 Hi, I'm Zor. Welcome to Unisor Education. Today we will continue talking about radio transmission, in particular how the AM radio works. This lecture is part of the course called Physics 14's presented on Unisor.com. I suggest you to watch this lecture from the website because this lecture is just part of the course and the website contains menu and all the lectures are logically connected plus every lecture has detailed notes, there are some problems solving. There is a prerequisite course called Mass 14's on the same website which I do recommend you to take. You do need a lot of mass when starting Physics. Okay, so back to radio transmission. So AM radio, it stands for Amplitude Modulation Radio. And we are talking about radio in terms of transmitting sound. Actually, obviously right now radio waves are electromagnetic waves and they are delivering television images, data, etc., etc. But just for our purposes it's quite sufficient to talk about sound. Now, what is at the heart of any kind of radio transmission and receiving? Well, we were all studying so-called LC circuit, which is basically a capacitor and inductor connected into a circuit. And the main property of this circuit is that with proper arrangement, additional sources of energy, etc., the oscillations of electric current will occur in this circuit in both directions with certain periodicity, with certain frequency. Now, in many cases there is also a resistor here, so sometimes it's called RLC circuit. But in any case, it's the inductor and capacitor which are actually playing this oscillation role. So it's capable of oscillating at certain frequency, which depends on parameters of these two devices. And if we ignore for a while the resistor, the angular frequency will be calculated using this formula. That's what we know. So it has certain properties. Now, if we will make this a variable capacitor, then we can change this and we can change this resonant frequency, which means that it can serve as a receiver. Now, as far as transmitting, well, obviously a receiver, if we will have some kind of antenna here connected maybe with another inductor through some kind of inductive transformation of energy. So with an antenna it serves as a receiver. Now, with an antenna and a good source of energy, some kind of batteries or whatever, the oscillations of this particular circuit, oscillation current in this circuit, again using much more powerful of this antenna, can serve as a transmitter of the oscillations. Now, in practice, we have many different sources of electromagnetic oscillations, many different transmitters. And whenever we want to receive a particular signal, now all these are transmitting at different frequencies, which depends on characteristics of circuits in any case. And this one can, using the tuning, can basically tune to this or this or this. And that's how we receive a particular signal from a particular transmitter. Okay, that's all great and dandy. And what's the purpose of all this? I mean, why do we have to transmit oscillations of certain frequency? They don't carry any information. They don't carry so far any sound. We have to somehow impose another input, a sound, to this transmission. So it will somehow be received and somehow interpret it and somehow produce the sound which we can hear. So this is a completely different task. So one task is just to transmit certain frequency from this guy, another frequency from this, another frequency from this. It's called carrier frequency. The signal is carried by these frequencies. And a completely different story is how to impose sound. So we need some kind of good idea. And the idea came in terms of amplitude modulation. So here is basically the story. Let's just assume that these oscillations are a significantly high frequency. Now, the sound has a different frequency. Usually the sound which human ear senses is from 20 hertz, which means oscillations per second. So it's actually one over second, the dimension, to 20,000 hertz. So let's assume that this range is significantly slower, I mean less frequent. These numbers are less than the frequency of this guy. Which means on the graph it will look something like this. The idea of amplitude modulation is let's reduce the amplitude of these oscillations of carrier, base carrier frequency. In sync with these. So in this particular case I will impose increasing or decreasing the amplitude of these oscillations in sync with this guy. And as a result I will have something like this. So the oscillation will still have exactly the same frequency. Frequency will be the same. But the amplitude, this is the constant amplitude, which we were talking about before. Now, with a sound imposing on these oscillations as far as their amplitude is, it's the same frequency, but different amplitude. Let's not talk about how. What's important is the idea. So this idea came to somebody's mind and we are not talking about who that was. There was a big discussion about this, who was the first guy. But whoever the first guy was, he basically came up with this idea. And since oscillations are transmitted and can be tuned to this particular radio station for example, we will have corresponding oscillations in the LC circuit on the receiver side. Now, if these oscillations are of variable amplitude, these oscillations will be of variable amplitude, obviously. I mean, we can somehow make it stronger or whatever it is. The amplifiers can be involved. These are all fringes. These are less important than the main idea. So the main idea is we will impose this sound to change the amplitude. And this change of amplitude will be transmitted to a receiver. The receiver will tune in because tuning is based on the frequency only. So if it's tuned to this particular radio station, it will always receive these signals. But the signals which we receive as a resonance in the receiver LC circuit will have different amplitude. And these amplitudes will be, again, in sync with the sound waves which have been put at the source of the whole thing. So this is the idea. Okay, now, what's next? Okay, the frequency which is used in AM radio, the frequency of the base of the carrier frequency are from 540,000 Hz to 1,600 Hz. But usually people are using kilohertz, which is 1,000 Hz, which is 540,000 Hz to 1,600,000 Hz. So these are different frequencies which are allocated for different radio stations which would like to transmit in the AM radio. There are other principles. There is a frequency modulator, which is FM radio, that would be the subject of the next lecture. But right now for the AM radio, the standard, at least for the United States, I'm not sure about other countries, but for the United States these are the minimum and maximum frequencies these AM radio transmitters can use. So just for example, there is a WCBS radio, which I can tune in in New York. It has 880 kilohertz between these two. That's what that particular other stations have. The different frequencies are assigned to different radio stations. They are probably applying for some kind of a license. They're supposed to pay something, I guess, for this. So they have assigned frequency. Usually I think it's increments of 10, like 880 kilohertz and 890 maybe. I'm not sure exactly. But anyway, it's assigned. These frequencies are fixed and assigned to different radio stations. If all the different frequencies are already taken, that means that there is no more new stations unless they will buy something, some other old station with their frequencies. So as you see, these frequencies, any of them, even the smallest one, is significantly greater than the highest sound frequency. Now, why is this important? Well, I mean, obviously the precision of the sound, the quality of the sound depends on how closely this curve on the sound will be transmitted and received and interpreted by the receiver side. No matter how good your microphone is on this side and how well it translates the sound into electric current, unless this thing is of very high quality, we will lose the details because the sound waves are not really such smooth sound waves. To tell you the truth, the sound waves are really something like this. Because any sound contains many different frequencies which are kind of super positioned onto each other. So it's a combination of, if you wish, different trigonometric functions like sine of 2x plus 3sine of 1.5x, etc. So each one has its own frequency because the voice produced by the person, for example, is not just one fixed frequency on which there is a sound. Even that has probably many different octaves combined into one sound. So it's oscillations with different frequencies and these different frequencies are superimposed onto each other resulting current in the electrical, after the microphone, the resulting current will have really very, very broken kind of a line, if you wish. And considering there are many details, you need many very high frequencies of carrier to basically reflect with each amplitude to reflect corresponding sound. So the more broken kind of line of the sound, the more frequent I need the carrier to get into each, with certain oscillation to get into each peak or trough of the sound. So that's why it's very important to have much higher frequency in the carrier than the frequency of the sound itself. Now, let me just give you an example. There is one particular note, I think it's called high A. And its high A is what? Okay, high A is 440, high A is 440 Hz. So there is a pitch, one particular note on the piano, for example, and this particular note is an oscillation with this particular frequency. I mean, it's just a standard, basically, for musical instruments, for example. Now, if you take, for instance, this carrier frequency and this note, what's the difference? This is hertz and this is kilohertz, so it's 1000, so it's 2000 times more frequent, this one than this. Which means that for each oscillation of the sound, I have 2000 oscillations of the carrier symbol. So that's relatively good quality. However, as you go into higher pitches, for instance, the very, very high pitch, which is 20,000 Hz, 20 kilohertz, the difference is between 20 kilohertz and 80 kilohertz, it's only 440 times. So this is 2000, this is 440. It's lower quality. So the high pitches will be represented by a smaller number of bass oscillation, carrier waves oscillations, which means it's not as precise, let's say. So that's why AM transmission is considered to be of a lower quality and something like a hi-fi sound usually is not transmitted through AM. There are different technologies which we will talk about next, actually. But this was the first and I would say the most understandable, at least for me, quite frankly, how the amplitude modulation can be used to transmit the sound. Now, another interesting point is the wavelength. Now, this is a frequency, so it's 880,000 oscillations per second. Now, the radio waves are propagating with the speed of light, which is 300,000 kilometers per second. That's the speed. So this is the speed. Every second the light goes by 300 million meters. Now, every second we have this many oscillations. So we will divide C by F, we will have lambda, the length of the wave, the wavelength. That's one period how long it is. This is the period from 2. Now, so in this particular case, for this particular frequency and this approximate, this is approximate speed of light, the lambda would be 341 meter. So the period, I mean the length of the wave is 341 meter. It's relatively long, which means it could go around large objects. So that's why an M-radio is traveling on certain long distances. More than that, in certain cases around the Earth we have so-called ionosphere. So it's basically electrically charged particles. And they present like a mirror, so to speak, and the electromagnetic waves of this type, of this length, they are reflected from ionosphere. Which means that if this is the Earth, and this is the source of electromagnetic waves, and this is ionosphere, it can actually go relatively far. I mean obviously it's getting weaker and weaker the signal. But in any case, I do remember basically when I lived in the Soviet Union, I was using the short waves radio to basically have signals from Great Britain, from the United States, from France, etc. So there are different ranges, and the range for AM radio is usually sufficient to transmit, but it's called long waves, middle range waves, and short waves, as opposite to ultra short waves, which can be received only within the direct, non-obstructed, so to speak, distance. So that's basically all about AM radio. Now the last question is how to implement this modulation. How to convert sound into a different amplitude. So how can we modulate a base frequency using the input from the sound? Well, this is something which can be done again many, many different waves, and basically the simplest thing is to introduce a variable resistor. So if you have a microphone, here comes the microphone, and obviously the result of the microphone is certain current in certain circuit. Now if this current somehow is triggering a variable resistor, and that variable resistor somehow affects the, this variable resistor somehow is part of this circuit, which is producing the high frequency carrier waves, then obviously since it's a resistor, it will change the amplitude of the current in this particular circuit without changing the frequency of oscillations, because frequency depends on primarily on these two. I mean, resistors also kind of affects the frequency, but we can really manipulate it in some way so that it's not really a big factor. But what's a big factor for a resistor is it will change the amplitude, because according to the Ohm's law, the current is inversely proportional to the resistor. You increase the resistor, the current goes down. So that's how you modulate, that's how you change the amplitude in the transmitting circuit. And in the receiving circuit, since it's received through the antenna, the weaker signal will be weaker, and the stronger will be stronger. So that's how you transmit. And then obviously you need amplifiers and filters and all that. Let me tell you one thing. Whatever I'm talking about and making even some circuits on the board, it has absolutely no comparison in complexity with the circuits which are really used. I mean, the radio technology has developed for, what, 150 years or something like that or more. And obviously the quality and the speed and all these convenience is significantly changed since the time when it was invented. But my purpose is not to go into the details of contemporary implementation of radio. This is for radio engineers, this is for specials. This course is basically for people who are interested in physics in general. And that's why I'm presenting principles, how it started actually. Because the principle remains, this circuit is still there. It's implemented not using some kind of archaic, length-based triodes or whatever. It's based on contemporary integrated circuits, etc. So the technology has grown up. But the principles, and that's exactly what I'm trying to convey to you, principles are still there. And it's very important to understand the principle before you go into the details and that depends on your profession, etc. It's only for professionals, obviously. Principles can actually be shared with everybody. That's it. I do suggest you to read the notes for this lecture on the website, Unison.com. So you go to Physics 14's course. This is the part which is called Waves. And then in this part I have a radio part, radio topic, so to speak. And this is one of the lectures, amplitude modulation, one of the lectures about radio. That's it. Thanks very much. And good luck.