FM radio receiver with reduced noise - by causing intermediate frequency filter centre frequency to track received frequency

摘要

The radio receiver, e.g. a car radio, has an input stage (2), a mixer (3) with oscillator (4), an IF filter (5) following the mixer, and an IF amplifier (6) with FM demodulator following the IF filter. The centre frequency of the IF filter (8) follows the instantaneous frequency of the IF signal. The LF output voltage from the FM demodulator (10) is passed to the IF filter such that the IF filter's centre frequency follows the instantaneous frequency of the IF signal. The IF filter contains a varactor diode (D) that alters its centre frequency. The advantage lies in being able to use an IF filter of relatively narrow band (ie narrower than the bandwidth of the received FM signal).