TUNING CONTROL SYSTEM

摘要

PURPOSE:To set intermediate frequency to the center frequency of a ceramic filter and to receive signals at the optimum tuning point by changing the oscillation frequency of a reference oscillation of a PLL every change of received frequency. CONSTITUTION:A mixing circuit 2 mixes a received signal from an antenna 1 with the local oscillation frequency of the PLL consisting of the shown functional circuits 8-10, 13-15 and sends the mixed intermediate frequency signal to an intermediate frequency amplifier 4 through the ceramic filter 3. A control circuit 19 controls a variable capacitor diode 12 in an oscillation circuit 10 through a DA converter 18 and raises or drops the oscillation frequency of the oscillation circuit 10. Receiving the peak level detecting signal of the intermediate frequency signal from a detecting circuit 17, the control circuit 19 also sets up the oscillation frequency of the reference oscillation circuit 10 so that the peak level detection signal is maximized in addition to the circuit 19 and makes the intermediate frequency coincide with the center frequency of the ceramic filter 3.