Modulation — Page 48, Lesson 43

We're starting a brand-new topic now: modulation. This is the heart of how radio navigation actually works, so I want to build it up carefully from the ground floor. Modulation is the name given to the process of adding information to a radio wave, or the formatting of radio waves for other purposes. Think of the radio wave as a blank carrier — modulation is how we write information onto it. Of the main forms of modulation, five have application in aviation: Keyed Modulation, Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation, and Pulse Modulation. Now, the modulation of a radio frequency is generally associated with the transmission of audio information. But I want you to note that it's not only for audio — the transmission of data, including that in satellite navigation systems, and the determination of bearing in VOR, for example, require modulation for other purposes. So modulation is a general tool, not just for voice. Before an audio signal can be added to a radio wave, it must be converted to an electrical signal. This is achieved by the use of a microphone, which is quite simply a device that converts sound waves to an electrical current. For AM and FM, we'll assume this conversion has already been accomplished. Let's start with the simplest form: Keyed Modulation. The simplest way to put information onto a carrier wave is to quite simply interrupt the wave to give short and long bursts of energy. By arranging the transmissions into short and long periods of carrier wave transmission, we can send information using the 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, or NDBs, for identification — we'll discuss that further in Chapter 7. Now let's move to the big one: Amplitude Modulation, or AM. In AM, the amplitude of the audio frequency — the AF — modifies the amplitude of the radio frequency — the RF. Look at the diagram: positive amplitude in the AF gives an increase in amplitude in the RF, and negative amplitude in the AF gives a decrease in amplitude in the RF. So the audio signal is literally shaping the height of the radio wave. The process of combining a radio frequency with a current at audio frequencies is known as heterodyning. Let's look at this in detail, because it's the key mechanism. 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 a concrete example. An audio frequency of 3 kHz is used to amplitude modulate a radio frequency of 2182 kHz. The RF remains unchanged, but the AF is now split into 2 sidebands. There's an upper sideband, the USB, extending upwards from 2182.001 kHz to 2185 kHz. And there's a lower sideband, the LSB, extending downwards from 2181.999 kHz to 2179 kHz. The spread of frequencies is from 2179 kHz to 2185 kHz, giving a bandwidth of 6 kHz — that is, double the audio frequency used. So the bandwidth is exactly twice the audio frequency. That's a relationship worth holding onto: 3 kHz audio gives 6 kHz bandwidth. The carrier itself stays at 2182 kHz, and the information is carried in those two sidebands around it. That's the core of AM — the carrier plus the upper and lower sidebands, with the bandwidth equal to twice the modulating audio frequency.