| 摘要 |
PURPOSE:To narrow the reception band width with a simple circuit and to improve the reception sensitivity by switching sequentially periodically a frequency of a local oscillating wave of a receiver so as to stop the change of the frequency of the local oscillating wave when a carrier detection circuit outputs an output of a prescribed value or over. CONSTITUTION:The oscillating frequency of the local oscillation circit 30 mixed by the 2nd mixing circuit 25 of the receiver is changed cyclicly by a local oscillating frequency switching circuit 35 as f1, f2...f7, f1.... Its output is amplified (33) through a band-pass filter 32 of an intermediate frequency amplifier section 31. When the carrier detection circuit 34 detects an output of the amplifier circuit 33 to be a prescribed level or over at, e.g., f2, its output stops the frequency changeover of the local oscillating frequency switching circuit 35, the frequency f2 is demodulated (27) and outputted through an LPF28. When the output of the f2 is decreased, the changeove of the local frequency is restarted again. Thus, an expensive highly stable oscillator is not required for both the transmission side and the reception side, the cost is decreased and high sensitive reception is obtained. |
