It d dt t t. In frequency modulation the frequency of the carrier voltage is varied according to the instantaneous value of the modulating voltage. Both C and f can be omitted to simplify the equation by changing C to 1 and f to 0.
In this expression f c denotes the frequency of unmodulated carrier and the constant k f represents the frequency sensitivity of the modulator.
Carrier frequency in Hertz is equal to oc 2 p. F is the phase of the signal at the start of the reference time. However we cannot simply substitute this expression into the general equation for a sinusoid to get the FM equation. The frequency at minimum amplitude of the message signal f c Df.