Information on Frequency Modulation - Direct Method
In the direct method of frequency modulation (FM), a circuit
whose reactance can be controlled by the amplitude of the
modulating signal is connected in parallel with a variable-frequency LO.
When the modulating signal voltage is zero, the effective capacitance
of the variable-reactance circuit is such that the oscillation frequency
is equal to the unmodulated carrier frequency. Variations in the
modulating signal voltage vary the effective capacitance of the reactance
circuit, which in turn will frequency-modulate the carrier. Due to
the variable tuning required, crystal oscillators cannot be used so
automatic frequency control (AFC) must be applied to enhance the
inherent stability of an L-C oscillator.