WEBVTT 00:00:04.310 --> 00:00:09.610 Lot's of things in our lives transmit signals. From your cell phone when it's making a call, 00:00:09.610 --> 00:00:15.660 to your computer when it's sending an email, to your local radio station when it's broadcasting. 00:00:15.660 --> 00:00:20.369 Here you see two objects that receive signals - they are radio receivers. Have you ever 00:00:20.369 --> 00:00:24.580 wondered how they pick out the signal they want out of all of the radio waves around 00:00:24.580 --> 00:00:29.029 them? In this video, we're going to find out. 00:00:29.029 --> 00:00:35.180 This video is part of the information flow video series. A system is shaped and changed 00:00:35.180 --> 00:00:40.310 by the nature and flow of information into, within, and out of the system. 00:00:40.310 --> 00:00:45.360 Hi, my name is Elena Glassman, and I am a graduate student in the Electrical Engineering 00:00:45.360 --> 00:00:49.440 and Computer Science Department at MIT. 00:00:49.440 --> 00:00:54.490 Before watching this video, you should be familiar with basic electrical circuits, and 00:00:54.490 --> 00:00:59.540 how inductors, capacitors, and AM radio transmitters work. 00:00:59.540 --> 00:01:03.770 After watching this video, you will be able to explain how a basic radio receiver circuit 00:01:03.770 --> 00:01:10.770 functions to select particular radio frequencies. 00:01:13.049 --> 00:01:19.049 This Hammarlund HQ120 communications receiver was introduced in 1938. It tunes in radio 00:01:19.049 --> 00:01:25.590 waves ranging from 540 thousand cycles per second to 30 million cycles per second. It 00:01:25.590 --> 00:01:31.439 can decode signals encoded with Amplitude Modulation, abbreviated as AM, or "continuous 00:01:31.439 --> 00:01:35.408 wave" modulation, abbreviated as CW. 00:01:35.408 --> 00:01:41.749 This Drake R-4 was introduced later, in 1964, and is optimized for amateur radio bands within 00:01:41.749 --> 00:01:47.819 the same frequency range covered by the HQ120. Both of these radios were popular with amateur 00:01:47.819 --> 00:01:52.530 radio operators. Amateur radio operators are radio enthusiasts 00:01:52.530 --> 00:01:57.399 who pass U.S. Federal Communications Commission tests to receive official government call 00:01:57.399 --> 00:02:01.170 signs and the right to broadcast on select bands. 00:02:01.170 --> 00:02:06.009 Amateur radio operators are anything but amateur, serving as part of an important worldwide 00:02:06.009 --> 00:02:11.890 network. When cell phones, the internet, or other communication networks are down, amateur 00:02:11.890 --> 00:02:17.810 radio operators can help pass along important information. Amateur radio operators have 00:02:17.810 --> 00:02:22.650 historically experimented with and advanced cutting-edge radio technologies. (viewers 00:02:22.650 --> 00:02:25.640 hear radio making some noise) 00:02:25.640 --> 00:02:30.099 Let's delve deeper into how these radios receive information. 00:02:30.099 --> 00:02:35.599 Recall that radio waves are electromagnetic waves whose frequencies fall in a certain 00:02:35.599 --> 00:02:42.500 range. A variety of data types can be transmitted over radio waves by systematically modulating 00:02:42.500 --> 00:02:44.730 some property of the wave. 00:02:44.730 --> 00:02:50.739 The information we want to transmit--say speech or music--has a much lower frequency than 00:02:50.739 --> 00:02:56.709 the carrier radio wave. We can encode this lower frequency by modulating the amplitude 00:02:56.709 --> 00:03:03.140 of the carrier wave. This is called Amplitude Modulation, which is used in AM radio. 00:03:03.140 --> 00:03:09.629 CW radio stands for continuous wave modulation and is traditionally used to transmit Morse 00:03:09.629 --> 00:03:12.870 code by switching the carrier signal on and off. 00:03:12.870 --> 00:03:19.870 All of the signals being transmitted around the globe are superimposed on to each other. 00:03:20.360 --> 00:03:26.299 Our goal is to understand how a basic radio circuit, called the regenerative circuit, 00:03:26.299 --> 00:03:32.150 works to pick out one frequency. This circuit was a breakthrough in radio technology both 00:03:32.150 --> 00:03:35.930 in terms of amplification and selectivity. 00:03:35.930 --> 00:03:42.329 It was invented in 1914 by American electrical engineer Edwin Armstrong when he was an undergraduate 00:03:42.329 --> 00:03:44.700 at Columbia University. 00:03:44.700 --> 00:03:51.379 This circuit was widely used in radio receivers, called regenerative receivers, between 1920 00:03:51.379 --> 00:03:58.379 and World War II. They are still used in low-cost electronic equipment such as garage door openers. 00:03:58.890 --> 00:04:05.890 Our Hammerlund and Drake radios use more sophisticated circuitry, also invented by Armstrong. However, 00:04:05.890 --> 00:04:12.890 we're going focus on the more basic and still powerful regenerative circuit. 00:04:15.939 --> 00:04:22.939 The regenerative circuit seen here is a classical, elegant electrical circuit that amplifies 00:04:23.229 --> 00:04:27.250 while selecting a particular radio wave frequency. 00:04:27.250 --> 00:04:33.800 Let's start by identifying the elements from this diagram in our actual radio. This is 00:04:33.800 --> 00:04:38.340 the antenna for the radio. And this is the symbol for that antenna in 00:04:38.340 --> 00:04:43.220 our diagram. This dial allows us to control the frequency 00:04:43.220 --> 00:04:48.470 we're selecting. And this is the variable capacitance element that determines which 00:04:48.470 --> 00:04:53.000 frequency this basic regenerative receiver circuit will select for. 00:04:53.000 --> 00:04:55.490 This is a vacuum tube in the radio. 00:04:55.490 --> 00:05:01.680 And in the diagram. It is going to act as an amplifier. 00:05:01.680 --> 00:05:06.400 Together the volume control and the speaker enable us to hear the information decoded 00:05:06.400 --> 00:05:08.360 from the radio signal. 00:05:08.360 --> 00:05:14.020 These are represented by a variable resistor and the headphone cartoon in our diagram. 00:05:14.020 --> 00:05:19.569 Now let's understand how this circuit works! Radio waves induce waves of alternating current 00:05:19.569 --> 00:05:24.720 through the antenna. By the phenomenon of inductive coupling, the radio wave energy 00:05:24.720 --> 00:05:30.300 is picked up by this tuned circuit, which is just an inductor and variable capacitor 00:05:30.300 --> 00:05:36.300 in parallel. This circuit acts like an echo chamber for radio waves. 00:05:36.300 --> 00:05:41.110 To get a better sense of how this parallel inductor and capacitor work, let's look at 00:05:41.110 --> 00:05:44.580 a system with which you may be more familiar--brass instruments. 00:05:44.580 --> 00:05:50.500 Brass players create lots of sound frequencies when they blow through their mouthpieces. 00:05:50.500 --> 00:05:54.729 [sound of buzzing] The brass tube acts as a resonant cavity or 00:05:54.729 --> 00:06:01.139 echo chamber. Sound waves bounce back and forth between each end of the tube. 00:06:01.139 --> 00:06:06.099 Particular frequencies constructively interfere or reinforce each other, while other frequencies 00:06:06.099 --> 00:06:11.370 destructively interfere, and therefore have their amplitude diminished. 00:06:11.370 --> 00:06:17.259 As the slide is moved, the length of the tube changes, and that changes which frequencies 00:06:17.259 --> 00:06:23.680 are reinforced and which frequencies are damped. This parallel inductor and capacitor act the 00:06:23.680 --> 00:06:30.680 same way for radio waves. Changing the capacitance of the capacitor is equivalent to changing 00:06:30.780 --> 00:06:34.430 the length of the trombone's tube by moving the slide. 00:06:34.430 --> 00:06:40.050 The noise produced by buzzing into the mouthpiece and filtering it with the brass tube is sufficient 00:06:40.050 --> 00:06:42.199 to produce audible sound. 00:06:42.199 --> 00:06:48.990 But radio waves may be coming from transmitters very far away, and may have very small amplitudes 00:06:48.990 --> 00:06:55.909 by the time they reach us. It's not uncommon for these receivers to pick up radio stations 00:06:55.909 --> 00:07:02.810 part way around the globe. How are we going to deal with this? We need amplification! 00:07:02.810 --> 00:07:08.099 The radio waves echoing in the tuned circuit here are applied to the input of this vacuum 00:07:08.099 --> 00:07:14.810 tube, which acts as an amplifier. An amplifier reproduces the signal applied to its input, 00:07:14.810 --> 00:07:21.639 but with greater magnitude. The added energy comes from a local power source, like a battery. 00:07:21.639 --> 00:07:26.409 That might be enough amplification, but we can do even better using positive feedback! 00:07:26.409 --> 00:07:33.409 The tickler, located here, takes the amplified radio signal from the vacuum tube and feeds 00:07:34.110 --> 00:07:40.009 it back into the echo chamber, to reinforce new radio waves coming in from the antenna 00:07:40.009 --> 00:07:42.050 at that same frequency! 00:07:42.050 --> 00:07:48.229 A frequency is strengthened by the echo chamber alone, so it becomes even larger in amplitude 00:07:48.229 --> 00:07:54.569 after amplification and positive feedback into the same echo chamber! Likewise, any 00:07:54.569 --> 00:07:59.889 frequencies that did not resonate well in the echo chamber will be diminished further 00:07:59.889 --> 00:08:06.590 with each circulation back through the loop. This echo chamber with its output amplified 00:08:06.590 --> 00:08:12.800 and fed back into itself is what is responsible for achieving our stated goal of selecting 00:08:12.800 --> 00:08:19.800 only one frequency to tune in to, in the presence of all the other frequencies carrying information. 00:08:20.289 --> 00:08:25.590 Now that we have selected and amplified the carrier frequency, we still need to extract 00:08:25.590 --> 00:08:31.129 the original information. 00:08:31.129 --> 00:08:37.940 You might expect our signal after amplification to look like this. But the information encoded 00:08:37.940 --> 00:08:44.940 in the amplitude would be hard to decode because the average amplitude is constant everywhere. 00:08:45.010 --> 00:08:50.860 Luckily for us, the vacuum tube is a nonlinear amplifier. 00:08:50.860 --> 00:08:57.760 It accentuates the lower half of the signal and diminishes the top half, giving us a signal 00:08:57.760 --> 00:09:04.760 that looks more like this. Since headphones are natural smoothing filters 00:09:05.350 --> 00:09:11.510 that convert variations in current to air pressure waves, the user hears the original 00:09:11.510 --> 00:09:18.510 information encoded in the AM signal. If we hadn't accentuated the lower half of the signal 00:09:18.700 --> 00:09:21.149 before smoothing, you wouldn't hear anything. There's one last aspect of this simple regenerative 00:09:21.149 --> 00:09:21.250 receiver we'd like to point out. 00:09:21.250 --> 00:09:21.480 After all this selective amplification of the carrier frequency, we must still extract 00:09:21.480 --> 00:09:21.630 the information carried by the envelope of its amplitude. 00:09:21.630 --> 00:09:21.890 We take advantage of the imperfect amplification of the vacuum tube to accentuate the lower 00:09:21.890 --> 00:09:21.959 half of the signal. 00:09:21.959 --> 00:09:22.180 Since headphones are natural smoothing filters that convert variations in current to air 00:09:22.180 --> 00:09:22.410 pressure waves, the user hears the original information encoded in the AM signal. 00:09:22.410 --> 00:09:29.410 It was extremely clever to use a single tube to amplify, provide selectivity, and demodulate 00:09:30.760 --> 00:09:37.470 an AM signal at a time when tubes were very expensive and considered cutting edge. 00:09:37.470 --> 00:09:43.120 This Drake R-4 radio uses more sophisticated circuitry but even modern radio receivers 00:09:43.120 --> 00:09:50.120 are built with digital circuits that operate on the same basic principles! 00:09:54.180 --> 00:10:00.800 Radio is a cool, handy way to disseminate information, so humans have created lots and 00:10:00.800 --> 00:10:07.290 lots of electromagnetic waves and transmitted them into the air at various frequencies. 00:10:07.290 --> 00:10:11.320 We've even standardized who can transmit and at what frequency. 00:10:11.320 --> 00:10:18.320 Here's the radio spectrum that is available. You see AM and FM broadcast within this spectrum. 00:10:19.120 --> 00:10:25.779 This radio spectrum has been divided up even further for different uses: mobile phones, 00:10:25.779 --> 00:10:31.750 amateur astronomy, satellites, space research, amateur radio, and Earth research. 00:10:31.750 --> 00:10:38.060 Zooming out further we see that lots of different communities have their own spot allocated. 00:10:38.060 --> 00:10:43.470 Out of all of that radio activity, we can now pull out what we personally want. Maybe 00:10:43.470 --> 00:10:49.220 it's finding a friend or fellow amateur radio operator transmitting from another continent, 00:10:49.220 --> 00:10:56.220 or maybe it's the BBC, broadcasting international news. 00:10:58.339 --> 00:11:04.970 To summarize, in this video, we have explained how the components in the regenerative receiver 00:11:04.970 --> 00:11:11.970 circuit work together to dampen unwanted frequencies, while selectively amplifying and demodulating 00:11:12.540 --> 00:11:19.540 a desired AM signal. This is no small feat, given the amount of radio waves humans broadcast! 00:11:19.990 --> 00:11:25.660 We've also demonstrated the tuning process with a Drake R-4 radio receiver built for 00:11:25.660 --> 00:11:32.459 amateur radio operators in the 1960s. Learning about these radio receivers with my dad when 00:11:32.459 --> 00:11:38.649 I was a kid was instrumental in developing my own interest in electrical engineering! 00:11:38.649 --> 00:11:43.160 It was really fun to share some of the circuitry with you. If you want to try something a little 00:11:43.160 --> 00:11:49.320 more hands-on, I recommend checking out the local amateur radio community or signing up 00:11:49.320 --> 00:11:55.740 for a course in electrical engineering!