Automatic frequency control

The automatic frequency control (AFC, of Engl. Automatic Frequency Control ) acts as a radio or television reception via a superheterodyne receiver to keep the tuned receiver (tuner ) to the station selection automatically on this.

General

Due to thermal influences on the frequency-determining components of the resonant circuit of the mixing oscillator of a superheterodyne receiver is the frequency can change. The consequence of this is that the difference between reception frequency and the oscillator frequency no longer the intermediate frequency (IF ) corresponds.

Is she with FM receivers for FM by, for example, instead of 10.7 MHz (typical setpoint) at 10.8 MHz, it is on the edge of the passband of the IF filter and the edge of the linearity range of the frequency. So that good reception is no longer possible.

Another function of the AFC is to increase the comfort when operated by the user: If the receiving frequency taken only inaccurate by manual tuning, can be achieved by subsequent connection of the AFC accurate compensation. With modern instruments, this sequence is automated.

The AFC for the necessary control voltage (control signal ) is obtained with FM receivers from the FM demodulator. In television receivers to use the Demulator of the FM - modulated audio signal (typically on the carrier frequency difference between 5.5 and 6.5 MHz). The demodulator signal is fed to a low pass, by the symmetry of the audio signal thus results in correct matching a control signal of "zero". The control signal is for either tuning the oscillator (VCO ) is added and supplied to the frequency-determining capacitance diode, or to an additional capacitance diode in it.

Thus, the oscillator is continuously tuned automatically mean that the intermediate frequency, the IF filter passband and the acceptance range of the demodulator is true. Oscillator, demodulator and capacitance diode form a control loop. This mechanism would also succeed in unstable transmitter frequency, but this is the reception of radio stations irrelevant, since they have very stable frequency.

The AFC is either separately switched on and off or is deactivated automatically when pressing the channel selection to allow for pre - tuning without the control that holds the receiver to the previous station.

In present-day television receivers with channel change occurs, the entire process AFC disable - program location change to the programmed channel reception - AFC activate automatically.

To prevent "jumping" to a frequency- adjacent stations, it requires a certain basic stability of the oscillator frequency and a certain minimum level of the received carrier frequency.

The demodulator in AM receivers does not provide a control voltage is applied here AFR.

• Radio receiver technology