| 摘要 |
PURPOSE:To reduce the tuning error by stopping the sweep when the AFT voltage from an AFT circuit is inverted so as to keep the tuning frequency constant thereby applying the tuning operation in a short time accurately and varying the AFT inversion threshold value. CONSTITUTION:The output of the AFT circuit 6 is inverted by taking the optimum tuning frequency of an electronic tuner 2 as the border. In desiring to change the reception channel, when an UP or DOWN position of a channel selection switch 6 is depressed, a channel selection control circuit 7 receiving the command controls a tuning voltage generating circuit 8, which starts the sweep of the tuning frequency. In sweeping the frequency in the positive direction, when the frequency of a desired broadcast signal is reached, the AFT circuit 5 outputs an H level and the H level is inverted into an L level after the optimum tuning frequency. The channel selection control circuit 7 starts the sweep at a high speed by the instruction of the channel selection switch 6, and in detecting the inversion of the output from the AFT circuit 5 into the L level from the H level, the sweep is stopped tentatively. After a prescribed time elapses, the tuning voltage generating circuit 8 is controlled to start the sweep at a high speed again.
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