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Valve Radio and Audio Repair Handbook by CHAS MILLER

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Chapter 15

Frequency modulation

The principle of frequency modulation (FM) is that the ćarrier wave of a transmitter receives a constant level of modulation, but that its frequency deviates by a predetermined amount above and below the nominal figure. The standard deviation has been for many years 75 kc/s. The applied modulation decides the number of deviations that occur per second, e.g. an applied modulation of 400 c/s produces 400 deviations every second, and so on. The system is capable of carrying the full AF range with ease. This method of modulation had been tested as long ago as just before the First World War, when it seemed likely to prove easier to achieve with the primitive equipment then available, but improvements to valves and other ...

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