Another method of superimposing information signals on to a carrier signal is
    frequency modulation (FM) in which the modulating signal varies the frequency
    of a carrier wave. This has a number of advantages over amplitude modulation
    (AM). Frequency modulation is used for sound broadcasting in the VHF band,
    for the sound signal of  625-line television broadcasting, for some mobile
    systems,  and for multi-channel telephony systems operating in the UHF band.
    Frequency modulation is therefore used for all analog communications via
    satellites. FM therefore has a wider bandwidth compared to AM.

    When a sinusoidal carrier wave is frequency modulated, its instantaneous
    frequency is caused to vary in accordance with the characteristics of the
    modulating signal. The modulated carrier frequency must vary either side of
    its nominal unmodulated frequency a number of times per second equal to the
    modulating frequency. The magnitude of the variation - known as the
    frequency deviation - is proportional to the amplitude of the modulating signal
    voltage.

    When the modulating signal is of sinusoidal waveform, the frequency of the
    modulated carrier wave will vary sinusoidally ; this is illustrated by Fig. 9

  

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