| 摘要 |
PURPOSE:To set up 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 oscillator 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 controlling means 21 controls a variable capacity diode 12 through a DA converter 20 and raises or drops the oscillation frequency of the oscillation circuit 10. Simultaneously, the controlling means 21 receives a peak level detecting signal and a signal strength discriminating signal of the intermediate frequency signal from a detecting circuit 17 and a discriminating the circuit 18 and sets up the oscillation frequency of the reference oscillation circuit 10 so that the peak level detecting signal is maximized together with the circuits 17, 18. |
