Frequency fine tuning circuit for superheterodyne receiver - has second AFC circuit deriving control signal after first intermediate frequency band pass filter and controlling RF input circuits

摘要

The receiver's input circuit comprises a preliminary circuit, at least one input circuit tuned to the received frequency, an oscillator circuit tuned to the oscillator frequency, and a mixer amplifier. The latter is followed by an i.f. band-pass filter and further signal processing stages. The receiver has an automatic frequency control circuit (AFA 1) which controls the oscillator frequency so that the required i.f. is obtained. Automatic gain control (AVR) may also be provided. The receiver has a further AFC circuit (AFA 2) controlling only the input or preliminary circuit. It is controlled by signals derived after the first i.f. band-pass filter, whose amplitude is changed by shifting of the resonance curves in the above circuits.