| 摘要 |
PURPOSE:To narrower a tuning band and to reduce the effect of drift of the circuit by providing two detuning discriminating circuits having different threshold values so as to select the detuning discriminating circuits having the large and small threshold values depending on sweeping and non-sweeping state. CONSTITUTION:While a tuning frequency is swept during the automatic channel selection, a signal from a controller 8 changes over a switch circuit 9 to the position of the detuning discriminating circuit 10a. The threshold value of the circuit 10a is smaller than the threshold value of the detuning discriminating circuit 10b and the tuning band is narrowed. A sweeping frequency is increased or decreased at each DELTAf of one channel, larger than the threshold value of the level discriminating circuit l5 at a tuning frequency f0 and smaller than the threshold value of the circuit 10a, then it is discriminated as tuning, an output of an AND circuit 7 transmits a signal of sweep stop to the controller 8 and the sweeping is stopped. Then the control signal from the controller 8 to the circuit 9 is stopped, the circuit 9 is changed over to the circuit 10b, thereby widening the tuning band. |
