| 摘要 |
PURPOSE:To improve the reliability of an AFC circuit and to constitute it at low cost by realizing AFC stop in a purely electronic way. CONSTITUTION:When a tuning state is beyond acquisition, the muting signal of IF amplifying circuit 5 is at H and while transistor Tr13 turns on, Tr14 turns off, thereby turning on Tr18. As a result, the output voltage of integrating network 9 is fixed at a reference voltage supplied from point X and AFC operation stops. Because it is out of tuning range at this time, the output of level detecting circuit 8 is at H and while Tr15 turn on, Tr16 turns off, so that tuning display LED turns on. Next, when the tuning state is placed in the acquisition range, the muting signal is at L, and while Tr13 turns off, Tr18 turns on, but since the charging current of capacitor 17 is supplied to the base of Tr18, which stays on. Therefore, while capacitor 17 is charged, the AFC operation stops. If the tuning state is set during the period, the output of circuit 8 is at L and LED21 displays the tuning state. |
