FM HIGH FREQUENCY TUNING CIRCUIT

PURPOSE:To prevent cross modulation and intermodulation due to an interference wave, by using a varactor diode as an input coupling capacitance connected to a tuning circuit and controlling the capacitor with a voltage proportional to a received electric field strength. CONSTITUTION:An input applied to an antenna input terminal 1 is given to a tuning coil 3 via a DC-cut capacitor 27 and a varactor diode 28. On the other hand, a broad band AGC voltage from a level detector 24 is applied to a connection point with the capacitor 27 as a control voltage for the level detector 24 via a resistor 18. As the output voltage of the level detector 24 increases, the capacitance of the varactoradiode 28 is reduced, the Q of the tuning circuit is increased and the pass band characteristics can be narrowed. Thus, the attenuation of the interference wave is increased to a desire signal, allowing to improve the cross modulation and intermodulation.