The automatic frequency correction (AFC) is incorporated in FM transmitter to keep carrier frequency stable.
The discriminator reacts only to small changes in the carrier frequency but not to the frequency deviations in the carrier (since it is too fast).
Suppose frequency of the carrier increases. This higher frequency is fed to the mixer for which the other input frequency is from the stable crystal oscillator. A some what higher frequency will be fed to the discriminator.
Since the discriminator is tuned to the correct frequency difference which should exist between the LC. oscillator and crystal oscillator, and its input frequency is now somewhat higher, the discriminator will develop a positive D.C voltage.
This voltage is applied to the reactance modulator whose transconductance is increased by the positive voltage developed by the discriminator. This increases the equivalent capacitance of the reactance modulator thereby decreasing the oscillator frequency.
The frequency increase in the carrier frequency is thus lowered and brought to the correct value.
The correcting d.c voltage developed by the discriminator may be fed to a varactor diode connected across the tank circuit of the oscillator and be used for Automatic Frequency Correction AFC purposes.
In directly modulated FM transmitters, many times the frequency modulation is carried out at a lower frequency and with a smaller frequency deviation. Then passing this frequency modulated wave through frequency multiplier circuit, the desired carrier frequency and desired frequency deviation is achieved.
In this case, it may happen that the frequency multiplication required for getting the desired carrier frequency from a smaller carrier frequency may not tum out to be exactly the same as required for getting the desired frequency deviation from the original smaller deviation. In such situation mixer is used to achieve the final values.
Direct FM Transmitters
Here we will discuss two transmitters that generate FM by modulating the carrier frequency directly.