| 摘要 |
PURPOSE:To solve the problem of unstableness in tuning without decreasing the precision of channel search by making the integral time constant of a low-pass filter large during channel search and small during tuning. CONSTITUTION:An electronic channel selecting circuit consists of a control circuit 6, channel selection voltage generating circuit 9, tuner 1, AFC detecting circuit 3, integral time constant switching type low-pass filter 4', and voltage comparing circuit 5. The control circuit 6 starts sweeping a channel selection voltage in response to a channel searching command and also switch the integral time constant of the low-pass filter 4' to a larger value. Further, the sweeping of the channel selection voltage is stopped in response to an in-tuning output from the voltage comparing circuit 5, and the integral time constant of the low-pass filter 4' is switched to a smaller value. Further, the channel selection voltage is varied in response to the output of the voltage comparator 5 to eliminate a shift between the frequency of an intermediate frequency signal from the tuner and a normal carrier frequency. |
