1 00:00:00,040 --> 00:00:02,470 The following content is provided under a Creative 2 00:00:02,470 --> 00:00:03,880 Commons license. 3 00:00:03,880 --> 00:00:06,920 Your support will help MIT OpenCourseWare continue to 4 00:00:06,920 --> 00:00:10,570 offer high quality educational resources for free. 5 00:00:10,570 --> 00:00:13,470 To make a donation or view additional materials from 6 00:00:13,470 --> 00:00:16,046 hundreds of MIT courses, visit MITOpenCourseWare at 7 00:00:16,046 --> 00:00:17,296 ocw.mit.edu. 8 00:01:04,540 --> 00:01:06,990 PROFESSOR: Well, today, we have a chance to put away the 9 00:01:06,990 --> 00:01:08,970 equations and have some fun. 10 00:01:08,970 --> 00:01:12,370 With me is Professor Sandy Hill from the University of 11 00:01:12,370 --> 00:01:13,970 Massachusetts at Amherst. 12 00:01:13,970 --> 00:01:17,870 And Sandy, maybe you could just give us a quick tour of 13 00:01:17,870 --> 00:01:18,860 what we have here. 14 00:01:18,860 --> 00:01:19,590 SANDY HILL: OK. 15 00:01:19,590 --> 00:01:22,070 We'll be dealing with about four instruments today. 16 00:01:22,070 --> 00:01:24,800 Two of them generate signals, and two of them display and 17 00:01:24,800 --> 00:01:26,400 analyze signals. 18 00:01:26,400 --> 00:01:30,180 The first, and perhaps simplest, is simply an RC 19 00:01:30,180 --> 00:01:34,440 audio generator, that will put out a variety wave shapes. 20 00:01:34,440 --> 00:01:36,510 And we'll look at that in a moment. 21 00:01:36,510 --> 00:01:39,340 Over to the right is a device that will allow us to study 22 00:01:39,340 --> 00:01:42,590 amplitude modulation and the various flavors of it. 23 00:01:42,590 --> 00:01:44,950 Down below is a standard oscilloscope to look at the 24 00:01:44,950 --> 00:01:47,850 signals in the time domain so we can see their shape. 25 00:01:47,850 --> 00:01:50,790 And then, finally the spectrum analyzer will analyze the 26 00:01:50,790 --> 00:01:52,750 signal into its Fourier components. 27 00:01:52,750 --> 00:01:55,250 And we'll display those, so you get to see the spectral 28 00:01:55,250 --> 00:01:57,650 content of a signal. 29 00:01:57,650 --> 00:02:00,490 PROFESSOR: Now, we'll be seeing all of these in a fair 30 00:02:00,490 --> 00:02:04,760 amount of detail as we go through the demonstrations. 31 00:02:04,760 --> 00:02:07,770 But maybe we can begin just with the signal generator. 32 00:02:07,770 --> 00:02:11,030 SANDY HILL: OK, let's look quickly at the 33 00:02:11,030 --> 00:02:12,390 various buttons involved. 34 00:02:12,390 --> 00:02:14,730 There's a lot of flexibility with this device. 35 00:02:14,730 --> 00:02:18,590 The top three buttons here allow us to change from one 36 00:02:18,590 --> 00:02:21,530 simple wave form to another one, a sine wave, a triangle 37 00:02:21,530 --> 00:02:23,330 wave, a square wave. 38 00:02:23,330 --> 00:02:26,490 And right below it is a button that will allow us to change 39 00:02:26,490 --> 00:02:28,390 the size of the signal. 40 00:02:28,390 --> 00:02:32,290 Moving over towards the left, we can change the DC offset 41 00:02:32,290 --> 00:02:36,790 which varies the signal, the level upon 42 00:02:36,790 --> 00:02:38,550 which the signal rides. 43 00:02:38,550 --> 00:02:40,390 PROFESSOR: And we'll be using that actually when we 44 00:02:40,390 --> 00:02:42,090 demonstrate amplitude modulation. 45 00:02:42,090 --> 00:02:44,540 SANDY HILL: That's right And then finally, there's a way of 46 00:02:44,540 --> 00:02:47,350 changing the frequency of the signal, either changing it by 47 00:02:47,350 --> 00:02:50,770 a factor of 10 or a vernier adjustment, if you want very 48 00:02:50,770 --> 00:02:53,850 specific adjustments, very delicate changes. 49 00:02:53,850 --> 00:02:54,190 PROFESSOR: OK. 50 00:02:54,190 --> 00:02:58,090 So maybe we can just vary some of this and see what a little 51 00:02:58,090 --> 00:02:59,040 bit of it looks like. 52 00:02:59,040 --> 00:02:59,870 SANDY HILL: All right. 53 00:02:59,870 --> 00:03:04,130 What we have displayed at this point is the effect of pushing 54 00:03:04,130 --> 00:03:05,300 the sine wave button. 55 00:03:05,300 --> 00:03:09,060 And so displayed down here is a 500 Hertz sine wave. 56 00:03:09,060 --> 00:03:10,240 In fact, let's listen to that. 57 00:03:10,240 --> 00:03:11,490 PROFESSOR: Great. 58 00:03:13,360 --> 00:03:16,790 SANDY HILL: And the scope is said to have one millisecond 59 00:03:16,790 --> 00:03:19,260 for each division here. 60 00:03:19,260 --> 00:03:21,900 By varying the amplitude knob can make the sine 61 00:03:21,900 --> 00:03:23,410 wave smaller or larger. 62 00:03:23,410 --> 00:03:26,010 The ear isn't too sensitive to that. 63 00:03:26,010 --> 00:03:28,400 By changing the DC offset, it simply rides 64 00:03:28,400 --> 00:03:29,515 on a different level. 65 00:03:29,515 --> 00:03:32,260 It's like putting a battery in series. 66 00:03:32,260 --> 00:03:36,360 And changing the frequency, you get a very perceptible 67 00:03:36,360 --> 00:03:38,230 change, a lower frequency. 68 00:03:38,230 --> 00:03:40,570 It takes longer to sweep through a period. 69 00:03:40,570 --> 00:03:44,256 Higher frequency, it takes a shorter amount of time. 70 00:03:44,256 --> 00:03:48,060 And also, we can change the wave shape itself from a sine 71 00:03:48,060 --> 00:03:51,890 wave to a triangle wave-- 72 00:03:51,890 --> 00:03:54,840 and you notice that has a richer sound to it-- 73 00:03:54,840 --> 00:03:58,210 and then finally to a square wave, which is an even 74 00:03:58,210 --> 00:04:00,510 brighter sound. 75 00:04:00,510 --> 00:04:04,640 PROFESSOR: And actually, as you realize from the previous 76 00:04:04,640 --> 00:04:07,980 lectures, with the triangle and square wave with the same 77 00:04:07,980 --> 00:04:11,740 fundamental as the sine wave, the richness comes in because 78 00:04:11,740 --> 00:04:17,880 of the higher harmonics in the Fourier series representation. 79 00:04:17,880 --> 00:04:20,790 OK, well let's go back to the sine wave. 80 00:04:20,790 --> 00:04:23,770 And we'll use that to take a closer look at 81 00:04:23,770 --> 00:04:25,190 the spectrum analyzer. 82 00:04:25,190 --> 00:04:25,720 SANDY HILL: OK. 83 00:04:25,720 --> 00:04:28,140 PROFESSOR: We'll look at a block diagram of the spectrum 84 00:04:28,140 --> 00:04:32,190 analyzer shortly, but first let's just look at a few quick 85 00:04:32,190 --> 00:04:34,360 things to get a feel for it. 86 00:04:34,360 --> 00:04:36,970 We have, of course, the time domain 87 00:04:36,970 --> 00:04:39,890 display of the sine wave. 88 00:04:39,890 --> 00:04:44,230 And the spectrum analyzer gives us a frequency domain 89 00:04:44,230 --> 00:04:46,450 analysis and display. 90 00:04:46,450 --> 00:04:52,540 And the vertical axis is the amplitude. 91 00:04:52,540 --> 00:04:55,070 The horizontal axis is frequency. 92 00:04:55,070 --> 00:05:00,150 And as it's set currently, the frequency axis goes from DC-- 93 00:05:00,150 --> 00:05:01,150 zero-- 94 00:05:01,150 --> 00:05:02,480 up to 2 kilohertz. 95 00:05:02,480 --> 00:05:04,620 And that, of course, can be varied on 96 00:05:04,620 --> 00:05:06,950 the spectrum analyzer. 97 00:05:06,950 --> 00:05:10,120 And since we have a sine wave input, we get, as we would 98 00:05:10,120 --> 00:05:13,820 expect, a line spectrum, corresponding to the frequency 99 00:05:13,820 --> 00:05:15,160 of the sine wave. 100 00:05:15,160 --> 00:05:18,970 Now, in fact, we can measure that frequency because the 101 00:05:18,970 --> 00:05:23,840 spectrum analyzer has a cursor associated with it, which I've 102 00:05:23,840 --> 00:05:25,600 just enabled. 103 00:05:25,600 --> 00:05:28,900 And this line is the cursor line. 104 00:05:28,900 --> 00:05:32,480 There is a read out for the frequency and a read out for 105 00:05:32,480 --> 00:05:34,500 the amplitude. 106 00:05:34,500 --> 00:05:37,390 And let's position the cursor on the 107 00:05:37,390 --> 00:05:40,320 frequency of the sine wave. 108 00:05:40,320 --> 00:05:44,520 And we're getting close. 109 00:05:44,520 --> 00:05:49,530 And there we are at the frequency of the sine wave. 110 00:05:49,530 --> 00:05:53,350 And we can read out, in fact, that it's 500 cycles. 111 00:05:53,350 --> 00:05:55,260 And this is the amplitude. 112 00:05:55,260 --> 00:05:58,910 And, of course, you could verify the frequency also in 113 00:05:58,910 --> 00:06:01,900 the time domain display, simply by measuring the 114 00:06:01,900 --> 00:06:05,690 frequency or the period. 115 00:06:05,690 --> 00:06:08,310 SANDY HILL: This is set at still one millisecond per 116 00:06:08,310 --> 00:06:08,940 centimeter. 117 00:06:08,940 --> 00:06:10,330 So that is 500 Hertz. 118 00:06:10,330 --> 00:06:11,640 PROFESSOR: OK. 119 00:06:11,640 --> 00:06:16,330 Well now maybe what we can do is vary the frequency. 120 00:06:16,330 --> 00:06:18,950 Let's vary the frequency of the sine wave generator. 121 00:06:18,950 --> 00:06:20,030 Maybe also listen to it? 122 00:06:20,030 --> 00:06:21,110 SANDY HILL: I'll turn on the tone. 123 00:06:21,110 --> 00:06:23,130 Yep. 124 00:06:23,130 --> 00:06:25,550 OK, I'll make the frequency higher. 125 00:06:25,550 --> 00:06:26,330 And you'll see a 126 00:06:26,330 --> 00:06:28,810 correspondence in the time domain. 127 00:06:28,810 --> 00:06:33,040 The frequency clearly goes up, and that line of the spectrum 128 00:06:33,040 --> 00:06:35,560 also migrates up away from the cursor. 129 00:06:35,560 --> 00:06:37,690 PROFESSOR: OK, and let me just point out again that this is 130 00:06:37,690 --> 00:06:38,610 the cursor line. 131 00:06:38,610 --> 00:06:40,680 This isn't the spectra line. 132 00:06:40,680 --> 00:06:46,560 And this is the frequency content of the sine wave. 133 00:06:46,560 --> 00:06:49,230 SANDY HILL: We can also change the amplitude of that line by 134 00:06:49,230 --> 00:06:53,240 making the sine wave itself smaller. 135 00:06:53,240 --> 00:06:55,880 And you notice the intensity of it goes-- 136 00:06:55,880 --> 00:06:57,420 PROFESSOR: While you do that, let me just position the 137 00:06:57,420 --> 00:07:01,790 cursor if I can on the sine wave. 138 00:07:01,790 --> 00:07:04,880 And now as you vary the amplitude, we should, in fact, 139 00:07:04,880 --> 00:07:08,685 see this vary. 140 00:07:08,685 --> 00:07:10,641 SANDY HILL: Well, that's right. 141 00:07:10,641 --> 00:07:11,130 PROFESSOR: OK. 142 00:07:11,130 --> 00:07:15,460 Now as Sandy varied the frequency, he did it slowly 143 00:07:15,460 --> 00:07:19,590 and we saw the line move as a single line. 144 00:07:19,590 --> 00:07:21,500 Maybe now, Sandy, you could vary the 145 00:07:21,500 --> 00:07:22,970 frequency more rapidly. 146 00:07:22,970 --> 00:07:29,583 And what will happen is that, in fact, what we'll get if I-- 147 00:07:29,583 --> 00:07:31,865 SANDY HILL: There'll be a spreading as we go-- 148 00:07:31,865 --> 00:07:33,430 PROFESSOR: Let me turn the cursor off. 149 00:07:33,430 --> 00:07:37,870 And what we'll see is a spreading of the line, so that 150 00:07:37,870 --> 00:07:39,840 the frequency content is richer. 151 00:07:39,840 --> 00:07:42,770 And, in fact, the analysis of this is considerably more 152 00:07:42,770 --> 00:07:43,560 complicated. 153 00:07:43,560 --> 00:07:48,050 It corresponds to frequency modulation, where Sandy is now 154 00:07:48,050 --> 00:07:49,175 the modulating signal. 155 00:07:49,175 --> 00:07:49,910 SANDY HILL: That's right. 156 00:07:49,910 --> 00:07:53,890 PROFESSOR: And we won't really be going into issues of 157 00:07:53,890 --> 00:07:57,870 frequency modulation in any more detail in this 158 00:07:57,870 --> 00:07:58,710 demonstration. 159 00:07:58,710 --> 00:08:00,670 SANDY HILL: Right. 160 00:08:00,670 --> 00:08:03,240 PROFESSOR: OK, well now let's go to the overhead projector 161 00:08:03,240 --> 00:08:05,460 and take a look at a block diagram of 162 00:08:05,460 --> 00:08:06,710 the spectrum analyzer. 163 00:08:10,040 --> 00:08:14,040 The specific spectrum analyzer that we're using in the 164 00:08:14,040 --> 00:08:18,420 demonstration is made by Rockland Systems, referred to 165 00:08:18,420 --> 00:08:22,320 as the Rockland Systems Model FFT 512. 166 00:08:22,320 --> 00:08:29,980 And basically, the idea is to sample the incoming wave form, 167 00:08:29,980 --> 00:08:33,919 convert that into digital form, and then the spectrum, 168 00:08:33,919 --> 00:08:38,929 in fact, is computed digitally using a microprocessor. 169 00:08:38,929 --> 00:08:45,330 So the overall system block diagram first consists of a 170 00:08:45,330 --> 00:08:48,280 system which is a low pass filter. 171 00:08:48,280 --> 00:08:54,100 And this low pass filter is used in advance of the 172 00:08:54,100 --> 00:08:59,580 sampling process to basically reduce the artifacts that are 173 00:08:59,580 --> 00:09:01,470 introduced due to sampling. 174 00:09:01,470 --> 00:09:05,490 And although we haven't talked yet about sampling and the 175 00:09:05,490 --> 00:09:09,200 associated artifacts, basically, as we'll see in the 176 00:09:09,200 --> 00:09:14,480 upcoming lectures, in order to sample a wave form and convert 177 00:09:14,480 --> 00:09:17,870 it into digital form, it requires that the wave form 178 00:09:17,870 --> 00:09:19,990 first be low pass filtered. 179 00:09:19,990 --> 00:09:24,440 So this low pass filter is referred to-- and will be in 180 00:09:24,440 --> 00:09:25,280 later lectures-- 181 00:09:25,280 --> 00:09:27,800 as an anti-aliasing filter. 182 00:09:27,800 --> 00:09:32,840 And this low pass filtered wave form is then converted to 183 00:09:32,840 --> 00:09:34,750 a sequence. 184 00:09:34,750 --> 00:09:39,730 And so a sequence is generated for which the sequence values 185 00:09:39,730 --> 00:09:46,590 are simply samples of the low pass filtered input. 186 00:09:46,590 --> 00:09:50,620 This low pass filtered input is then put 187 00:09:50,620 --> 00:09:52,340 into digital memory. 188 00:09:52,340 --> 00:09:56,730 Basically, a time block of it is put into digital memory, 189 00:09:56,730 --> 00:09:59,620 and so that's what we have down here. 190 00:09:59,620 --> 00:10:02,340 Here is the sequence. 191 00:10:02,340 --> 00:10:06,520 The sequence is put into digital memory. 192 00:10:06,520 --> 00:10:11,470 And then an arithmetic processor computes for the 193 00:10:11,470 --> 00:10:13,770 samples in this memory. 194 00:10:13,770 --> 00:10:18,430 It computes the Fourier transform or the spectrum. 195 00:10:18,430 --> 00:10:22,810 And after the spectrum is computed and put either into 196 00:10:22,810 --> 00:10:26,860 the same or a different memory, that is then put 197 00:10:26,860 --> 00:10:31,560 through a conversion process back to a continuous time 198 00:10:31,560 --> 00:10:37,210 signal, and finally, put out on the display that we've been 199 00:10:37,210 --> 00:10:39,000 seeing in the demonstration. 200 00:10:39,000 --> 00:10:43,190 And so this is just indicative of the display. 201 00:10:43,190 --> 00:10:46,900 So basically, the idea then is that the input 202 00:10:46,900 --> 00:10:49,410 wave form comes in. 203 00:10:49,410 --> 00:10:53,950 It's filtered and sampled and captured on a block basis, put 204 00:10:53,950 --> 00:10:58,480 into a digital memory, and then, a digital computer or 205 00:10:58,480 --> 00:11:02,250 microprocessor computes the Fourier transform. 206 00:11:02,250 --> 00:11:05,530 And then that Fourier transform is what we see on 207 00:11:05,530 --> 00:11:06,930 the display. 208 00:11:06,930 --> 00:11:10,840 So what we're computing, of course, are samples of the 209 00:11:10,840 --> 00:11:12,830 Fourier transform. 210 00:11:12,830 --> 00:11:16,040 And so, for example, if the input-- let's say-- 211 00:11:16,040 --> 00:11:20,680 was a rectangular pulse whose Fourier transform is of the 212 00:11:20,680 --> 00:11:24,030 form of a sine(x) over x function, what we would, in 213 00:11:24,030 --> 00:11:28,700 fact, see on the display are samples of that at discrete 214 00:11:28,700 --> 00:11:29,900 frequencies. 215 00:11:29,900 --> 00:11:36,140 Or if as we have an input which is a square wave, what 216 00:11:36,140 --> 00:11:40,560 will generate through the spectrum analyzer are the 217 00:11:40,560 --> 00:11:45,760 Fourier series coefficients or equivalently, the harmonics 218 00:11:45,760 --> 00:11:48,720 associated with the square wave. 219 00:11:48,720 --> 00:11:53,950 OK, well, let's now go back to the equipment and look at the 220 00:11:53,950 --> 00:11:55,200 spectrum analyzer. 221 00:11:57,700 --> 00:12:01,640 We'll look at the square wave through the spectrum analyzer 222 00:12:01,640 --> 00:12:06,040 shortly, but first what we have is what we saw before, 223 00:12:06,040 --> 00:12:07,920 which is the sine wave. 224 00:12:07,920 --> 00:12:12,450 And just to point out, again, the fact that the sine wave 225 00:12:12,450 --> 00:12:16,550 spectrum, of course, is just a single line corresponding to 226 00:12:16,550 --> 00:12:19,080 the fundamental frequency. 227 00:12:19,080 --> 00:12:24,070 And here we have a frequency scale now that goes from 0 to 228 00:12:24,070 --> 00:12:25,640 5 kilohertz. 229 00:12:25,640 --> 00:12:29,680 And this is then the 500 cycle sine wave. 230 00:12:29,680 --> 00:12:35,240 And, in fact, we can flip the cursor on and I happen to just 231 00:12:35,240 --> 00:12:37,110 magically have it positioned correctly. 232 00:12:37,110 --> 00:12:41,530 And we see that it's a 500 cycle sine wave. 233 00:12:41,530 --> 00:12:43,030 SANDY HILL: Might be interesting, Al, to go and 234 00:12:43,030 --> 00:12:45,770 look at a richer set of signals, such as the triangle 235 00:12:45,770 --> 00:12:48,730 wave and the square wave-- things that are conveniently 236 00:12:48,730 --> 00:12:50,650 on the signal generator itself. 237 00:12:50,650 --> 00:12:54,000 I'll switch over to a square wave now. 238 00:12:54,000 --> 00:12:57,340 And what you see is all the harmonics 239 00:12:57,340 --> 00:13:00,064 coming up, being displayed. 240 00:13:00,064 --> 00:13:02,460 PROFESSOR: OK, it's actually interesting to point out, I 241 00:13:02,460 --> 00:13:07,460 think, that the square wave, as we know, is an 242 00:13:07,460 --> 00:13:08,940 odd harmonic function. 243 00:13:08,940 --> 00:13:14,470 And so, in fact, the even harmonics are missing in the 244 00:13:14,470 --> 00:13:15,220 square wave. 245 00:13:15,220 --> 00:13:17,030 So this is the fundamental. 246 00:13:17,030 --> 00:13:20,320 This is the third harmonic, fifth harmonic, et cetera. 247 00:13:20,320 --> 00:13:24,940 And then the amplitude of the square wave decays 248 00:13:24,940 --> 00:13:27,870 proportional to 1 over f, which is the kind of analysis 249 00:13:27,870 --> 00:13:30,000 that we've gone through in looking at Fourier series. 250 00:13:30,000 --> 00:13:33,150 SANDY HILL: Right, and if we switch to the triangle wave, 251 00:13:33,150 --> 00:13:36,730 instead of their decaying as 1 over f, the harmonics decay as 252 00:13:36,730 --> 00:13:37,800 1 over f squared. 253 00:13:37,800 --> 00:13:39,980 And you see they drop off much more quickly. 254 00:13:39,980 --> 00:13:41,310 PROFESSOR: And again, of course, it's an 255 00:13:41,310 --> 00:13:43,020 odd harmonic signal. 256 00:13:43,020 --> 00:13:47,730 And so the even numbered harmonics are missing. 257 00:13:47,730 --> 00:13:52,450 And maybe just to kind of emphasize the point, we can 258 00:13:52,450 --> 00:13:55,720 show the sine wave again and the square wave again. 259 00:13:55,720 --> 00:13:59,360 SANDY HILL: We'll go through from the most bland to the 260 00:13:59,360 --> 00:14:01,510 richer to the richest. 261 00:14:01,510 --> 00:14:02,750 PROFESSOR: OK, and it's really kind of 262 00:14:02,750 --> 00:14:03,750 interesting and dramatic-- 263 00:14:03,750 --> 00:14:04,650 SANDY HILL: It's fun to do that. 264 00:14:04,650 --> 00:14:06,130 PROFESSOR: --to see the harmonics pop in. 265 00:14:06,130 --> 00:14:07,900 SANDY HILL: While we're at the square wave, let me fiddle 266 00:14:07,900 --> 00:14:10,100 with the frequency of the square wave, and we can see 267 00:14:10,100 --> 00:14:13,970 the duality of the time and frequency domains. 268 00:14:13,970 --> 00:14:16,480 That is, as you compress things in the time domain, 269 00:14:16,480 --> 00:14:19,920 such as this, going to a higher frequency square wave, 270 00:14:19,920 --> 00:14:22,620 the harmonics wonder further away from each other. 271 00:14:22,620 --> 00:14:25,950 SANDY HILL: So this is the fundamental. 272 00:14:25,950 --> 00:14:28,810 The second harmonic is missing. 273 00:14:28,810 --> 00:14:30,290 This is the third harmonic. 274 00:14:30,290 --> 00:14:31,730 And of course, the fifth. 275 00:14:31,730 --> 00:14:34,290 SANDY HILL: And as we take this to an extreme by going to 276 00:14:34,290 --> 00:14:37,500 very low repetition rate square waves, all those 277 00:14:37,500 --> 00:14:43,370 harmonics come scurrying in and cluster together near DC. 278 00:14:43,370 --> 00:14:45,265 PROFESSOR: Kind of fun with Fourier transforms. 279 00:14:48,010 --> 00:14:53,170 Well, speaking of time and frequency scaling, recall that 280 00:14:53,170 --> 00:14:57,280 we had demonstrated time and frequency scaling previously 281 00:14:57,280 --> 00:14:59,880 with the glockenspiel. 282 00:14:59,880 --> 00:15:03,840 And what we had done there was to record a particular 283 00:15:03,840 --> 00:15:08,110 glockenspiel note, and then we played that 284 00:15:08,110 --> 00:15:10,870 back at half speed. 285 00:15:10,870 --> 00:15:14,880 And what we had done in that case is expanded things in 286 00:15:14,880 --> 00:15:17,740 time, consequently compress them in frequency. 287 00:15:17,740 --> 00:15:24,800 And so, comparing that with a note an octave lower than we 288 00:15:24,800 --> 00:15:27,340 saw that, in fact, the time scaling had led 289 00:15:27,340 --> 00:15:29,010 to a frequency scaling. 290 00:15:29,010 --> 00:15:33,330 And then we also played the same note back at twice speed. 291 00:15:33,330 --> 00:15:36,120 And in that case, the frequencies were all scaled up 292 00:15:36,120 --> 00:15:37,010 by a factor of 2. 293 00:15:37,010 --> 00:15:40,800 And again, we illustrated that by comparing with the 294 00:15:40,800 --> 00:15:43,660 glockenspiel note a full octave up. 295 00:15:43,660 --> 00:15:46,890 Now when we did that, we didn't actually look at the 296 00:15:46,890 --> 00:15:49,600 time wave forms or spectra. 297 00:15:49,600 --> 00:15:52,230 And having the equipment that we have here gives us kind of 298 00:15:52,230 --> 00:15:54,420 a nice opportunity to do that. 299 00:15:54,420 --> 00:15:59,110 So what I have is the tape that we had originally made of 300 00:15:59,110 --> 00:16:01,810 the glockenspiel, the original note that we recorded. 301 00:16:01,810 --> 00:16:05,670 And what we'll do is look at the spectrum of that, and then 302 00:16:05,670 --> 00:16:10,560 compare that spectrum when we play the tape at half speed 303 00:16:10,560 --> 00:16:13,540 and also play it at twice speed. 304 00:16:13,540 --> 00:16:16,240 So let's play the tape. 305 00:16:16,240 --> 00:16:22,160 And what we have is the glockenspiel right now 306 00:16:22,160 --> 00:16:26,540 displayed on a frequency scale from 0 to 5 kilohertz. 307 00:16:26,540 --> 00:16:31,210 Let's just change that to zero to 10 kilohertz. 308 00:16:31,210 --> 00:16:33,620 So here is this spectrum. 309 00:16:33,620 --> 00:16:36,950 Over here we have the time wave form. 310 00:16:36,950 --> 00:16:40,740 And here is then the first spectral line, and we can see 311 00:16:40,740 --> 00:16:44,710 where that is by setting up the cursor. 312 00:16:44,710 --> 00:16:48,290 And magically, once again, I have the cursor positioned at 313 00:16:48,290 --> 00:16:49,650 just the right spot. 314 00:16:49,650 --> 00:16:55,120 The first spectral line is at 1.775 kilohertz. 315 00:16:55,120 --> 00:17:00,590 And so this is the spectrum then of the original 316 00:17:00,590 --> 00:17:02,881 glockenspiel note. 317 00:17:02,881 --> 00:17:03,320 All right. 318 00:17:03,320 --> 00:17:07,890 Let's stop the tape and rewind it. 319 00:17:07,890 --> 00:17:13,660 And now what we want to do is play that back at half speed. 320 00:17:13,660 --> 00:17:16,849 Played at half speed, the frequencies 321 00:17:16,849 --> 00:17:18,599 should be scaled down. 322 00:17:18,599 --> 00:17:22,560 And, in particular then the first spectra line should be 323 00:17:22,560 --> 00:17:24,760 at a lower frequency. 324 00:17:24,760 --> 00:17:27,730 So let's play that now. 325 00:17:27,730 --> 00:17:29,320 SANDY HILL: There are really very complicated signals, 326 00:17:29,320 --> 00:17:30,200 aren't they? 327 00:17:30,200 --> 00:17:32,760 PROFESSOR: They really are. 328 00:17:32,760 --> 00:17:37,580 Here we have the first spectral line. 329 00:17:37,580 --> 00:17:41,690 And we can compare that with the first spectral line that 330 00:17:41,690 --> 00:17:46,570 we had before which was at 1.775 kilohertz. 331 00:17:46,570 --> 00:17:51,030 And, once again, you see that the time wave form over here 332 00:17:51,030 --> 00:17:55,800 has been scaled by a factor of 2. 333 00:17:55,800 --> 00:17:59,210 So, once again, we see that time and frequency scaling 334 00:17:59,210 --> 00:18:00,820 really works. 335 00:18:00,820 --> 00:18:05,340 Incidentally as Sandy pointed out, and rightfully so, the 336 00:18:05,340 --> 00:18:10,200 glockenspiel really is a pretty complicated signal, as 337 00:18:10,200 --> 00:18:13,870 a graduate student and I found out when we were preparing the 338 00:18:13,870 --> 00:18:16,120 original glockenspiel demo. 339 00:18:16,120 --> 00:18:18,380 SANDY HILL: Speaking of complicated signals, one of my 340 00:18:18,380 --> 00:18:20,760 favorites is to look at speech. 341 00:18:20,760 --> 00:18:23,810 I set this up so that what's coming into my microphone is 342 00:18:23,810 --> 00:18:26,780 indeed what you're going to see on the two screens. 343 00:18:26,780 --> 00:18:29,660 The telephone company thinks of speech, basically in terms 344 00:18:29,660 --> 00:18:32,100 of bandwidth, that it extends from about 300 345 00:18:32,100 --> 00:18:33,930 Hertz to 3,300 Hertz. 346 00:18:33,930 --> 00:18:35,880 But, as we'll see in the spectrum analyzer, there's a 347 00:18:35,880 --> 00:18:37,720 lot of leakage outside of that. 348 00:18:37,720 --> 00:18:40,860 The telephone company just filters out everything outside 349 00:18:40,860 --> 00:18:43,130 of that and things of that as a speech signal. 350 00:18:43,130 --> 00:18:46,220 So it doesn't have the high fidelity that you might have 351 00:18:46,220 --> 00:18:47,630 on high fire equipment. 352 00:18:47,630 --> 00:18:51,800 As we look at the scope, again, the time wave form is 353 00:18:51,800 --> 00:18:55,460 extremely complicated, seems to have some periodicities in 354 00:18:55,460 --> 00:18:57,670 it, although they're short-lived, and then it goes 355 00:18:57,670 --> 00:19:00,120 on to some other periodic chunk. 356 00:19:00,120 --> 00:19:02,950 And over in the frequency domain, you can see as we 357 00:19:02,950 --> 00:19:08,020 extend from 0 to 10 kilohertz, that as I speak, there are 358 00:19:08,020 --> 00:19:11,750 trenchants that have spectral content in them, covering that 359 00:19:11,750 --> 00:19:12,990 entire band. 360 00:19:12,990 --> 00:19:14,460 I can try some simpler signals. 361 00:19:14,460 --> 00:19:17,550 A whistle is almost a sinusoid, but as you'll see 362 00:19:17,550 --> 00:19:19,170 isn't terribly sinusoidal. 363 00:19:19,170 --> 00:19:22,920 [HIGH-PITCHED WHISTLE] 364 00:19:22,920 --> 00:19:24,310 That's the best I can do. 365 00:19:24,310 --> 00:19:28,176 I can sing, a little bit embarrassedly, a B, 366 00:19:28,176 --> 00:19:28,670 [HIGH-PITCHED] 367 00:19:28,670 --> 00:19:30,220 Boo, and things like that. 368 00:19:30,220 --> 00:19:32,720 PROFESSOR: They don't ask me to, Sandy. 369 00:19:32,720 --> 00:19:35,440 SANDY HILL: And then different vowel sounds have a lot of 370 00:19:35,440 --> 00:19:40,280 energy in, like the letter A. (SUNG) A. Whereas some of the 371 00:19:40,280 --> 00:19:42,800 others are very impulsive, like-- 372 00:19:42,800 --> 00:19:43,210 [HISSING] 373 00:19:43,210 --> 00:19:45,540 Or puh and tuh. 374 00:19:45,540 --> 00:19:48,160 And it's a little hard to grab them at the right time. 375 00:19:48,160 --> 00:19:51,390 But what's fascinating is just to stare at equipment like 376 00:19:51,390 --> 00:19:54,410 this and try different speech sounds, and you begin to get 377 00:19:54,410 --> 00:19:58,300 sort of a sense of the complicated nature of them. 378 00:19:58,300 --> 00:20:02,040 PROFESSOR: OK, well that's a look at some spectra signals. 379 00:20:02,040 --> 00:20:08,780 And now what we'd like to focus on is the modulator and 380 00:20:08,780 --> 00:20:12,215 talk a little bit about modulation and demodulation 381 00:20:12,215 --> 00:20:13,800 and demonstrate it. 382 00:20:13,800 --> 00:20:18,510 And let's begin that by first taking a look at a block 383 00:20:18,510 --> 00:20:20,940 diagram of the modulator system. 384 00:20:24,170 --> 00:20:27,260 Well, as we've discussed in a previous lecture, amplitude 385 00:20:27,260 --> 00:20:32,790 modulation basically consists of multiplying the modulating 386 00:20:32,790 --> 00:20:36,900 signal by an appropriate carrier, illustrated here, as 387 00:20:36,900 --> 00:20:41,870 we've seen previously, for the case of a sinusoidal carrier. 388 00:20:41,870 --> 00:20:45,930 And then, specifically for sinusoidal amplitude 389 00:20:45,930 --> 00:20:51,580 modulation, we may or may not inject some carrier signal-- 390 00:20:51,580 --> 00:20:53,660 A times the carrier. 391 00:20:53,660 --> 00:20:59,610 Or equivalently, if we look at the modulated output, the 392 00:20:59,610 --> 00:21:03,800 injection of the carrier is equivalent, mathematically, to 393 00:21:03,800 --> 00:21:07,310 simply adding a DC offset or a constant to 394 00:21:07,310 --> 00:21:09,250 the modulating signal. 395 00:21:09,250 --> 00:21:14,490 And, as you recall when we talked about this, the idea of 396 00:21:14,490 --> 00:21:19,310 injecting a carrier or not is related to the issue of 397 00:21:19,310 --> 00:21:22,860 whether or not we want to do synchronous or asynchronous 398 00:21:22,860 --> 00:21:24,290 demodulation. 399 00:21:24,290 --> 00:21:27,540 The asynchronous demodulation corresponding to the simple 400 00:21:27,540 --> 00:21:30,050 use of an envelope detector. 401 00:21:30,050 --> 00:21:34,250 And to remind you of the wave forms that are involved, 402 00:21:34,250 --> 00:21:38,230 again, I show two that we saw previously. 403 00:21:38,230 --> 00:21:44,360 And for the case, this is for one value of the amount of 404 00:21:44,360 --> 00:21:45,720 carrier that's injected. 405 00:21:45,720 --> 00:21:50,170 And this is for an amount of carrier injected that's less. 406 00:21:50,170 --> 00:21:54,560 And this, in fact, corresponds to what we refer to as 50% 407 00:21:54,560 --> 00:22:00,020 modulation, and this is the case of 100% modulation. 408 00:22:00,020 --> 00:22:04,810 Well, the modulating system that we're using in this 409 00:22:04,810 --> 00:22:08,630 demonstration is basically of the form that 410 00:22:08,630 --> 00:22:10,160 we're indicating here. 411 00:22:10,160 --> 00:22:16,020 And a simple block diagram for it is more or less identical 412 00:22:16,020 --> 00:22:17,820 to what we just saw. 413 00:22:17,820 --> 00:22:22,400 Specifically, the modulating signal is 414 00:22:22,400 --> 00:22:25,010 multiplied by the carrier. 415 00:22:25,010 --> 00:22:29,300 And there also is the capability of injecting some 416 00:22:29,300 --> 00:22:33,420 additional carrier, meaning adding it to the output of 417 00:22:33,420 --> 00:22:34,430 this product. 418 00:22:34,430 --> 00:22:38,900 And so the modulated output can have a variable percent 419 00:22:38,900 --> 00:22:39,770 modulation-- 420 00:22:39,770 --> 00:22:44,000 the percent modulation being changed, depending on how we 421 00:22:44,000 --> 00:22:46,690 set this variable gain. 422 00:22:46,690 --> 00:22:50,450 Now, in addition to sinusoidal modulation-- 423 00:22:50,450 --> 00:22:53,980 in fact, for the particular system that we're using, we 424 00:22:53,980 --> 00:22:56,350 have somewhat more flexibility. 425 00:22:56,350 --> 00:22:59,340 We can use, in addition to a sinusoidal carrier at this 426 00:22:59,340 --> 00:23:04,640 point, we can alternatively choose a square wave carrier 427 00:23:04,640 --> 00:23:07,540 or a triangular carrier. 428 00:23:07,540 --> 00:23:10,600 And, as we'll indicate in a moment when we illustrate 429 00:23:10,600 --> 00:23:14,280 this, there are some specific advantages to using, for 430 00:23:14,280 --> 00:23:17,300 example, a square wave carrier. 431 00:23:17,300 --> 00:23:21,890 So this is a somewhat simplified version of-- 432 00:23:21,890 --> 00:23:26,730 or rather block diagram of the modulating system that we're 433 00:23:26,730 --> 00:23:27,950 demonstrating. 434 00:23:27,950 --> 00:23:32,010 The external modulating input here, a choice of carrier with 435 00:23:32,010 --> 00:23:35,100 also the capability for injecting some additional 436 00:23:35,100 --> 00:23:37,350 carrier into the output. 437 00:23:37,350 --> 00:23:39,490 OK, now let's go back to the equipment and 438 00:23:39,490 --> 00:23:42,100 take a look at this. 439 00:23:42,100 --> 00:23:45,290 Well, Sandy, maybe to begin, you can just point out what 440 00:23:45,290 --> 00:23:47,800 some of the controls are on the modulator box. 441 00:23:47,800 --> 00:23:50,180 SANDY HILL: OK, there are some interesting 442 00:23:50,180 --> 00:23:52,230 points to look at here. 443 00:23:52,230 --> 00:23:54,860 This is an input for an external signal. 444 00:23:54,860 --> 00:23:57,330 We'll be taking a signal right out of the signal generator 445 00:23:57,330 --> 00:23:58,670 and putting it in here. 446 00:23:58,670 --> 00:24:01,960 And that will be the signal that will be modulated, 447 00:24:01,960 --> 00:24:03,560 according to the carrier. 448 00:24:03,560 --> 00:24:06,450 The carrier is generated internally in this device. 449 00:24:06,450 --> 00:24:08,870 And there are several ways of controlling it. 450 00:24:08,870 --> 00:24:12,170 One is the amount of carrier injection. 451 00:24:12,170 --> 00:24:16,410 One is the wave form of the carrier itself-- and this is 452 00:24:16,410 --> 00:24:19,530 typically sinusoidal in the broadcast industry, but others 453 00:24:19,530 --> 00:24:21,710 are interesting to look at as well. 454 00:24:21,710 --> 00:24:24,110 The output that will be displaying on both the 455 00:24:24,110 --> 00:24:27,310 spectrum analyzer and scope comes out here. 456 00:24:27,310 --> 00:24:30,580 And then farther over to the left, there are some knobs for 457 00:24:30,580 --> 00:24:33,440 changing the frequency of the carrier signal. 458 00:24:33,440 --> 00:24:37,340 PROFESSOR: OK, let me also just point out again for 459 00:24:37,340 --> 00:24:40,780 emphasis that changing the carrier level, as we've talked 460 00:24:40,780 --> 00:24:43,800 about, is mathematically equivalent to changing the DC 461 00:24:43,800 --> 00:24:45,990 level of the modulating signal. 462 00:24:45,990 --> 00:24:49,545 And that's also what affects the percent modulation as we 463 00:24:49,545 --> 00:24:51,320 had just discussed. 464 00:24:51,320 --> 00:24:56,930 Now what we have set up is a sinusoidal modulating signal 465 00:24:56,930 --> 00:24:59,050 and a sinusoidal little carrier. 466 00:24:59,050 --> 00:25:04,120 And, as usual, we have the time wave form displayed here. 467 00:25:04,120 --> 00:25:06,840 And so this is the modulated signal. 468 00:25:06,840 --> 00:25:10,070 And then on the spectrum analyzer, we have 469 00:25:10,070 --> 00:25:11,870 the spectral display. 470 00:25:11,870 --> 00:25:14,750 And this is the carrier signal. 471 00:25:14,750 --> 00:25:16,620 That's the carrier frequency. 472 00:25:16,620 --> 00:25:21,070 And these side bands then correspond to the side bands 473 00:25:21,070 --> 00:25:23,270 associated with the modulating signal. 474 00:25:23,270 --> 00:25:27,730 So this is the spectrum of the total modulated output, right? 475 00:25:27,730 --> 00:25:28,480 SANDY HILL: Right. 476 00:25:28,480 --> 00:25:30,990 It's interesting to vary some of these parameters. 477 00:25:30,990 --> 00:25:33,560 You can see the sinusoidal modulating 478 00:25:33,560 --> 00:25:34,970 shape at this point. 479 00:25:34,970 --> 00:25:39,010 Let me switch that to a triangular shape. 480 00:25:39,010 --> 00:25:42,500 And, again, it's a little hard to sink in both the modulating 481 00:25:42,500 --> 00:25:45,150 signal and the carrier, so you see the carrier kind of 482 00:25:45,150 --> 00:25:47,630 wondering by, but there it is. 483 00:25:47,630 --> 00:25:49,950 Another thing that can be varied is the amplitude of the 484 00:25:49,950 --> 00:25:50,950 modulating signal. 485 00:25:50,950 --> 00:25:52,280 I'll make it smaller-- 486 00:25:52,280 --> 00:25:54,960 you'll see the side bands go away-- 487 00:25:54,960 --> 00:25:58,070 until finally we have a pure sinusoidal carrier-- 488 00:25:58,070 --> 00:26:00,040 as I bring them back in. 489 00:26:00,040 --> 00:26:01,440 Again, this is triangular. 490 00:26:01,440 --> 00:26:04,160 There would be harmonics there that maybe they're a little 491 00:26:04,160 --> 00:26:06,560 hard to see in the spectrum analyzer. 492 00:26:06,560 --> 00:26:10,570 I'll go back first to a square wave modulating signal, which 493 00:26:10,570 --> 00:26:14,630 is, again, you can see the square wave on top and one on 494 00:26:14,630 --> 00:26:15,740 the bottom. 495 00:26:15,740 --> 00:26:18,480 PROFESSOR: And so all of this, then, represents side bands. 496 00:26:18,480 --> 00:26:19,050 Is that-- 497 00:26:19,050 --> 00:26:19,330 SANDY HILL: That's right. 498 00:26:19,330 --> 00:26:23,450 Those are the side bands due to the square wave modulation. 499 00:26:23,450 --> 00:26:26,130 Going back to the simplest, the sinusoidal carrier. 500 00:26:26,130 --> 00:26:30,330 Another thing we can do is to vary the frequency of the 501 00:26:30,330 --> 00:26:32,790 sinusoidal signal that's modulating it. 502 00:26:32,790 --> 00:26:34,890 I'll make it higher. 503 00:26:34,890 --> 00:26:37,650 And you'll notice that the side bands wander away from 504 00:26:37,650 --> 00:26:40,230 the carrier signal in the spectrum. 505 00:26:40,230 --> 00:26:43,130 The spectrum, the carrier signal of that, doesn't have a 506 00:26:43,130 --> 00:26:46,040 frequency that's changing as I do this. 507 00:26:46,040 --> 00:26:48,900 It's just the width of the band, due to 508 00:26:48,900 --> 00:26:49,990 the modulating signal. 509 00:26:49,990 --> 00:26:51,350 PROFESSOR: So this is the carrier. 510 00:26:51,350 --> 00:26:52,740 And these are the side bands. 511 00:26:52,740 --> 00:26:53,280 SANDY HILL: That's right. 512 00:26:53,280 --> 00:26:56,570 Now as we go out very far, those side bands get gobbled 513 00:26:56,570 --> 00:26:58,215 up by the carrier itself. 514 00:26:58,215 --> 00:27:01,510 I'll come back to a nice reasonable point there. 515 00:27:01,510 --> 00:27:09,060 PROFESSOR: OK, now, we can change also the carrier 516 00:27:09,060 --> 00:27:13,790 frequency and, as Sandy's indicated, various parameters. 517 00:27:13,790 --> 00:27:17,690 Now let me just point out, since I didn't previously, 518 00:27:17,690 --> 00:27:21,380 that the frequency scale that we're looking at here is a 519 00:27:21,380 --> 00:27:24,790 frequency scale out to 20 kilohertz. 520 00:27:24,790 --> 00:27:28,770 And we had talked about-- or Sandy had indicated-- 521 00:27:28,770 --> 00:27:33,210 that we can change the carrier signal shape. 522 00:27:33,210 --> 00:27:38,550 And let me change the carrier signal from a sine wave to a 523 00:27:38,550 --> 00:27:39,970 square wave. 524 00:27:39,970 --> 00:27:43,190 Now you haven't seen any change on this particular 525 00:27:43,190 --> 00:27:47,680 display , but let me change the frequency scale. 526 00:27:47,680 --> 00:27:54,060 And what you see now is the modulating signal showing up 527 00:27:54,060 --> 00:27:58,090 around harmonics associated with the carrier. 528 00:27:58,090 --> 00:28:03,100 And those harmonics will go away when we go back to a 529 00:28:03,100 --> 00:28:04,630 sinusoidal carrier. 530 00:28:04,630 --> 00:28:06,300 SANDY HILL: This is actually called a ring modulator. 531 00:28:06,300 --> 00:28:09,600 It's very simple to multiply a signal by a square wave. 532 00:28:09,600 --> 00:28:11,220 It's just a chopping process. 533 00:28:11,220 --> 00:28:13,770 And so a square wave carrier signal is very 534 00:28:13,770 --> 00:28:15,380 convenient to generate. 535 00:28:15,380 --> 00:28:19,676 And then you simply filter out the higher order harmonics. 536 00:28:19,676 --> 00:28:21,660 PROFESSOR: Right, I'll go back to the 537 00:28:21,660 --> 00:28:23,850 scale that we had before. 538 00:28:23,850 --> 00:28:28,160 And Sandy had also commented that we can 539 00:28:28,160 --> 00:28:31,190 change the carrier level. 540 00:28:31,190 --> 00:28:33,870 And let's do that on the modulator box. 541 00:28:33,870 --> 00:28:38,140 We could do that either by changing a DC offset on the 542 00:28:38,140 --> 00:28:40,700 modulating signal or by changing the amount of carrier 543 00:28:40,700 --> 00:28:41,950 that's injected. 544 00:28:41,950 --> 00:28:46,170 And as we decrease the amount of carrier, in fact, going 545 00:28:46,170 --> 00:28:49,550 down to no carrier at all or almost no carrier. 546 00:28:49,550 --> 00:28:51,280 That's suppressed carrier. 547 00:28:51,280 --> 00:28:55,340 We have only the two side bands and the signal is now 548 00:28:55,340 --> 00:28:58,320 highly over-modulated. 549 00:28:58,320 --> 00:29:05,390 We bring the carrier back up and when we do that, then we 550 00:29:05,390 --> 00:29:11,350 are reducing the percent modulation and simultaneously 551 00:29:11,350 --> 00:29:15,420 obviously related to that is changing the carrier level. 552 00:29:15,420 --> 00:29:21,230 Now I had indicated in previous lectures that 553 00:29:21,230 --> 00:29:24,270 reducing the carrier is efficient in terms of power 554 00:29:24,270 --> 00:29:30,250 transmission, but requires a synchronous demodulator, 555 00:29:30,250 --> 00:29:34,730 whereas if there's carrier injected, as we have here, so 556 00:29:34,730 --> 00:29:36,200 that we're not over-modulating-- 557 00:29:36,200 --> 00:29:38,700 the percent modulation is less than 100-- 558 00:29:38,700 --> 00:29:42,180 then because of the shape of the time wave form, as you can 559 00:29:42,180 --> 00:29:47,480 see here, you can do the demodulation with a more or 560 00:29:47,480 --> 00:29:50,520 less simple envelope detector. 561 00:29:50,520 --> 00:29:54,500 And in fact, a envelope detector of the type that 562 00:29:54,500 --> 00:29:57,720 we've talked about before is exactly what is used in AM 563 00:29:57,720 --> 00:30:01,560 radios, because of the fact that it's so inexpensive. 564 00:30:01,560 --> 00:30:05,650 And so here the power transmitted required is 565 00:30:05,650 --> 00:30:09,800 higher, but the demodulator, based on using an envelope 566 00:30:09,800 --> 00:30:12,280 detector, is considerably simpler. 567 00:30:15,150 --> 00:30:18,900 Well, speaking of AM radios, what we'd like to now 568 00:30:18,900 --> 00:30:24,030 demonstrate is modulation and demodulation with an AM radio, 569 00:30:24,030 --> 00:30:27,710 which Sandy happens to have here. 570 00:30:27,710 --> 00:30:31,670 And let's see, I guess, what you're going to do, Sandy, is 571 00:30:31,670 --> 00:30:32,815 take it apart for us, right? 572 00:30:32,815 --> 00:30:33,070 SANDY HILL: Right. 573 00:30:33,070 --> 00:30:34,930 I've taken off the back. 574 00:30:34,930 --> 00:30:39,260 And I'm just now going to slip out the guts of it all, and 575 00:30:39,260 --> 00:30:42,940 try and set it up so you can see it conveniently, such as 576 00:30:42,940 --> 00:30:44,846 right there. 577 00:30:44,846 --> 00:30:47,140 PROFESSOR: Now, this is your daughter's radio that you 578 00:30:47,140 --> 00:30:47,750 promised to give back.? 579 00:30:47,750 --> 00:30:50,480 SANDY HILL: I would say was my daughter's radio. 580 00:30:50,480 --> 00:30:53,250 And now let's start attaching clip leads, because what we're 581 00:30:53,250 --> 00:30:56,480 going to want to do is to be able to see the signals coming 582 00:30:56,480 --> 00:30:57,580 out of the radio. 583 00:30:57,580 --> 00:31:00,890 We want to see both the audio and then the modulated signal 584 00:31:00,890 --> 00:31:02,800 and try and see them on the oscilloscope. 585 00:31:02,800 --> 00:31:05,005 So I'll start setting that up now. 586 00:31:05,005 --> 00:31:05,700 PROFESSOR: OK. 587 00:31:05,700 --> 00:31:06,760 Let me-- 588 00:31:06,760 --> 00:31:11,220 while you're doing that-- also just comment that the kind of 589 00:31:11,220 --> 00:31:13,790 AM radio that we're looking at here is called a 590 00:31:13,790 --> 00:31:15,350 superheterodyne receiver. 591 00:31:15,350 --> 00:31:19,380 And the way that it does the demodulation is not exactly 592 00:31:19,380 --> 00:31:20,950 the way we've talked about it in the lectures. 593 00:31:20,950 --> 00:31:22,320 It's very close. 594 00:31:22,320 --> 00:31:25,770 The idea is this the RF signal, the radio frequency 595 00:31:25,770 --> 00:31:29,790 signal, first gets modulated down to what's called an IF 596 00:31:29,790 --> 00:31:32,320 stage-- the intermediate frequency stage. 597 00:31:32,320 --> 00:31:36,840 And then it's that signal that goes to an envelope detector 598 00:31:36,840 --> 00:31:39,280 of the type we've talked about to generate 599 00:31:39,280 --> 00:31:41,930 the demodulated signal. 600 00:31:41,930 --> 00:31:45,900 So, I guess what Sandy has some probes on are the RF 601 00:31:45,900 --> 00:31:48,540 input and a ground-- 602 00:31:48,540 --> 00:31:49,260 SANDY HILL: That's right. 603 00:31:49,260 --> 00:31:50,960 The red lead here is a ground. 604 00:31:50,960 --> 00:31:53,650 It's common for both the signals we'll be looking at. 605 00:31:53,650 --> 00:31:54,520 This is at the audio. 606 00:31:54,520 --> 00:31:56,660 We're actually looking at the signal going directly into the 607 00:31:56,660 --> 00:31:58,720 speaker across these two leads. 608 00:31:58,720 --> 00:32:01,810 And then this probe down here, which I found by hunting and 609 00:32:01,810 --> 00:32:05,740 pecking, is the IF signal, the intermediate frequency signal, 610 00:32:05,740 --> 00:32:08,490 with a station on it that I'll now try and get something that 611 00:32:08,490 --> 00:32:09,380 sounds reasonable. 612 00:32:09,380 --> 00:32:12,146 [RADIO FEEDBACK] 613 00:32:12,146 --> 00:32:12,610 SANDY HILL: OK. 614 00:32:12,610 --> 00:32:13,710 A little sports. 615 00:32:13,710 --> 00:32:15,755 PROFESSOR: Never quite know what you're going to get when 616 00:32:15,755 --> 00:32:16,910 you turn this on. 617 00:32:16,910 --> 00:32:18,160 SANDY HILL: That's right, you don't. 618 00:32:22,380 --> 00:32:23,960 Make some adjustments there. 619 00:32:23,960 --> 00:32:27,500 And what you can see right here is the audio signal. 620 00:32:27,500 --> 00:32:31,480 I'll give it a little bit more game. 621 00:32:31,480 --> 00:32:34,510 And this is the actual intermediate frequency, 622 00:32:34,510 --> 00:32:36,380 modulated by the audio. 623 00:32:36,380 --> 00:32:39,010 And there's a correspondence between them that goes by 624 00:32:39,010 --> 00:32:42,180 awfully fast, but I think it's pretty simple to see. 625 00:32:42,180 --> 00:32:45,550 PROFESSOR: So basically the top trace is the envelope of 626 00:32:45,550 --> 00:32:46,170 the bottom right. 627 00:32:46,170 --> 00:32:47,000 SANDY HILL: That's right. 628 00:32:47,000 --> 00:32:49,860 This is what comes out of the envelope detector. 629 00:32:49,860 --> 00:32:52,800 And this is the signal into the envelope detector, in this 630 00:32:52,800 --> 00:32:55,930 particular radio. 631 00:32:55,930 --> 00:32:59,850 What I'd like to do for a last experiment is what we've been 632 00:32:59,850 --> 00:33:00,700 doing at this point-- 633 00:33:00,700 --> 00:33:02,470 I'll turn this off for a moment-- 634 00:33:02,470 --> 00:33:05,190 is we're taking something that's coming in over the 635 00:33:05,190 --> 00:33:08,350 airwaves, as they used to say, and we're simply viewing it. 636 00:33:08,350 --> 00:33:10,690 What I'd like to do now is generate our own radio 637 00:33:10,690 --> 00:33:13,390 frequency signal, tune it up so this radio is 638 00:33:13,390 --> 00:33:15,160 going to hear it-- 639 00:33:15,160 --> 00:33:16,300 whatever that means-- 640 00:33:16,300 --> 00:33:18,710 and display it here. 641 00:33:18,710 --> 00:33:23,170 What we're going to do then is use the audio oscillator, 642 00:33:23,170 --> 00:33:26,710 along with the amplitude modulator. 643 00:33:26,710 --> 00:33:30,060 And we're going to take from the output now the lead here. 644 00:33:30,060 --> 00:33:32,020 And this will act as the antenna. 645 00:33:32,020 --> 00:33:34,900 This device was not designed to put out a lot of power. 646 00:33:34,900 --> 00:33:37,550 But fortunately we don't have a lot of distance to go over. 647 00:33:37,550 --> 00:33:40,480 What I'm going to do is turn myself into an antenna-- 648 00:33:40,480 --> 00:33:43,250 I'm basically 150 pounds of salt water-- 649 00:33:43,250 --> 00:33:46,380 and when I touch this, the signal coming in here, which 650 00:33:46,380 --> 00:33:49,550 is radiating a little from the wire, is going to suddenly 651 00:33:49,550 --> 00:33:52,150 radiate from my whole body and hopefully will be enough 652 00:33:52,150 --> 00:33:53,450 to be picked up. 653 00:33:53,450 --> 00:33:58,130 So what I'll do is I'm going to turn this on-- 654 00:33:58,130 --> 00:33:59,800 excuse the noise. 655 00:33:59,800 --> 00:34:02,850 Now I've set this up so it's tuned to a particular radio 656 00:34:02,850 --> 00:34:04,900 station, and I hope I can find it now. 657 00:34:07,850 --> 00:34:09,100 It takes a little fussing. 658 00:34:14,179 --> 00:34:15,429 That's not it. 659 00:34:18,060 --> 00:34:19,710 That sounds like it now. 660 00:34:19,710 --> 00:34:22,210 OK. 661 00:34:22,210 --> 00:34:25,929 PROFESSOR: So this gets billed as Sandy Hill, human antenna. 662 00:34:25,929 --> 00:34:27,580 SANDY HILL: That's right. 663 00:34:27,580 --> 00:34:30,920 What you're looking at here is I've got the antenna at this 664 00:34:30,920 --> 00:34:33,630 point, and what we're going to do is you're 665 00:34:33,630 --> 00:34:35,110 seeing the audio signal. 666 00:34:35,110 --> 00:34:37,980 This is coming from the signal generator up here. 667 00:34:37,980 --> 00:34:42,699 And this is the intermediate frequency signal again. 668 00:34:42,699 --> 00:34:45,389 You can hear it's much stronger when my body becomes 669 00:34:45,389 --> 00:34:47,250 the antenna. 670 00:34:47,250 --> 00:34:48,909 And I'll leave it on for a second. 671 00:34:48,909 --> 00:34:51,872 And I can switch to square wave-- 672 00:34:51,872 --> 00:34:54,199 doesn't look much like a square wave. 673 00:34:54,199 --> 00:34:56,590 Various things. 674 00:34:56,590 --> 00:34:59,780 There's significant distortion, because of all the 675 00:34:59,780 --> 00:35:02,290 transformations that the signal is going through. 676 00:35:02,290 --> 00:35:06,870 But it does indeed work, and we're picking up a frequency 677 00:35:06,870 --> 00:35:09,360 that the radio is tuned to. 678 00:35:09,360 --> 00:35:10,170 And that's it. 679 00:35:10,170 --> 00:35:13,590 PROFESSOR: So Sandy has now been modulated and 680 00:35:13,590 --> 00:35:14,080 demodulated. 681 00:35:14,080 --> 00:35:16,280 SANDY HILL: That's right. 682 00:35:16,280 --> 00:35:19,090 PROFESSOR: Well, hopefully, all the things that we've gone 683 00:35:19,090 --> 00:35:23,800 through in the process of this tape and this set of 684 00:35:23,800 --> 00:35:27,250 demonstrations gives you a feel for at least some of the 685 00:35:27,250 --> 00:35:30,920 things that we've talked about so far in the course. 686 00:35:30,920 --> 00:35:35,430 And Sandy, I'd really like to thank you for joining us, for 687 00:35:35,430 --> 00:35:38,260 sharing your insights with us, as well as sharing your 688 00:35:38,260 --> 00:35:38,940 equipment with us. 689 00:35:38,940 --> 00:35:39,300 Thanks a lot. 690 00:35:39,300 --> 00:35:40,210 SANDY HILL: It was a treat. 691 00:35:40,210 --> 00:35:41,460 Thank you.