| 摘要 |
First, second and third manual control switches are arranged side by side in close proximity to each other, the first switch for initiating the upward sweep of frequency at a low sweep rate, the third switch for initiating the downward sweep of frequency at the same sweep rate, and the second or intermediate switch for switching the sweep rate to a higher rate. A counter having up- and down-counting modes of operation is provided, the modes of which are under the control of the first and third switches. The up-down counter is supplied with pulses at a low repetition frequency when either the first or third switch is operated and with pulses at a high repetition frequency when the second switch is operated. The local oscillations of a radio receiver are under the control of the output of a comparator connected in a phase-locked loop including the local oscillator of a radio receiver, a programmable counter and the comparator. The programmable counter is under the control of the output signal from the up-down counter to vary the number of peaks of the local oscillation to be counted thereby for each output thereof, the output frequency from the programmable counter is compared with a reference frequency which is a fraction of the frequency to be tuned to produce a voltage for controlling the frequency of the local oscillator.
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