FM Direct Method

Transmitters and Receivers Glossary

Transmitters and Receivers — FM Direct Method

Information on FM 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.

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