| 摘要 |
PURPOSE:To enable to accurate tuning set automatically, by counting the sum or difference between the local oscillating frequency and intermediate frequency, counting the output of the clock oscillator, and controlling the local oscillator through the comparison of both the count values. CONSTITUTION:When the oscillating frequency fL of the local oscillator 13 is given to the counter circuit 16, the circuit 16 outputs the BCD signal S1 corresponding to the sum or difference between the frequency fL and the intermediate frequency fip given in advance to the display unit 17 and the comparator CM18. When the automatic channel selection switch 21 is closed, the clock pulse of the clock generating circuit 22 is input to the counter 20 via the gate circuit 23. The counter 20 counts the clock pulse and inputs BCD signal S2 to the CM18. The CM18 produces the control signal Vc from the signals S1 and S2 and gives it to the varactor diode of the oscillator 13 to control the frequency fL so that the signals S1 and S2 are equal. When the frequency fL is tuned to the frequency of the specified boradcasting station, the circuit 23 is OFF with the squelch signal of the tuning detecting circuit 24, the supplt of the clock pulse is stopped, and automatic channel selection for desired station can be made. |
