WEBVTT 00:00:00.040 --> 00:00:02.470 The following content is provided under a Creative 00:00:02.470 --> 00:00:03.880 Commons license. 00:00:03.880 --> 00:00:06.920 Your support will help MIT OpenCourseWare continue to 00:00:06.920 --> 00:00:10.570 offer high quality educational resources for free. 00:00:10.570 --> 00:00:13.470 To make a donation or view additional materials from 00:00:13.470 --> 00:00:16.046 hundreds of MIT courses, visit MITOpenCourseWare at 00:00:16.046 --> 00:00:17.296 ocw.mit.edu. 00:01:04.540 --> 00:01:06.990 PROFESSOR: Well, today, we have a chance to put away the 00:01:06.990 --> 00:01:08.970 equations and have some fun. 00:01:08.970 --> 00:01:12.370 With me is Professor Sandy Hill from the University of 00:01:12.370 --> 00:01:13.970 Massachusetts at Amherst. 00:01:13.970 --> 00:01:17.870 And Sandy, maybe you could just give us a quick tour of 00:01:17.870 --> 00:01:18.860 what we have here. 00:01:18.860 --> 00:01:19.590 SANDY HILL: OK. 00:01:19.590 --> 00:01:22.070 We'll be dealing with about four instruments today. 00:01:22.070 --> 00:01:24.800 Two of them generate signals, and two of them display and 00:01:24.800 --> 00:01:26.400 analyze signals. 00:01:26.400 --> 00:01:30.180 The first, and perhaps simplest, is simply an RC 00:01:30.180 --> 00:01:34.440 audio generator, that will put out a variety wave shapes. 00:01:34.440 --> 00:01:36.510 And we'll look at that in a moment. 00:01:36.510 --> 00:01:39.340 Over to the right is a device that will allow us to study 00:01:39.340 --> 00:01:42.590 amplitude modulation and the various flavors of it. 00:01:42.590 --> 00:01:44.950 Down below is a standard oscilloscope to look at the 00:01:44.950 --> 00:01:47.850 signals in the time domain so we can see their shape. 00:01:47.850 --> 00:01:50.790 And then, finally the spectrum analyzer will analyze the 00:01:50.790 --> 00:01:52.750 signal into its Fourier components. 00:01:52.750 --> 00:01:55.250 And we'll display those, so you get to see the spectral 00:01:55.250 --> 00:01:57.650 content of a signal. 00:01:57.650 --> 00:02:00.490 PROFESSOR: Now, we'll be seeing all of these in a fair 00:02:00.490 --> 00:02:04.760 amount of detail as we go through the demonstrations. 00:02:04.760 --> 00:02:07.770 But maybe we can begin just with the signal generator. 00:02:07.770 --> 00:02:11.030 SANDY HILL: OK, let's look quickly at the 00:02:11.030 --> 00:02:12.390 various buttons involved. 00:02:12.390 --> 00:02:14.730 There's a lot of flexibility with this device. 00:02:14.730 --> 00:02:18.590 The top three buttons here allow us to change from one 00:02:18.590 --> 00:02:21.530 simple wave form to another one, a sine wave, a triangle 00:02:21.530 --> 00:02:23.330 wave, a square wave. 00:02:23.330 --> 00:02:26.490 And right below it is a button that will allow us to change 00:02:26.490 --> 00:02:28.390 the size of the signal. 00:02:28.390 --> 00:02:32.290 Moving over towards the left, we can change the DC offset 00:02:32.290 --> 00:02:36.790 which varies the signal, the level upon 00:02:36.790 --> 00:02:38.550 which the signal rides. 00:02:38.550 --> 00:02:40.390 PROFESSOR: And we'll be using that actually when we 00:02:40.390 --> 00:02:42.090 demonstrate amplitude modulation. 00:02:42.090 --> 00:02:44.540 SANDY HILL: That's right And then finally, there's a way of 00:02:44.540 --> 00:02:47.350 changing the frequency of the signal, either changing it by 00:02:47.350 --> 00:02:50.770 a factor of 10 or a vernier adjustment, if you want very 00:02:50.770 --> 00:02:53.850 specific adjustments, very delicate changes. 00:02:53.850 --> 00:02:54.190 PROFESSOR: OK. 00:02:54.190 --> 00:02:58.090 So maybe we can just vary some of this and see what a little 00:02:58.090 --> 00:02:59.040 bit of it looks like. 00:02:59.040 --> 00:02:59.870 SANDY HILL: All right. 00:02:59.870 --> 00:03:04.130 What we have displayed at this point is the effect of pushing 00:03:04.130 --> 00:03:05.300 the sine wave button. 00:03:05.300 --> 00:03:09.060 And so displayed down here is a 500 Hertz sine wave. 00:03:09.060 --> 00:03:10.240 In fact, let's listen to that. 00:03:10.240 --> 00:03:11.490 PROFESSOR: Great. 00:03:13.360 --> 00:03:16.790 SANDY HILL: And the scope is said to have one millisecond 00:03:16.790 --> 00:03:19.260 for each division here. 00:03:19.260 --> 00:03:21.900 By varying the amplitude knob can make the sine 00:03:21.900 --> 00:03:23.410 wave smaller or larger. 00:03:23.410 --> 00:03:26.010 The ear isn't too sensitive to that. 00:03:26.010 --> 00:03:28.400 By changing the DC offset, it simply rides 00:03:28.400 --> 00:03:29.515 on a different level. 00:03:29.515 --> 00:03:32.260 It's like putting a battery in series. 00:03:32.260 --> 00:03:36.360 And changing the frequency, you get a very perceptible 00:03:36.360 --> 00:03:38.230 change, a lower frequency. 00:03:38.230 --> 00:03:40.570 It takes longer to sweep through a period. 00:03:40.570 --> 00:03:44.256 Higher frequency, it takes a shorter amount of time. 00:03:44.256 --> 00:03:48.060 And also, we can change the wave shape itself from a sine 00:03:48.060 --> 00:03:51.890 wave to a triangle wave-- 00:03:51.890 --> 00:03:54.840 and you notice that has a richer sound to it-- 00:03:54.840 --> 00:03:58.210 and then finally to a square wave, which is an even 00:03:58.210 --> 00:04:00.510 brighter sound. 00:04:00.510 --> 00:04:04.640 PROFESSOR: And actually, as you realize from the previous 00:04:04.640 --> 00:04:07.980 lectures, with the triangle and square wave with the same 00:04:07.980 --> 00:04:11.740 fundamental as the sine wave, the richness comes in because 00:04:11.740 --> 00:04:17.880 of the higher harmonics in the Fourier series representation. 00:04:17.880 --> 00:04:20.790 OK, well let's go back to the sine wave. 00:04:20.790 --> 00:04:23.770 And we'll use that to take a closer look at 00:04:23.770 --> 00:04:25.190 the spectrum analyzer. 00:04:25.190 --> 00:04:25.720 SANDY HILL: OK. 00:04:25.720 --> 00:04:28.140 PROFESSOR: We'll look at a block diagram of the spectrum 00:04:28.140 --> 00:04:32.190 analyzer shortly, but first let's just look at a few quick 00:04:32.190 --> 00:04:34.360 things to get a feel for it. 00:04:34.360 --> 00:04:36.970 We have, of course, the time domain 00:04:36.970 --> 00:04:39.890 display of the sine wave. 00:04:39.890 --> 00:04:44.230 And the spectrum analyzer gives us a frequency domain 00:04:44.230 --> 00:04:46.450 analysis and display. 00:04:46.450 --> 00:04:52.540 And the vertical axis is the amplitude. 00:04:52.540 --> 00:04:55.070 The horizontal axis is frequency. 00:04:55.070 --> 00:05:00.150 And as it's set currently, the frequency axis goes from DC-- 00:05:00.150 --> 00:05:01.150 zero-- 00:05:01.150 --> 00:05:02.480 up to 2 kilohertz. 00:05:02.480 --> 00:05:04.620 And that, of course, can be varied on 00:05:04.620 --> 00:05:06.950 the spectrum analyzer. 00:05:06.950 --> 00:05:10.120 And since we have a sine wave input, we get, as we would 00:05:10.120 --> 00:05:13.820 expect, a line spectrum, corresponding to the frequency 00:05:13.820 --> 00:05:15.160 of the sine wave. 00:05:15.160 --> 00:05:18.970 Now, in fact, we can measure that frequency because the 00:05:18.970 --> 00:05:23.840 spectrum analyzer has a cursor associated with it, which I've 00:05:23.840 --> 00:05:25.600 just enabled. 00:05:25.600 --> 00:05:28.900 And this line is the cursor line. 00:05:28.900 --> 00:05:32.480 There is a read out for the frequency and a read out for 00:05:32.480 --> 00:05:34.500 the amplitude. 00:05:34.500 --> 00:05:37.390 And let's position the cursor on the 00:05:37.390 --> 00:05:40.320 frequency of the sine wave. 00:05:40.320 --> 00:05:44.520 And we're getting close. 00:05:44.520 --> 00:05:49.530 And there we are at the frequency of the sine wave. 00:05:49.530 --> 00:05:53.350 And we can read out, in fact, that it's 500 cycles. 00:05:53.350 --> 00:05:55.260 And this is the amplitude. 00:05:55.260 --> 00:05:58.910 And, of course, you could verify the frequency also in 00:05:58.910 --> 00:06:01.900 the time domain display, simply by measuring the 00:06:01.900 --> 00:06:05.690 frequency or the period. 00:06:05.690 --> 00:06:08.310 SANDY HILL: This is set at still one millisecond per 00:06:08.310 --> 00:06:08.940 centimeter. 00:06:08.940 --> 00:06:10.330 So that is 500 Hertz. 00:06:10.330 --> 00:06:11.640 PROFESSOR: OK. 00:06:11.640 --> 00:06:16.330 Well now maybe what we can do is vary the frequency. 00:06:16.330 --> 00:06:18.950 Let's vary the frequency of the sine wave generator. 00:06:18.950 --> 00:06:20.030 Maybe also listen to it? 00:06:20.030 --> 00:06:21.110 SANDY HILL: I'll turn on the tone. 00:06:21.110 --> 00:06:23.130 Yep. 00:06:23.130 --> 00:06:25.550 OK, I'll make the frequency higher. 00:06:25.550 --> 00:06:26.330 And you'll see a 00:06:26.330 --> 00:06:28.810 correspondence in the time domain. 00:06:28.810 --> 00:06:33.040 The frequency clearly goes up, and that line of the spectrum 00:06:33.040 --> 00:06:35.560 also migrates up away from the cursor. 00:06:35.560 --> 00:06:37.690 PROFESSOR: OK, and let me just point out again that this is 00:06:37.690 --> 00:06:38.610 the cursor line. 00:06:38.610 --> 00:06:40.680 This isn't the spectra line. 00:06:40.680 --> 00:06:46.560 And this is the frequency content of the sine wave. 00:06:46.560 --> 00:06:49.230 SANDY HILL: We can also change the amplitude of that line by 00:06:49.230 --> 00:06:53.240 making the sine wave itself smaller. 00:06:53.240 --> 00:06:55.880 And you notice the intensity of it goes-- 00:06:55.880 --> 00:06:57.420 PROFESSOR: While you do that, let me just position the 00:06:57.420 --> 00:07:01.790 cursor if I can on the sine wave. 00:07:01.790 --> 00:07:04.880 And now as you vary the amplitude, we should, in fact, 00:07:04.880 --> 00:07:08.685 see this vary. 00:07:08.685 --> 00:07:10.641 SANDY HILL: Well, that's right. 00:07:10.641 --> 00:07:11.130 PROFESSOR: OK. 00:07:11.130 --> 00:07:15.460 Now as Sandy varied the frequency, he did it slowly 00:07:15.460 --> 00:07:19.590 and we saw the line move as a single line. 00:07:19.590 --> 00:07:21.500 Maybe now, Sandy, you could vary the 00:07:21.500 --> 00:07:22.970 frequency more rapidly. 00:07:22.970 --> 00:07:29.583 And what will happen is that, in fact, what we'll get if I-- 00:07:29.583 --> 00:07:31.865 SANDY HILL: There'll be a spreading as we go-- 00:07:31.865 --> 00:07:33.430 PROFESSOR: Let me turn the cursor off. 00:07:33.430 --> 00:07:37.870 And what we'll see is a spreading of the line, so that 00:07:37.870 --> 00:07:39.840 the frequency content is richer. 00:07:39.840 --> 00:07:42.770 And, in fact, the analysis of this is considerably more 00:07:42.770 --> 00:07:43.560 complicated. 00:07:43.560 --> 00:07:48.050 It corresponds to frequency modulation, where Sandy is now 00:07:48.050 --> 00:07:49.175 the modulating signal. 00:07:49.175 --> 00:07:49.910 SANDY HILL: That's right. 00:07:49.910 --> 00:07:53.890 PROFESSOR: And we won't really be going into issues of 00:07:53.890 --> 00:07:57.870 frequency modulation in any more detail in this 00:07:57.870 --> 00:07:58.710 demonstration. 00:07:58.710 --> 00:08:00.670 SANDY HILL: Right. 00:08:00.670 --> 00:08:03.240 PROFESSOR: OK, well now let's go to the overhead projector 00:08:03.240 --> 00:08:05.460 and take a look at a block diagram of 00:08:05.460 --> 00:08:06.710 the spectrum analyzer. 00:08:10.040 --> 00:08:14.040 The specific spectrum analyzer that we're using in the 00:08:14.040 --> 00:08:18.420 demonstration is made by Rockland Systems, referred to 00:08:18.420 --> 00:08:22.320 as the Rockland Systems Model FFT 512. 00:08:22.320 --> 00:08:29.980 And basically, the idea is to sample the incoming wave form, 00:08:29.980 --> 00:08:33.919 convert that into digital form, and then the spectrum, 00:08:33.919 --> 00:08:38.929 in fact, is computed digitally using a microprocessor. 00:08:38.929 --> 00:08:45.330 So the overall system block diagram first consists of a 00:08:45.330 --> 00:08:48.280 system which is a low pass filter. 00:08:48.280 --> 00:08:54.100 And this low pass filter is used in advance of the 00:08:54.100 --> 00:08:59.580 sampling process to basically reduce the artifacts that are 00:08:59.580 --> 00:09:01.470 introduced due to sampling. 00:09:01.470 --> 00:09:05.490 And although we haven't talked yet about sampling and the 00:09:05.490 --> 00:09:09.200 associated artifacts, basically, as we'll see in the 00:09:09.200 --> 00:09:14.480 upcoming lectures, in order to sample a wave form and convert 00:09:14.480 --> 00:09:17.870 it into digital form, it requires that the wave form 00:09:17.870 --> 00:09:19.990 first be low pass filtered. 00:09:19.990 --> 00:09:24.440 So this low pass filter is referred to-- and will be in 00:09:24.440 --> 00:09:25.280 later lectures-- 00:09:25.280 --> 00:09:27.800 as an anti-aliasing filter. 00:09:27.800 --> 00:09:32.840 And this low pass filtered wave form is then converted to 00:09:32.840 --> 00:09:34.750 a sequence. 00:09:34.750 --> 00:09:39.730 And so a sequence is generated for which the sequence values 00:09:39.730 --> 00:09:46.590 are simply samples of the low pass filtered input. 00:09:46.590 --> 00:09:50.620 This low pass filtered input is then put 00:09:50.620 --> 00:09:52.340 into digital memory. 00:09:52.340 --> 00:09:56.730 Basically, a time block of it is put into digital memory, 00:09:56.730 --> 00:09:59.620 and so that's what we have down here. 00:09:59.620 --> 00:10:02.340 Here is the sequence. 00:10:02.340 --> 00:10:06.520 The sequence is put into digital memory. 00:10:06.520 --> 00:10:11.470 And then an arithmetic processor computes for the 00:10:11.470 --> 00:10:13.770 samples in this memory. 00:10:13.770 --> 00:10:18.430 It computes the Fourier transform or the spectrum. 00:10:18.430 --> 00:10:22.810 And after the spectrum is computed and put either into 00:10:22.810 --> 00:10:26.860 the same or a different memory, that is then put 00:10:26.860 --> 00:10:31.560 through a conversion process back to a continuous time 00:10:31.560 --> 00:10:37.210 signal, and finally, put out on the display that we've been 00:10:37.210 --> 00:10:39.000 seeing in the demonstration. 00:10:39.000 --> 00:10:43.190 And so this is just indicative of the display. 00:10:43.190 --> 00:10:46.900 So basically, the idea then is that the input 00:10:46.900 --> 00:10:49.410 wave form comes in. 00:10:49.410 --> 00:10:53.950 It's filtered and sampled and captured on a block basis, put 00:10:53.950 --> 00:10:58.480 into a digital memory, and then, a digital computer or 00:10:58.480 --> 00:11:02.250 microprocessor computes the Fourier transform. 00:11:02.250 --> 00:11:05.530 And then that Fourier transform is what we see on 00:11:05.530 --> 00:11:06.930 the display. 00:11:06.930 --> 00:11:10.840 So what we're computing, of course, are samples of the 00:11:10.840 --> 00:11:12.830 Fourier transform. 00:11:12.830 --> 00:11:16.040 And so, for example, if the input-- let's say-- 00:11:16.040 --> 00:11:20.680 was a rectangular pulse whose Fourier transform is of the 00:11:20.680 --> 00:11:24.030 form of a sine(x) over x function, what we would, in 00:11:24.030 --> 00:11:28.700 fact, see on the display are samples of that at discrete 00:11:28.700 --> 00:11:29.900 frequencies. 00:11:29.900 --> 00:11:36.140 Or if as we have an input which is a square wave, what 00:11:36.140 --> 00:11:40.560 will generate through the spectrum analyzer are the 00:11:40.560 --> 00:11:45.760 Fourier series coefficients or equivalently, the harmonics 00:11:45.760 --> 00:11:48.720 associated with the square wave. 00:11:48.720 --> 00:11:53.950 OK, well, let's now go back to the equipment and look at the 00:11:53.950 --> 00:11:55.200 spectrum analyzer. 00:11:57.700 --> 00:12:01.640 We'll look at the square wave through the spectrum analyzer 00:12:01.640 --> 00:12:06.040 shortly, but first what we have is what we saw before, 00:12:06.040 --> 00:12:07.920 which is the sine wave. 00:12:07.920 --> 00:12:12.450 And just to point out, again, the fact that the sine wave 00:12:12.450 --> 00:12:16.550 spectrum, of course, is just a single line corresponding to 00:12:16.550 --> 00:12:19.080 the fundamental frequency. 00:12:19.080 --> 00:12:24.070 And here we have a frequency scale now that goes from 0 to 00:12:24.070 --> 00:12:25.640 5 kilohertz. 00:12:25.640 --> 00:12:29.680 And this is then the 500 cycle sine wave. 00:12:29.680 --> 00:12:35.240 And, in fact, we can flip the cursor on and I happen to just 00:12:35.240 --> 00:12:37.110 magically have it positioned correctly. 00:12:37.110 --> 00:12:41.530 And we see that it's a 500 cycle sine wave. 00:12:41.530 --> 00:12:43.030 SANDY HILL: Might be interesting, Al, to go and 00:12:43.030 --> 00:12:45.770 look at a richer set of signals, such as the triangle 00:12:45.770 --> 00:12:48.730 wave and the square wave-- things that are conveniently 00:12:48.730 --> 00:12:50.650 on the signal generator itself. 00:12:50.650 --> 00:12:54.000 I'll switch over to a square wave now. 00:12:54.000 --> 00:12:57.340 And what you see is all the harmonics 00:12:57.340 --> 00:13:00.064 coming up, being displayed. 00:13:00.064 --> 00:13:02.460 PROFESSOR: OK, it's actually interesting to point out, I 00:13:02.460 --> 00:13:07.460 think, that the square wave, as we know, is an 00:13:07.460 --> 00:13:08.940 odd harmonic function. 00:13:08.940 --> 00:13:14.470 And so, in fact, the even harmonics are missing in the 00:13:14.470 --> 00:13:15.220 square wave. 00:13:15.220 --> 00:13:17.030 So this is the fundamental. 00:13:17.030 --> 00:13:20.320 This is the third harmonic, fifth harmonic, et cetera. 00:13:20.320 --> 00:13:24.940 And then the amplitude of the square wave decays 00:13:24.940 --> 00:13:27.870 proportional to 1 over f, which is the kind of analysis 00:13:27.870 --> 00:13:30.000 that we've gone through in looking at Fourier series. 00:13:30.000 --> 00:13:33.150 SANDY HILL: Right, and if we switch to the triangle wave, 00:13:33.150 --> 00:13:36.730 instead of their decaying as 1 over f, the harmonics decay as 00:13:36.730 --> 00:13:37.800 1 over f squared. 00:13:37.800 --> 00:13:39.980 And you see they drop off much more quickly. 00:13:39.980 --> 00:13:41.310 PROFESSOR: And again, of course, it's an 00:13:41.310 --> 00:13:43.020 odd harmonic signal. 00:13:43.020 --> 00:13:47.730 And so the even numbered harmonics are missing. 00:13:47.730 --> 00:13:52.450 And maybe just to kind of emphasize the point, we can 00:13:52.450 --> 00:13:55.720 show the sine wave again and the square wave again. 00:13:55.720 --> 00:13:59.360 SANDY HILL: We'll go through from the most bland to the 00:13:59.360 --> 00:14:01.510 richer to the richest. 00:14:01.510 --> 00:14:02.750 PROFESSOR: OK, and it's really kind of 00:14:02.750 --> 00:14:03.750 interesting and dramatic-- 00:14:03.750 --> 00:14:04.650 SANDY HILL: It's fun to do that. 00:14:04.650 --> 00:14:06.130 PROFESSOR: --to see the harmonics pop in. 00:14:06.130 --> 00:14:07.900 SANDY HILL: While we're at the square wave, let me fiddle 00:14:07.900 --> 00:14:10.100 with the frequency of the square wave, and we can see 00:14:10.100 --> 00:14:13.970 the duality of the time and frequency domains. 00:14:13.970 --> 00:14:16.480 That is, as you compress things in the time domain, 00:14:16.480 --> 00:14:19.920 such as this, going to a higher frequency square wave, 00:14:19.920 --> 00:14:22.620 the harmonics wonder further away from each other. 00:14:22.620 --> 00:14:25.950 SANDY HILL: So this is the fundamental. 00:14:25.950 --> 00:14:28.810 The second harmonic is missing. 00:14:28.810 --> 00:14:30.290 This is the third harmonic. 00:14:30.290 --> 00:14:31.730 And of course, the fifth. 00:14:31.730 --> 00:14:34.290 SANDY HILL: And as we take this to an extreme by going to 00:14:34.290 --> 00:14:37.500 very low repetition rate square waves, all those 00:14:37.500 --> 00:14:43.370 harmonics come scurrying in and cluster together near DC. 00:14:43.370 --> 00:14:45.265 PROFESSOR: Kind of fun with Fourier transforms. 00:14:48.010 --> 00:14:53.170 Well, speaking of time and frequency scaling, recall that 00:14:53.170 --> 00:14:57.280 we had demonstrated time and frequency scaling previously 00:14:57.280 --> 00:14:59.880 with the glockenspiel. 00:14:59.880 --> 00:15:03.840 And what we had done there was to record a particular 00:15:03.840 --> 00:15:08.110 glockenspiel note, and then we played that 00:15:08.110 --> 00:15:10.870 back at half speed. 00:15:10.870 --> 00:15:14.880 And what we had done in that case is expanded things in 00:15:14.880 --> 00:15:17.740 time, consequently compress them in frequency. 00:15:17.740 --> 00:15:24.800 And so, comparing that with a note an octave lower than we 00:15:24.800 --> 00:15:27.340 saw that, in fact, the time scaling had led 00:15:27.340 --> 00:15:29.010 to a frequency scaling. 00:15:29.010 --> 00:15:33.330 And then we also played the same note back at twice speed. 00:15:33.330 --> 00:15:36.120 And in that case, the frequencies were all scaled up 00:15:36.120 --> 00:15:37.010 by a factor of 2. 00:15:37.010 --> 00:15:40.800 And again, we illustrated that by comparing with the 00:15:40.800 --> 00:15:43.660 glockenspiel note a full octave up. 00:15:43.660 --> 00:15:46.890 Now when we did that, we didn't actually look at the 00:15:46.890 --> 00:15:49.600 time wave forms or spectra. 00:15:49.600 --> 00:15:52.230 And having the equipment that we have here gives us kind of 00:15:52.230 --> 00:15:54.420 a nice opportunity to do that. 00:15:54.420 --> 00:15:59.110 So what I have is the tape that we had originally made of 00:15:59.110 --> 00:16:01.810 the glockenspiel, the original note that we recorded. 00:16:01.810 --> 00:16:05.670 And what we'll do is look at the spectrum of that, and then 00:16:05.670 --> 00:16:10.560 compare that spectrum when we play the tape at half speed 00:16:10.560 --> 00:16:13.540 and also play it at twice speed. 00:16:13.540 --> 00:16:16.240 So let's play the tape. 00:16:16.240 --> 00:16:22.160 And what we have is the glockenspiel right now 00:16:22.160 --> 00:16:26.540 displayed on a frequency scale from 0 to 5 kilohertz. 00:16:26.540 --> 00:16:31.210 Let's just change that to zero to 10 kilohertz. 00:16:31.210 --> 00:16:33.620 So here is this spectrum. 00:16:33.620 --> 00:16:36.950 Over here we have the time wave form. 00:16:36.950 --> 00:16:40.740 And here is then the first spectral line, and we can see 00:16:40.740 --> 00:16:44.710 where that is by setting up the cursor. 00:16:44.710 --> 00:16:48.290 And magically, once again, I have the cursor positioned at 00:16:48.290 --> 00:16:49.650 just the right spot. 00:16:49.650 --> 00:16:55.120 The first spectral line is at 1.775 kilohertz. 00:16:55.120 --> 00:17:00.590 And so this is the spectrum then of the original 00:17:00.590 --> 00:17:02.881 glockenspiel note. 00:17:02.881 --> 00:17:03.320 All right. 00:17:03.320 --> 00:17:07.890 Let's stop the tape and rewind it. 00:17:07.890 --> 00:17:13.660 And now what we want to do is play that back at half speed. 00:17:13.660 --> 00:17:16.849 Played at half speed, the frequencies 00:17:16.849 --> 00:17:18.599 should be scaled down. 00:17:18.599 --> 00:17:22.560 And, in particular then the first spectra line should be 00:17:22.560 --> 00:17:24.760 at a lower frequency. 00:17:24.760 --> 00:17:27.730 So let's play that now. 00:17:27.730 --> 00:17:29.320 SANDY HILL: There are really very complicated signals, 00:17:29.320 --> 00:17:30.200 aren't they? 00:17:30.200 --> 00:17:32.760 PROFESSOR: They really are. 00:17:32.760 --> 00:17:37.580 Here we have the first spectral line. 00:17:37.580 --> 00:17:41.690 And we can compare that with the first spectral line that 00:17:41.690 --> 00:17:46.570 we had before which was at 1.775 kilohertz. 00:17:46.570 --> 00:17:51.030 And, once again, you see that the time wave form over here 00:17:51.030 --> 00:17:55.800 has been scaled by a factor of 2. 00:17:55.800 --> 00:17:59.210 So, once again, we see that time and frequency scaling 00:17:59.210 --> 00:18:00.820 really works. 00:18:00.820 --> 00:18:05.340 Incidentally as Sandy pointed out, and rightfully so, the 00:18:05.340 --> 00:18:10.200 glockenspiel really is a pretty complicated signal, as 00:18:10.200 --> 00:18:13.870 a graduate student and I found out when we were preparing the 00:18:13.870 --> 00:18:16.120 original glockenspiel demo. 00:18:16.120 --> 00:18:18.380 SANDY HILL: Speaking of complicated signals, one of my 00:18:18.380 --> 00:18:20.760 favorites is to look at speech. 00:18:20.760 --> 00:18:23.810 I set this up so that what's coming into my microphone is 00:18:23.810 --> 00:18:26.780 indeed what you're going to see on the two screens. 00:18:26.780 --> 00:18:29.660 The telephone company thinks of speech, basically in terms 00:18:29.660 --> 00:18:32.100 of bandwidth, that it extends from about 300 00:18:32.100 --> 00:18:33.930 Hertz to 3,300 Hertz. 00:18:33.930 --> 00:18:35.880 But, as we'll see in the spectrum analyzer, there's a 00:18:35.880 --> 00:18:37.720 lot of leakage outside of that. 00:18:37.720 --> 00:18:40.860 The telephone company just filters out everything outside 00:18:40.860 --> 00:18:43.130 of that and things of that as a speech signal. 00:18:43.130 --> 00:18:46.220 So it doesn't have the high fidelity that you might have 00:18:46.220 --> 00:18:47.630 on high fire equipment. 00:18:47.630 --> 00:18:51.800 As we look at the scope, again, the time wave form is 00:18:51.800 --> 00:18:55.460 extremely complicated, seems to have some periodicities in 00:18:55.460 --> 00:18:57.670 it, although they're short-lived, and then it goes 00:18:57.670 --> 00:19:00.120 on to some other periodic chunk. 00:19:00.120 --> 00:19:02.950 And over in the frequency domain, you can see as we 00:19:02.950 --> 00:19:08.020 extend from 0 to 10 kilohertz, that as I speak, there are 00:19:08.020 --> 00:19:11.750 trenchants that have spectral content in them, covering that 00:19:11.750 --> 00:19:12.990 entire band. 00:19:12.990 --> 00:19:14.460 I can try some simpler signals. 00:19:14.460 --> 00:19:17.550 A whistle is almost a sinusoid, but as you'll see 00:19:17.550 --> 00:19:19.170 isn't terribly sinusoidal. 00:19:19.170 --> 00:19:22.920 [HIGH-PITCHED WHISTLE] 00:19:22.920 --> 00:19:24.310 That's the best I can do. 00:19:24.310 --> 00:19:28.176 I can sing, a little bit embarrassedly, a B, 00:19:28.176 --> 00:19:28.670 [HIGH-PITCHED] 00:19:28.670 --> 00:19:30.220 Boo, and things like that. 00:19:30.220 --> 00:19:32.720 PROFESSOR: They don't ask me to, Sandy. 00:19:32.720 --> 00:19:35.440 SANDY HILL: And then different vowel sounds have a lot of 00:19:35.440 --> 00:19:40.280 energy in, like the letter A. (SUNG) A. Whereas some of the 00:19:40.280 --> 00:19:42.800 others are very impulsive, like-- 00:19:42.800 --> 00:19:43.210 [HISSING] 00:19:43.210 --> 00:19:45.540 Or puh and tuh. 00:19:45.540 --> 00:19:48.160 And it's a little hard to grab them at the right time. 00:19:48.160 --> 00:19:51.390 But what's fascinating is just to stare at equipment like 00:19:51.390 --> 00:19:54.410 this and try different speech sounds, and you begin to get 00:19:54.410 --> 00:19:58.300 sort of a sense of the complicated nature of them. 00:19:58.300 --> 00:20:02.040 PROFESSOR: OK, well that's a look at some spectra signals. 00:20:02.040 --> 00:20:08.780 And now what we'd like to focus on is the modulator and 00:20:08.780 --> 00:20:12.215 talk a little bit about modulation and demodulation 00:20:12.215 --> 00:20:13.800 and demonstrate it. 00:20:13.800 --> 00:20:18.510 And let's begin that by first taking a look at a block 00:20:18.510 --> 00:20:20.940 diagram of the modulator system. 00:20:24.170 --> 00:20:27.260 Well, as we've discussed in a previous lecture, amplitude 00:20:27.260 --> 00:20:32.790 modulation basically consists of multiplying the modulating 00:20:32.790 --> 00:20:36.900 signal by an appropriate carrier, illustrated here, as 00:20:36.900 --> 00:20:41.870 we've seen previously, for the case of a sinusoidal carrier. 00:20:41.870 --> 00:20:45.930 And then, specifically for sinusoidal amplitude 00:20:45.930 --> 00:20:51.580 modulation, we may or may not inject some carrier signal-- 00:20:51.580 --> 00:20:53.660 A times the carrier. 00:20:53.660 --> 00:20:59.610 Or equivalently, if we look at the modulated output, the 00:20:59.610 --> 00:21:03.800 injection of the carrier is equivalent, mathematically, to 00:21:03.800 --> 00:21:07.310 simply adding a DC offset or a constant to 00:21:07.310 --> 00:21:09.250 the modulating signal. 00:21:09.250 --> 00:21:14.490 And, as you recall when we talked about this, the idea of 00:21:14.490 --> 00:21:19.310 injecting a carrier or not is related to the issue of 00:21:19.310 --> 00:21:22.860 whether or not we want to do synchronous or asynchronous 00:21:22.860 --> 00:21:24.290 demodulation. 00:21:24.290 --> 00:21:27.540 The asynchronous demodulation corresponding to the simple 00:21:27.540 --> 00:21:30.050 use of an envelope detector. 00:21:30.050 --> 00:21:34.250 And to remind you of the wave forms that are involved, 00:21:34.250 --> 00:21:38.230 again, I show two that we saw previously. 00:21:38.230 --> 00:21:44.360 And for the case, this is for one value of the amount of 00:21:44.360 --> 00:21:45.720 carrier that's injected. 00:21:45.720 --> 00:21:50.170 And this is for an amount of carrier injected that's less. 00:21:50.170 --> 00:21:54.560 And this, in fact, corresponds to what we refer to as 50% 00:21:54.560 --> 00:22:00.020 modulation, and this is the case of 100% modulation. 00:22:00.020 --> 00:22:04.810 Well, the modulating system that we're using in this 00:22:04.810 --> 00:22:08.630 demonstration is basically of the form that 00:22:08.630 --> 00:22:10.160 we're indicating here. 00:22:10.160 --> 00:22:16.020 And a simple block diagram for it is more or less identical 00:22:16.020 --> 00:22:17.820 to what we just saw. 00:22:17.820 --> 00:22:22.400 Specifically, the modulating signal is 00:22:22.400 --> 00:22:25.010 multiplied by the carrier. 00:22:25.010 --> 00:22:29.300 And there also is the capability of injecting some 00:22:29.300 --> 00:22:33.420 additional carrier, meaning adding it to the output of 00:22:33.420 --> 00:22:34.430 this product. 00:22:34.430 --> 00:22:38.900 And so the modulated output can have a variable percent 00:22:38.900 --> 00:22:39.770 modulation-- 00:22:39.770 --> 00:22:44.000 the percent modulation being changed, depending on how we 00:22:44.000 --> 00:22:46.690 set this variable gain. 00:22:46.690 --> 00:22:50.450 Now, in addition to sinusoidal modulation-- 00:22:50.450 --> 00:22:53.980 in fact, for the particular system that we're using, we 00:22:53.980 --> 00:22:56.350 have somewhat more flexibility. 00:22:56.350 --> 00:22:59.340 We can use, in addition to a sinusoidal carrier at this 00:22:59.340 --> 00:23:04.640 point, we can alternatively choose a square wave carrier 00:23:04.640 --> 00:23:07.540 or a triangular carrier. 00:23:07.540 --> 00:23:10.600 And, as we'll indicate in a moment when we illustrate 00:23:10.600 --> 00:23:14.280 this, there are some specific advantages to using, for 00:23:14.280 --> 00:23:17.300 example, a square wave carrier. 00:23:17.300 --> 00:23:21.890 So this is a somewhat simplified version of-- 00:23:21.890 --> 00:23:26.730 or rather block diagram of the modulating system that we're 00:23:26.730 --> 00:23:27.950 demonstrating. 00:23:27.950 --> 00:23:32.010 The external modulating input here, a choice of carrier with 00:23:32.010 --> 00:23:35.100 also the capability for injecting some additional 00:23:35.100 --> 00:23:37.350 carrier into the output. 00:23:37.350 --> 00:23:39.490 OK, now let's go back to the equipment and 00:23:39.490 --> 00:23:42.100 take a look at this. 00:23:42.100 --> 00:23:45.290 Well, Sandy, maybe to begin, you can just point out what 00:23:45.290 --> 00:23:47.800 some of the controls are on the modulator box. 00:23:47.800 --> 00:23:50.180 SANDY HILL: OK, there are some interesting 00:23:50.180 --> 00:23:52.230 points to look at here. 00:23:52.230 --> 00:23:54.860 This is an input for an external signal. 00:23:54.860 --> 00:23:57.330 We'll be taking a signal right out of the signal generator 00:23:57.330 --> 00:23:58.670 and putting it in here. 00:23:58.670 --> 00:24:01.960 And that will be the signal that will be modulated, 00:24:01.960 --> 00:24:03.560 according to the carrier. 00:24:03.560 --> 00:24:06.450 The carrier is generated internally in this device. 00:24:06.450 --> 00:24:08.870 And there are several ways of controlling it. 00:24:08.870 --> 00:24:12.170 One is the amount of carrier injection. 00:24:12.170 --> 00:24:16.410 One is the wave form of the carrier itself-- and this is 00:24:16.410 --> 00:24:19.530 typically sinusoidal in the broadcast industry, but others 00:24:19.530 --> 00:24:21.710 are interesting to look at as well. 00:24:21.710 --> 00:24:24.110 The output that will be displaying on both the 00:24:24.110 --> 00:24:27.310 spectrum analyzer and scope comes out here. 00:24:27.310 --> 00:24:30.580 And then farther over to the left, there are some knobs for 00:24:30.580 --> 00:24:33.440 changing the frequency of the carrier signal. 00:24:33.440 --> 00:24:37.340 PROFESSOR: OK, let me also just point out again for 00:24:37.340 --> 00:24:40.780 emphasis that changing the carrier level, as we've talked 00:24:40.780 --> 00:24:43.800 about, is mathematically equivalent to changing the DC 00:24:43.800 --> 00:24:45.990 level of the modulating signal. 00:24:45.990 --> 00:24:49.545 And that's also what affects the percent modulation as we 00:24:49.545 --> 00:24:51.320 had just discussed. 00:24:51.320 --> 00:24:56.930 Now what we have set up is a sinusoidal modulating signal 00:24:56.930 --> 00:24:59.050 and a sinusoidal little carrier. 00:24:59.050 --> 00:25:04.120 And, as usual, we have the time wave form displayed here. 00:25:04.120 --> 00:25:06.840 And so this is the modulated signal. 00:25:06.840 --> 00:25:10.070 And then on the spectrum analyzer, we have 00:25:10.070 --> 00:25:11.870 the spectral display. 00:25:11.870 --> 00:25:14.750 And this is the carrier signal. 00:25:14.750 --> 00:25:16.620 That's the carrier frequency. 00:25:16.620 --> 00:25:21.070 And these side bands then correspond to the side bands 00:25:21.070 --> 00:25:23.270 associated with the modulating signal. 00:25:23.270 --> 00:25:27.730 So this is the spectrum of the total modulated output, right? 00:25:27.730 --> 00:25:28.480 SANDY HILL: Right. 00:25:28.480 --> 00:25:30.990 It's interesting to vary some of these parameters. 00:25:30.990 --> 00:25:33.560 You can see the sinusoidal modulating 00:25:33.560 --> 00:25:34.970 shape at this point. 00:25:34.970 --> 00:25:39.010 Let me switch that to a triangular shape. 00:25:39.010 --> 00:25:42.500 And, again, it's a little hard to sink in both the modulating 00:25:42.500 --> 00:25:45.150 signal and the carrier, so you see the carrier kind of 00:25:45.150 --> 00:25:47.630 wondering by, but there it is. 00:25:47.630 --> 00:25:49.950 Another thing that can be varied is the amplitude of the 00:25:49.950 --> 00:25:50.950 modulating signal. 00:25:50.950 --> 00:25:52.280 I'll make it smaller-- 00:25:52.280 --> 00:25:54.960 you'll see the side bands go away-- 00:25:54.960 --> 00:25:58.070 until finally we have a pure sinusoidal carrier-- 00:25:58.070 --> 00:26:00.040 as I bring them back in. 00:26:00.040 --> 00:26:01.440 Again, this is triangular. 00:26:01.440 --> 00:26:04.160 There would be harmonics there that maybe they're a little 00:26:04.160 --> 00:26:06.560 hard to see in the spectrum analyzer. 00:26:06.560 --> 00:26:10.570 I'll go back first to a square wave modulating signal, which 00:26:10.570 --> 00:26:14.630 is, again, you can see the square wave on top and one on 00:26:14.630 --> 00:26:15.740 the bottom. 00:26:15.740 --> 00:26:18.480 PROFESSOR: And so all of this, then, represents side bands. 00:26:18.480 --> 00:26:19.050 Is that-- 00:26:19.050 --> 00:26:19.330 SANDY HILL: That's right. 00:26:19.330 --> 00:26:23.450 Those are the side bands due to the square wave modulation. 00:26:23.450 --> 00:26:26.130 Going back to the simplest, the sinusoidal carrier. 00:26:26.130 --> 00:26:30.330 Another thing we can do is to vary the frequency of the 00:26:30.330 --> 00:26:32.790 sinusoidal signal that's modulating it. 00:26:32.790 --> 00:26:34.890 I'll make it higher. 00:26:34.890 --> 00:26:37.650 And you'll notice that the side bands wander away from 00:26:37.650 --> 00:26:40.230 the carrier signal in the spectrum. 00:26:40.230 --> 00:26:43.130 The spectrum, the carrier signal of that, doesn't have a 00:26:43.130 --> 00:26:46.040 frequency that's changing as I do this. 00:26:46.040 --> 00:26:48.900 It's just the width of the band, due to 00:26:48.900 --> 00:26:49.990 the modulating signal. 00:26:49.990 --> 00:26:51.350 PROFESSOR: So this is the carrier. 00:26:51.350 --> 00:26:52.740 And these are the side bands. 00:26:52.740 --> 00:26:53.280 SANDY HILL: That's right. 00:26:53.280 --> 00:26:56.570 Now as we go out very far, those side bands get gobbled 00:26:56.570 --> 00:26:58.215 up by the carrier itself. 00:26:58.215 --> 00:27:01.510 I'll come back to a nice reasonable point there. 00:27:01.510 --> 00:27:09.060 PROFESSOR: OK, now, we can change also the carrier 00:27:09.060 --> 00:27:13.790 frequency and, as Sandy's indicated, various parameters. 00:27:13.790 --> 00:27:17.690 Now let me just point out, since I didn't previously, 00:27:17.690 --> 00:27:21.380 that the frequency scale that we're looking at here is a 00:27:21.380 --> 00:27:24.790 frequency scale out to 20 kilohertz. 00:27:24.790 --> 00:27:28.770 And we had talked about-- or Sandy had indicated-- 00:27:28.770 --> 00:27:33.210 that we can change the carrier signal shape. 00:27:33.210 --> 00:27:38.550 And let me change the carrier signal from a sine wave to a 00:27:38.550 --> 00:27:39.970 square wave. 00:27:39.970 --> 00:27:43.190 Now you haven't seen any change on this particular 00:27:43.190 --> 00:27:47.680 display , but let me change the frequency scale. 00:27:47.680 --> 00:27:54.060 And what you see now is the modulating signal showing up 00:27:54.060 --> 00:27:58.090 around harmonics associated with the carrier. 00:27:58.090 --> 00:28:03.100 And those harmonics will go away when we go back to a 00:28:03.100 --> 00:28:04.630 sinusoidal carrier. 00:28:04.630 --> 00:28:06.300 SANDY HILL: This is actually called a ring modulator. 00:28:06.300 --> 00:28:09.600 It's very simple to multiply a signal by a square wave. 00:28:09.600 --> 00:28:11.220 It's just a chopping process. 00:28:11.220 --> 00:28:13.770 And so a square wave carrier signal is very 00:28:13.770 --> 00:28:15.380 convenient to generate. 00:28:15.380 --> 00:28:19.676 And then you simply filter out the higher order harmonics. 00:28:19.676 --> 00:28:21.660 PROFESSOR: Right, I'll go back to the 00:28:21.660 --> 00:28:23.850 scale that we had before. 00:28:23.850 --> 00:28:28.160 And Sandy had also commented that we can 00:28:28.160 --> 00:28:31.190 change the carrier level. 00:28:31.190 --> 00:28:33.870 And let's do that on the modulator box. 00:28:33.870 --> 00:28:38.140 We could do that either by changing a DC offset on the 00:28:38.140 --> 00:28:40.700 modulating signal or by changing the amount of carrier 00:28:40.700 --> 00:28:41.950 that's injected. 00:28:41.950 --> 00:28:46.170 And as we decrease the amount of carrier, in fact, going 00:28:46.170 --> 00:28:49.550 down to no carrier at all or almost no carrier. 00:28:49.550 --> 00:28:51.280 That's suppressed carrier. 00:28:51.280 --> 00:28:55.340 We have only the two side bands and the signal is now 00:28:55.340 --> 00:28:58.320 highly over-modulated. 00:28:58.320 --> 00:29:05.390 We bring the carrier back up and when we do that, then we 00:29:05.390 --> 00:29:11.350 are reducing the percent modulation and simultaneously 00:29:11.350 --> 00:29:15.420 obviously related to that is changing the carrier level. 00:29:15.420 --> 00:29:21.230 Now I had indicated in previous lectures that 00:29:21.230 --> 00:29:24.270 reducing the carrier is efficient in terms of power 00:29:24.270 --> 00:29:30.250 transmission, but requires a synchronous demodulator, 00:29:30.250 --> 00:29:34.730 whereas if there's carrier injected, as we have here, so 00:29:34.730 --> 00:29:36.200 that we're not over-modulating-- 00:29:36.200 --> 00:29:38.700 the percent modulation is less than 100-- 00:29:38.700 --> 00:29:42.180 then because of the shape of the time wave form, as you can 00:29:42.180 --> 00:29:47.480 see here, you can do the demodulation with a more or 00:29:47.480 --> 00:29:50.520 less simple envelope detector. 00:29:50.520 --> 00:29:54.500 And in fact, a envelope detector of the type that 00:29:54.500 --> 00:29:57.720 we've talked about before is exactly what is used in AM 00:29:57.720 --> 00:30:01.560 radios, because of the fact that it's so inexpensive. 00:30:01.560 --> 00:30:05.650 And so here the power transmitted required is 00:30:05.650 --> 00:30:09.800 higher, but the demodulator, based on using an envelope 00:30:09.800 --> 00:30:12.280 detector, is considerably simpler. 00:30:15.150 --> 00:30:18.900 Well, speaking of AM radios, what we'd like to now 00:30:18.900 --> 00:30:24.030 demonstrate is modulation and demodulation with an AM radio, 00:30:24.030 --> 00:30:27.710 which Sandy happens to have here. 00:30:27.710 --> 00:30:31.670 And let's see, I guess, what you're going to do, Sandy, is 00:30:31.670 --> 00:30:32.815 take it apart for us, right? 00:30:32.815 --> 00:30:33.070 SANDY HILL: Right. 00:30:33.070 --> 00:30:34.930 I've taken off the back. 00:30:34.930 --> 00:30:39.260 And I'm just now going to slip out the guts of it all, and 00:30:39.260 --> 00:30:42.940 try and set it up so you can see it conveniently, such as 00:30:42.940 --> 00:30:44.846 right there. 00:30:44.846 --> 00:30:47.140 PROFESSOR: Now, this is your daughter's radio that you 00:30:47.140 --> 00:30:47.750 promised to give back.? 00:30:47.750 --> 00:30:50.480 SANDY HILL: I would say was my daughter's radio. 00:30:50.480 --> 00:30:53.250 And now let's start attaching clip leads, because what we're 00:30:53.250 --> 00:30:56.480 going to want to do is to be able to see the signals coming 00:30:56.480 --> 00:30:57.580 out of the radio. 00:30:57.580 --> 00:31:00.890 We want to see both the audio and then the modulated signal 00:31:00.890 --> 00:31:02.800 and try and see them on the oscilloscope. 00:31:02.800 --> 00:31:05.005 So I'll start setting that up now. 00:31:05.005 --> 00:31:05.700 PROFESSOR: OK. 00:31:05.700 --> 00:31:06.760 Let me-- 00:31:06.760 --> 00:31:11.220 while you're doing that-- also just comment that the kind of 00:31:11.220 --> 00:31:13.790 AM radio that we're looking at here is called a 00:31:13.790 --> 00:31:15.350 superheterodyne receiver. 00:31:15.350 --> 00:31:19.380 And the way that it does the demodulation is not exactly 00:31:19.380 --> 00:31:20.950 the way we've talked about it in the lectures. 00:31:20.950 --> 00:31:22.320 It's very close. 00:31:22.320 --> 00:31:25.770 The idea is this the RF signal, the radio frequency 00:31:25.770 --> 00:31:29.790 signal, first gets modulated down to what's called an IF 00:31:29.790 --> 00:31:32.320 stage-- the intermediate frequency stage. 00:31:32.320 --> 00:31:36.840 And then it's that signal that goes to an envelope detector 00:31:36.840 --> 00:31:39.280 of the type we've talked about to generate 00:31:39.280 --> 00:31:41.930 the demodulated signal. 00:31:41.930 --> 00:31:45.900 So, I guess what Sandy has some probes on are the RF 00:31:45.900 --> 00:31:48.540 input and a ground-- 00:31:48.540 --> 00:31:49.260 SANDY HILL: That's right. 00:31:49.260 --> 00:31:50.960 The red lead here is a ground. 00:31:50.960 --> 00:31:53.650 It's common for both the signals we'll be looking at. 00:31:53.650 --> 00:31:54.520 This is at the audio. 00:31:54.520 --> 00:31:56.660 We're actually looking at the signal going directly into the 00:31:56.660 --> 00:31:58.720 speaker across these two leads. 00:31:58.720 --> 00:32:01.810 And then this probe down here, which I found by hunting and 00:32:01.810 --> 00:32:05.740 pecking, is the IF signal, the intermediate frequency signal, 00:32:05.740 --> 00:32:08.490 with a station on it that I'll now try and get something that 00:32:08.490 --> 00:32:09.380 sounds reasonable. 00:32:09.380 --> 00:32:12.146 [RADIO FEEDBACK] 00:32:12.146 --> 00:32:12.610 SANDY HILL: OK. 00:32:12.610 --> 00:32:13.710 A little sports. 00:32:13.710 --> 00:32:15.755 PROFESSOR: Never quite know what you're going to get when 00:32:15.755 --> 00:32:16.910 you turn this on. 00:32:16.910 --> 00:32:18.160 SANDY HILL: That's right, you don't. 00:32:22.380 --> 00:32:23.960 Make some adjustments there. 00:32:23.960 --> 00:32:27.500 And what you can see right here is the audio signal. 00:32:27.500 --> 00:32:31.480 I'll give it a little bit more game. 00:32:31.480 --> 00:32:34.510 And this is the actual intermediate frequency, 00:32:34.510 --> 00:32:36.380 modulated by the audio. 00:32:36.380 --> 00:32:39.010 And there's a correspondence between them that goes by 00:32:39.010 --> 00:32:42.180 awfully fast, but I think it's pretty simple to see. 00:32:42.180 --> 00:32:45.550 PROFESSOR: So basically the top trace is the envelope of 00:32:45.550 --> 00:32:46.170 the bottom right. 00:32:46.170 --> 00:32:47.000 SANDY HILL: That's right. 00:32:47.000 --> 00:32:49.860 This is what comes out of the envelope detector. 00:32:49.860 --> 00:32:52.800 And this is the signal into the envelope detector, in this 00:32:52.800 --> 00:32:55.930 particular radio. 00:32:55.930 --> 00:32:59.850 What I'd like to do for a last experiment is what we've been 00:32:59.850 --> 00:33:00.700 doing at this point-- 00:33:00.700 --> 00:33:02.470 I'll turn this off for a moment-- 00:33:02.470 --> 00:33:05.190 is we're taking something that's coming in over the 00:33:05.190 --> 00:33:08.350 airwaves, as they used to say, and we're simply viewing it. 00:33:08.350 --> 00:33:10.690 What I'd like to do now is generate our own radio 00:33:10.690 --> 00:33:13.390 frequency signal, tune it up so this radio is 00:33:13.390 --> 00:33:15.160 going to hear it-- 00:33:15.160 --> 00:33:16.300 whatever that means-- 00:33:16.300 --> 00:33:18.710 and display it here. 00:33:18.710 --> 00:33:23.170 What we're going to do then is use the audio oscillator, 00:33:23.170 --> 00:33:26.710 along with the amplitude modulator. 00:33:26.710 --> 00:33:30.060 And we're going to take from the output now the lead here. 00:33:30.060 --> 00:33:32.020 And this will act as the antenna. 00:33:32.020 --> 00:33:34.900 This device was not designed to put out a lot of power. 00:33:34.900 --> 00:33:37.550 But fortunately we don't have a lot of distance to go over. 00:33:37.550 --> 00:33:40.480 What I'm going to do is turn myself into an antenna-- 00:33:40.480 --> 00:33:43.250 I'm basically 150 pounds of salt water-- 00:33:43.250 --> 00:33:46.380 and when I touch this, the signal coming in here, which 00:33:46.380 --> 00:33:49.550 is radiating a little from the wire, is going to suddenly 00:33:49.550 --> 00:33:52.150 radiate from my whole body and hopefully will be enough 00:33:52.150 --> 00:33:53.450 to be picked up. 00:33:53.450 --> 00:33:58.130 So what I'll do is I'm going to turn this on-- 00:33:58.130 --> 00:33:59.800 excuse the noise. 00:33:59.800 --> 00:34:02.850 Now I've set this up so it's tuned to a particular radio 00:34:02.850 --> 00:34:04.900 station, and I hope I can find it now. 00:34:07.850 --> 00:34:09.100 It takes a little fussing. 00:34:14.179 --> 00:34:15.429 That's not it. 00:34:18.060 --> 00:34:19.710 That sounds like it now. 00:34:19.710 --> 00:34:22.210 OK. 00:34:22.210 --> 00:34:25.929 PROFESSOR: So this gets billed as Sandy Hill, human antenna. 00:34:25.929 --> 00:34:27.580 SANDY HILL: That's right. 00:34:27.580 --> 00:34:30.920 What you're looking at here is I've got the antenna at this 00:34:30.920 --> 00:34:33.630 point, and what we're going to do is you're 00:34:33.630 --> 00:34:35.110 seeing the audio signal. 00:34:35.110 --> 00:34:37.980 This is coming from the signal generator up here. 00:34:37.980 --> 00:34:42.699 And this is the intermediate frequency signal again. 00:34:42.699 --> 00:34:45.389 You can hear it's much stronger when my body becomes 00:34:45.389 --> 00:34:47.250 the antenna. 00:34:47.250 --> 00:34:48.909 And I'll leave it on for a second. 00:34:48.909 --> 00:34:51.872 And I can switch to square wave-- 00:34:51.872 --> 00:34:54.199 doesn't look much like a square wave. 00:34:54.199 --> 00:34:56.590 Various things. 00:34:56.590 --> 00:34:59.780 There's significant distortion, because of all the 00:34:59.780 --> 00:35:02.290 transformations that the signal is going through. 00:35:02.290 --> 00:35:06.870 But it does indeed work, and we're picking up a frequency 00:35:06.870 --> 00:35:09.360 that the radio is tuned to. 00:35:09.360 --> 00:35:10.170 And that's it. 00:35:10.170 --> 00:35:13.590 PROFESSOR: So Sandy has now been modulated and 00:35:13.590 --> 00:35:14.080 demodulated. 00:35:14.080 --> 00:35:16.280 SANDY HILL: That's right. 00:35:16.280 --> 00:35:19.090 PROFESSOR: Well, hopefully, all the things that we've gone 00:35:19.090 --> 00:35:23.800 through in the process of this tape and this set of 00:35:23.800 --> 00:35:27.250 demonstrations gives you a feel for at least some of the 00:35:27.250 --> 00:35:30.920 things that we've talked about so far in the course. 00:35:30.920 --> 00:35:35.430 And Sandy, I'd really like to thank you for joining us, for 00:35:35.430 --> 00:35:38.260 sharing your insights with us, as well as sharing your 00:35:38.260 --> 00:35:38.940 equipment with us. 00:35:38.940 --> 00:35:39.300 Thanks a lot. 00:35:39.300 --> 00:35:40.210 SANDY HILL: It was a treat. 00:35:40.210 --> 00:35:41.460 Thank you.