| 摘要 |
PURPOSE:To perform accurate tuning operation by comparing a frequency double as high as a modulated wave signal with the frequency of a demodulated signal from an amplitude modulated radio receiver and varying the frequency of a carrier signal so that the frequency of the demodulated signal is twice as high as the frequency of the modulated wave signal. CONSTITUTION:A comparing circuit 20 compares the voltage of a line 14 corresponding to the frequency of the demodulated signal from the radio receiver 4 with the voltage of a line 16 corresponding to the frequency double as high as the modulated wave signal from a modulated signal generating circuit 2. When the frequency of the carrier signal is lower than a tuning frequency f0, the voltage of the line 14 is lower than that voltage of the line 16, so the comparing circuit 20 outputs a low-level signal to a line 17. A CPU18 increases the frequency of the carrier signal of a carrier signal generating circuit 3 and when the output of the comparing circuit 20 falls to the zero level, it is judged that the frequency of the carrier signal from the carrier signal generating circuit 3 becomes equal to the tuning frequency f0 of the radio receiver 4 and the variation in the frequency of the carrier signal is stopped, thereby finishing the tuning operation.
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