwidth is by learning this theory. 00:01:04.856 --> 00:01:08.277 align:center line:-1 position:50% size:50% The bandwidth is defined by the point at which the frequency components 00:01:08.277 --> 00:01:12.948 align:center line:-1 position:50% size:50% passing through the probe are attenuated or decreased by 3 dB. 00:01:12.948 --> 00:01:19.454 align:center line:-1 position:50% size:58% In other words, the amplitude of your signal is reduced by 70.7% of what it should be. 00:01:19.454 --> 00:01:23.041 align:center line:-1 position:50% size:48% As an example, let's say we're working with a 20 MHz probe. 00:01:23.041 --> 00:01:27.629 align:center line:-1 position:50% size:56% To start out, I'm measuring a 1 MHz signal with 1 V peak-to-peak 00:01:27.629 --> 00:01:29.589 align:center line:-1 position:50% size:33% that's coming from my waveform generator. 00:01:29.589 --> 00:01:34.553 align:center line:-1 position:50% size:56% The peak-to-peak measurement you see on-screen is accurate at 1 V peak-to-peak 00:01:34.553 --> 00:01:38.015 align:center line:-1 position:50% size:57% because we're using a probe that has plenty of bandwidth for the signal. 00:01:38.015 --> 00:01:41.977 align:center line:-1 position:50% size:48% However, if I increase the frequency of the signal to 20 MHz, 00:01:41.977 --> 00:01:44.938 align:center line:-1 position:50% size:36% you can see we hit the 3 dB point of the probe 00:01:44.938 --> 00:01:51.153 align:center line:-1 position:50% size:59% which means we'll see a measurement error of -3 dB or -.3 V peak-to-peak. 00:01:51.153 --> 00:01:54.114 align:center line:-1 position:50% size:44% You can see in our peak-to-peak measurement here, 00:01:54.114 --> 00:01:59.328 align:center line:-1 position:50% size:53% our 1 V peak-to-peak is now reading off at .7 V peak-to-peak. 00:01:59.328 --> 00:02:03.665 align:center line:-1 position:50% size:65% You can fix this pitfall by calculating the correct bandwidth for your desired frequency. 00:02:03.665 --> 00:02:08.587 align:center line:-1 position:50% size:66% A common rule of thumb is your probe bandwidth should be at least three times the size 00:02:08.587 --> 00:02:10.714 align:center line:-1 position:50% size:29% of the sine wave you wish to measure. 00:02:10.714 --> 00:02:16.636 align:center line:-1 position:50% size:63% As a result, to measure this 20 MHz sine wave, we would want to probe with at least 60 MHz. 00:02:16.636 --> 00:02:19.681 align:center line:-1 position:50% size:43% Now, remember, this only works for perfect sine waves, 00:02:19.681 --> 00:02:23.060 align:center line:-1 position:50% size:43% but most analog measurements aren't perfect sine waves. 00:02:23.060 --> 00:02:26.772 align:center line:-1 position:50% size:50% If this is the case, the rise time theory is critical in determining 00:02:26.772 --> 00:02:28.440 align:center line:-1 position:50% size:43% the necessary probe bandwidth. 00:02:28.440 --> 00:02:31.443 align:center line:-1 position:50% size:46% To truly calculate the best probe bandwidth needed, 00:02:31.443 --> 00:02:33.570 align:center line:-1 position:50% size:41% you need to know the rise time of your signal. 00:02:33.570 --> 00:02:37.991 align:center line:-1 position:50% size:62% The rise time is the time it takes for your signal to get from the 10% level 00:02:37.991 --> 00:02:40.494 align:center line:-1 position:50% size:22% to the 90% level of a rising edge. 00:02:40.494 --> 00:02:46.875 align:center line:-1 position:50% size:66% This formula states that the bandwidth of a signal equals .34 divided by the rise time 00:02:46.875 --> 00:02:48.710 align:center line:-1 position:50% size:49% for a signal with Gaussian response, 00:02:48.710 --> 00:02:53.465 align:center line:-1 position:50% size:46% and, again, this is when evaluating rising edges from 10% to 90%. 00:02:53.465 --> 00:02:56.176 align:center line:-1 position:50% size:34% Many times, the rise time specification is listed 00:02:56.176 --> 00:02:59.179 align:center line:-1 position:50% size:41% along with the other bandwidth specs of your design. 00:02:59.179 --> 00:03:04.601 align:center line:-1 position:50% size:56% As an example, think about a clock signal with a frequency of 500 MHz. 00:03:04.601 --> 00:03:08.522 align:center line:-1 position:50% size:54% You may think, "All right, I just need a 1.5 GHz probe to measure that signal, 00:03:08.522 --> 00:03:13.527 align:center line:-1 position:50% size:58% according to what Erin said a minute ago," but your rise time could change everything. 00:03:13.527 --> 00:03:19.741 align:center line:-1 position:50% size:60% If your rise time for the 500 MHz clock signal is something like 350 ps, 00:03:19.741 --> 00:03:26.498 align:center line:-1 position:50% size:64% the calculated bandwidth of that signal based on the formula is actually equal to 1 GHz. 00:03:26.498 --> 00:03:31.795 align:center line:-1 position:50% size:50% This means that the real bandwidth of your signal is 1 GHz, not 500 MHz. 00:03:31.795 --> 00:03:35.841 align:center line:-1 position:50% size:44% 500 MHz is your clock frequency for an entire cycle, 00:03:35.841 --> 00:03:39.886 align:center line:-1 position:50% size:37% but your clock's rising edge is much faster at 1 GHz. 00:03:39.886 --> 00:03:45.183 align:center line:-1 position:50% size:65% This would mean that you would actually need a 3 GHz probe to measure that signal accurately. 00:03:45.183 --> 00:03:47.811 align:center line:-1 position:50% size:39% Avoid the pitfall of not having enough bandwidth 00:03:47.811 --> 00:03:50.856 align:center line:-1 position:50% size:37% by remembering to check your rise time specification. 00:03:50.856 --> 00:03:53.316 align:center line:-1 position:50% size:33% Another way to visualize the effects of bandwidth 00:03:53.316 --> 00:03:56.361 align:center line:-1 position:50% size:46% is by looking at the different harmonics of a signal. 00:03:56.361 --> 00:04:01.658 align:center line:-1 position:50% size:63% The yellow trace here shows the original signal and is made from the fundamental harmonics. 00:04:01.658 --> 00:04:04.786 align:center line:-1 position:50% size:42% The first harmonic can be seen as this green trace. 00:04:04.786 --> 00:04:08.665 align:center line:-1 position:50% size:49% You can see it has the same period and duty cycle as the original signal, 00:04:08.665 --> 00:04:12.586 align:center line:-1 position:50% size:47% but the rising edges are slower and the corners are more rounded. 00:04:12.586 --> 00:04:16.131 align:center line:-1 position:50% size:45% The first and third harmonic are combined in the purple signal 00:04:16.131 --> 00:04:20.093 align:center line:-1 position:50% size:50% and have a faster rising edge, and the corners are even more crisp, 00:04:20.093 --> 00:04:24.473 align:center line:-1 position:50% size:48% but the first, third, and fifth harmonic are combined in the pink trace 00:04:24.473 --> 00:04:28.393 align:center line:-1 position:50% size:50% and have faster edges, crisper corners, and even more detail 00:04:28.393 --> 00:04:30.979 align:center line:-1 position:50% size:30% on the top and bottom of the waveform. 00:04:30.979 --> 00:04:34.858 align:center line:-1 position:50% size:70% The higher the probe bandwidth, the more detailed your waveform is going to become 00:04:34.858 --> 00:04:37.319 align:center line:-1 position:50% size:39% because the more harmonics can be captured. 00:04:37.319 --> 00:04:40.822 align:center line:-1 position:50% size:56% Now, let's take a look at this harmonic knowledge a little bit further 00:04:40.822 --> 00:04:43.492 align:center line:-1 position:50% size:30% by looking at a 20 MHz clock wave. 00:04:43.492 --> 00:04:48.455 align:center line:-1 position:50% size:55% Here you can see the result of measuring a 20 MHz clock with a 20 MHz probe. 00:04:48.455 --> 00:04:52.918 align:center line:-1 position:50% size:51% Basically, we can't capture enough of the harmonics of the 20 MHz signal 00:04:52.918 --> 00:04:57.672 align:center line:-1 position:50% size:66% because we don't have enough probe bandwidth, and the result is going to look like a sine wave. 00:04:57.672 --> 00:05:00.675 align:center line:-1 position:50% size:53% Any measurements made on that signal will be inaccurate. 00:05:00.675 --> 00:05:04.179 align:center line:-1 position:50% size:41% If we measure the same signal using a 200 MHz probe, 00:05:04.179 --> 00:05:07.224 align:center line:-1 position:50% size:44% the result is a clean square wave that you can see here 00:05:07.224 --> 00:05:10.435 align:center line:-1 position:50% size:41% because the critical harmonics of the 20 MHz signal 00:05:10.435 --> 00:05:12.521 align:center line:-1 position:50% size:31% are able to be captured in the measurement. 00:05:12.521 --> 00:05:14.856 align:center line:-1 position:50% size:38% The last common pitfall when it comes to bandwidth 00:05:14.856 --> 00:05:18.151 align:center line:-1 position:50% size:39% is when engineers forget about the system bandwidth. 00:05:18.151 --> 00:05:21.780 align:center line:-1 position:50% size:51% Your probe combined with your scope creates the system, 00:05:21.780 --> 00:05:25.408 align:center line:-1 position:50% size:53% and the bandwidth of this whole system is lower than you may think. 00:05:25.408 --> 00:05:30.622 align:center line:-1 position:50% size:56% Let's say both your oscilloscope and your probe bandwidths are 500 MHz. 00:05:30.622 --> 00:05:35.794 align:center line:-1 position:50% size:56% Using this formula, the system bandwidth would be 353 MHz, 00:05:35.794 --> 00:05:39.005 align:center line:-1 position:50% size:42% so you can see that the system bandwidth is degraded greatly 00:05:39.005 --> 00:05:43.301 align:center line:-1 position:50% size:64% from the two individual bandwidth specifications of the probe and the scope, 00:05:43.301 --> 00:05:46.513 align:center line:-1 position:50% size:48% and this will just keep getting worse if you start using accessories, 00:05:46.513 --> 00:05:48.390 align:center line:-1 position:50% size:25% but I'll save that for a later episode. 00:05:48.390 --> 00:05:52.143 align:center line:-1 position:50% size:52% Obviously, system bandwidth is another important aspect to consider 00:05:52.143 --> 00:05:54.312 align:center line:-1 position:50% size:39% when you're selecting the correct probe bandwidth. 00:05:54.312 --> 00:05:56.565 align:center line:-1 position:50% size:34% Now that you understand these common pitfalls, 00:05:56.565 --> 00:06:00.318 align:center line:-1 position:50% size:57% you can understand why choosing a probe with adequate bandwidth is crucial 00:06:00.318 --> 00:06:02.404 align:center line:-1 position:50% size:48% for making accurate measurements. 00:06:02.404 --> 00:06:05.323 align:center line:-1 position:50% size:48% Having too little bandwidth means that you won't be able to see 00:06:05.323 --> 00:06:08.577 align:center line:-1 position:50% size:48% a true representation of your signal, making it difficult for you 00:06:08.577 --> 00:06:11.204 align:center line:-1 position:50% size:49% to make good engineering decisions while designing. 00:06:11.204 --> 00:06:14.040 align:center line:-1 position:50% size:39% To learn more about probing, click on the link on-screen 00:06:14.040 --> 00:06:18.169 align:center line:-1 position:50% size:68% and subscribe to our YouTube channel, and follow us on Facebook, Instagram, and Twitter. 00:06:21.214 --> 00:06:26.261 align:center line:-1 position:50% size:59% For anyone wondering about my cast, this is what happens when you steal probes.