| 摘要 |
PURPOSE:To increase the degree of freedom of fine adjustment of a frequency synthesizer reception system which uses a phase locked loop, by properly controlling the oscillation frequency of the voltage-controlled oscillating circuit of a tuner by a gate circuit for automatic tuning operation. CONSTITUTION:An automatic tuning AFT circuit 31 is supplied with the IF output of a tuner 11 to output a signal which corresponds to a shift in the IF signal. Then, a gate circuit 32 receives the output of the circuit 31 via one terminal and that of an LPF18 via the other terminal to select either one of the both, and the selected signal is applied to the oscillation frequency control terminal of a controlled oscillator in the tuner 11. A fine-adjustment signal generating circuit 20 performs pulse-width modulating operation and D/A converting operation and inputs an obtained DC voltage to the circuit by superposition. The output voltage of the circuit 31, when the circuit 32 is at a terminal 322 side, controls porperly the oscillation frequency of the voltage-controlled oscillator in the tuner 11. |
