A tuning circuit for a radio receiver

摘要

An improved tuner for a varactor-tuned local oscillator (54) in a radio receiver includes a winding of a pair of magnetically coupled coils (32), preferably constructed in the form of a transformer. Preferably, the first and second windings (30,34) of the transformer are magnetically coupled with reduced inter-winding capacitance by axially separating the windings on a common core. The second winding (34) is selectively coupled by a switch (48) to a capacitive load (40) in order to change the inductance of the first winding. The tuning range of the resonant circuit formed by the varactor diode set (26,28) and the first winding (30) is changed when the capacitive load (40) is coupled. The magnetic coupling between the transformer windings provides DC isolation between the oscillator and the switching circuit. Preferably, a tuning actuator has overlapping ranges of movement for selection of a signal within both the AM and FM frequency bands as the intermediate frequencies for the local oscillator are generated. <IMAGE>