As you have seen, modulation is the process of varying a parameter of a carrier signal with an information signal. Recall that in amplitude modulation the parameter of amplitude is varied. In frequency modulation (FM), the frequency of a carrier is varied above and below its normal or at-rest value by a modulating signal. This section provides a further look into FM and discusses the differences between an AM and an FM receiver.
In a frequency-modulated (FM) signal, the carrier frequency is increased or decreased according to the modulating signal. The amount of deviation above or
below the carrier frequency depends on the amplitude of the modulating signal. The rate at which the frequency deviation occurs depends on the frequency of the modulating signal.
A Basic Frequency Modulator
Frequency modulation is achieved by varying the frequency of an oscillator with the modulation signal. A voltage-controlled oscillator (VCO) is typically used for this purpose, as illustrated in figure 5.14.
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Figure 5.14 Frequency modulation by a voltage-controlled oscillator.
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Generally, a variable-reactance type of voltage-controlled oscillator is used in FM applications. The variable-reactance VCO uses the varactor diode as a voltage-variable capacitance, as illustrated in figure 5.15, where the capacitance is varied with the modulation voltage, Vm.
