Modulation — Page 48, Lesson 43

Let's start with the very name of the process. Modulation is the name given to adding information to a radio wave, or formatting radio waves for other purposes. Think of the radio wave as a blank carrier, and modulation is how we write information onto it. Now, of the main forms, five have application in aviation. I want you to hold these five in your head because they're the backbone of the whole subject: Keyed Modulation, Amplitude Modulation (which we call AM), Frequency Modulation (FM), Phase Modulation, and Pulse Modulation. Here's an important point before we go further. Modulation of a radio frequency is generally associated with transmitting audio information. But it's not only that. The transmission of data, including in satellite navigation systems, and the determination of bearing in VOR, for example, require modulation for other purposes. So don't lock modulation into "just voice" — it's used for data and navigation too. Now, before an audio signal can be added to a radio wave, it must be converted to an electrical signal. That's done with a microphone, which is simply a device that converts sound waves into an electrical current. For AM and FM, we'll assume that conversion has already happened. Let's look at the first form: Keyed Modulation. This is the simplest way to put information onto a carrier wave. You simply interrupt the wave to give short and long bursts of energy. By arranging those transmissions into short and long periods of carrier wave transmission, you can send information using Morse code. This is known as telegraphy, and until the development of other forms of modulation, it was the only means of passing information. Keyed modulation is still used by some non-directional beacons, the NDBs, for identification. We'll come back to that in Chapter 7. Now the big one: Amplitude Modulation, or AM. Here's the core idea. In AM, the amplitude of the audio frequency — the AF — modifies the amplitude of the radio frequency — the RF. So the audio signal's strength shapes the radio wave's strength. Look at the diagram. Positive amplitude in the AF gives an increase in amplitude in the RF. Negative amplitude in the AF gives a decrease in amplitude in the RF. So the audio waveform is literally traced onto the radio wave's envelope. Now, the process of combining a radio frequency with a current at audio frequencies is called heterodyning. Let me walk you through that in detail, because it's the heart of AM. The heterodyning process combines the two frequencies, leaving the RF unchanged, but producing new frequencies at the sum and difference of the RF and AF. Here's the concrete example. Take an audio frequency of 3 kHz used to amplitude modulate a radio frequency of 2182 kHz. The RF remains unchanged. But the AF is now split into two sidebands. There's an upper sideband, the USB, extending upwards from 2182.001 kHz to 2185 kHz. And a lower sideband, the LSB, extending downwards from 2181.999 kHz to 2179 kHz. So the spread of frequencies is from 2179 kHz to 2185 kHz. That gives a bandwidth of 6 kHz — which is double the audio frequency used. That's a key relationship to remember: the bandwidth is twice the audio frequency. Let me make sure you see the structure in the diagram. You have the RF carrier at 2182 kHz at 100 watts. The AF is at 3 kHz at 50 watts. Then you have the upper sideband at 2185 kHz at 25 watts, and the lower sideband at 2179 kHz at 25 watts. Notice the sidebands are at the sum and difference — 2182 plus 3 is 2185, and 2182 minus 3 is 2179. And the sidebands start just above and below the carrier, at 2182.001 and 2181.999 kHz. So the whole picture is this: the carrier stays put, and the audio information is carried in those two sidebands, one above and one below. That's amplitude modulation in its essence.