Q.Describe basic operation of a varactor diode FM generator.

A varactor (varicap) diode is a reverse-biased PN junction whose junction capacitance decreases as the reverse voltage increases. Connected across the tank circuit of the carrier oscillator, its capacitance is varied by the modulating signal, which shifts the oscillator frequency and produces FM directly. …

[!TLDR]

A varactor placed across the oscillator tank acts as a voltage-controlled capacitor; the message signal varies its capacitance and therefore the oscillator frequency, giving direct FM.

A varactor diode is a specially made PN junction operated in reverse bias. The depletion region acts as the dielectric of a capacitor whose plates are the P and N regions; as the reverse bias increases the depletion width increases and the junction (transition) capacitance $C_j$ decreases. Thus the diode behaves as a capacitor whose value is controlled by the applied dc voltage.

In the FM generator the varactor is connected in parallel with the inductor $L$ and capacitor of the tank circuit of an LC oscillator that produces the carrier. A dc reverse bias fixes the operating (rest) capacitance, which together with $L$ sets the carrier (rest) frequency

$f = \frac{1}{2\pi\sqrt{LC}}.$ …