Radio Experimenter Theory Notes

Frequency Modulation (FM)

With Amplitude Modulation, we saw that the information is encoded onto the carrier by varying it's amplitude. Frequency modulation operates by varying the frequency of the carrier in sympathy with the modulating audio signal, but it's amplitude never changes.

With FM, the carrier is known as the centre frequency and the modulation is more correctly called deviation. This is because the carrier frequency varies with the modulation. The rate at which the carrier frequency is deviated is equal to the frequency of the modulating signal. Circuitry at the receiving end detects the slight variations in frequency and converts them back to an audio signal for amplification and reproduction to a loudspeaker.

Wide-Band FM (WBFM) is the standard used for FM broadcast stations. The type of FM that we deal with is Narrow-Band FM (NBFM). They work in the same way, but NBFM uses a much narrower bandwidth which is sufficient for transmitting voice but music would lose it's quality unless using the bandwidth of WBFM.

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| Amplitude Modulation (AM) | Frequency Modulation (FM) | Single Sideband (SSB) |