Automatic frequency control
The automatic frequency control ( AFC , of Engl. Automatic Frequency Control ) is used in radio or TV reception by means of a heterodyne receiver to the receiving section ( tuner to hold) after the station selection automatically adjusted to this.
General
Thermal influences on the frequency-determining components of the resonant circuit of the mixer oscillator of a superposition receiver can change its frequency somewhat. The consequence of this is that the difference between the receiving frequency and the oscillator frequency no longer corresponds to the intermediate frequency (IF).
Is it for FM receivers for VHF thereby z. B. instead of 10.7 MHz (usual target value) at 10.8 MHz, it is at the edge of the pass band of the IF filter and at the edge of the linearity range of the frequency demodulator. Good reception is no longer possible.
Another function of the AFC is to increase the convenience of operation by the user: If the receiving frequency is only inaccurately determined after manual tuning, a more precise comparison can be achieved by subsequently switching on the AFC. In modern devices, this sequence is automated.
The control voltage (control signal) required for the AFC is obtained from the FM demodulator in FM receivers . In television receivers, the demodulator of the FM-modulated audio signal is used (usually on the carrier difference frequency 5.5 or 6.5 MHz). The demodulator signal is fed to a low-pass filter . Due to the symmetry of the useful signal, a control signal of "zero" results if the coordination is correct. The control signal is either added to the tuning voltage of the oscillator ( VCO ) and sent to its frequency-determining capacitance diode or to an additional capacitance diode in it.
As a result, the oscillator is constantly and automatically tuned so that the intermediate frequency meets the IF filter pass-through curve and the acceptance range of the demodulator. The oscillator, demodulator and capacitance diode form a control loop. This regulation would also work with an unstable transmitter frequency, but this is irrelevant when receiving radio transmitters, since these work with a very stable frequency.
The AFC can either be switched on and off separately or is automatically deactivated when the transmitter selection is activated in order to enable pre-tuning without the control system holding the receiver on the previously set transmitter.
With today's television receivers, the entire process of deactivating AFC takes place when changing channels - changing the program position to the programmed receiving channel - activating AFC automatically.
In order to prevent “jumping” to a neighboring FM transmitter in terms of frequency, sufficient selectivity in the IF amplifier and basic stability of the oscillator frequency as well as a certain minimum level of the received carrier frequency are required.
The demodulator in AM receivers does not provide a suitable control voltage; automatic frequency control (AFR) is used here.
Modern radio sets often use PLL tuners to generate the oscillator frequency , which often display the receiving frequency digitally and make manual activation of the AFC superfluous.
