AFC via the tuning diode

7-12 elektor july/august 1980 Some receivers have Varicap tuning, but no AFC. If this facility is desired, it seems a pity to have to add yet another Varicap. The principle of the circuit described here is that the voltage on the Varicap tuning diodes in a receiver is automatically affected by the AFC voltage. This is achieved by connecting the common pin of an integrated fixed voltage regulator with the AFC control voltage, instead of to ground. This not only causes the total output voltage to rise, but also enables it to be con- trolled. The AFC voltage from the demodu- lator is buffered with the aid of an opamp, after which it is fed to the voltage regulator. Part of the quiesc- ent current of the regulator flows through R3, and at the same time this resistor provides a defined terminating impedance for the opamp. The AFC voltage from most demodulators is roughly 4.5 V (within ± 0.5 V) and the quiescent current of the voltage regulator is approximately 3 mA. In order to contro...