Why the automatic frequency control function is usually not necessary in today's FM receivers?

Answer 1

AFC was used to provide an electronic means to stabilize the frequency of a circuit. Older receivers and transmitters used a tank circuit, made of an inductor and capacitor to select frequency. Since both the inductor and capacitor are physical objects, their sizes and properties vary with temperature, humidity, age and other factors. Also to be able to tune the frequency, at least one of them had to be deliberately made to be mechanically alterable to change it's electrical properties. These factors caused the frequency to slowly drift over time. Also, the tuning was usually accomplished in a continuously variable way, so it was difficult to confirm an exact tuning to the station which could lead to loss of sensitivity or distortion of the signal. AFC used electronic feedback to fine tune the tank circuit by a (usually) electrically tuned component.

Modern radio equipment tends to use a quartz crystal oscillator of a fixed frequency as a reference. Quartz crystals can easily be made stable in frequency to 50 parts per million. This is sufficiently accurate to not need to compare it with the frequency of the incoming signal, itself derived from a stable crystal source. The radio is usually tuned by a frequency synthesizer controlled by the crystal.

Both AFC and the frequency synthesizer method use an electrically tuned variable frequency oscillator to tune the radio. The difference is that AFC used characteristics of the received signal to measure and correct the mistuning, while frequency synthesizers compare the tuning with crystal reference. AFC cannot find the frequency you desire to tune, it only fine tunes it once you select it. The frequency synthesizer accurately tunes the radio to whatever frequency you ask it.