| 摘要 |
PURPOSE:To decrease the prescribed time for a channel selection data preset by using the 1st BPF so as to detect that an IF signal approaches a prescribed frequency. CONSTITUTION:A computer 30 changes a duty ratio of a PWM wave inputted to an oscillator 14 as a tuning signal via an LPF 26 quickly. When the oscillating frequency of the oscillator 14 approaches the frequency at the time of reception of a broadcast of a channel, the IP signal passes through the 1st BPF 36 and the 1st level detection circuit 40 detects it. The computer 30 increased the value of the PWM wave by a prescribed value by using a detection signal from the circuit 40 to vary the duty ratio change of the PWM wave from a PWM oscillator 44 at a low speed. When the oscillating frequency of the oscillator 14 reaches the frequency at the reception of broadcast of the channel 1, the IF signal passes through the 2nd BPF 38 and it is detected by the 2nd level detection circuit 42. The computer 30 recognizes the oscillating state of the oscillator 14 at the oscillating frequency at the reception of the channel 1 by using the detection signal.
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