| 摘要 |
PURPOSE:To keep the tuning voltage of the channel to the change of the electric field under reception, by dividing the state of the AFC voltage into three types, i.e. the charging, discharging and holding states and then keeping these three states to the tuning voltage. CONSTITUTION:The AFC output of AFC generating circit 15 is transmitted to transistors TR37, 39 which function as the voltage deciding circuit. And the AFC voltage is about 6V as long as the tuning voltage is proper after discontinuation of the sweep and the local oscillation frequency of the electronic tuner has a proper value. Thus TR37, 39 are turned off and on respectively. As a result, both TR38, 40 are turned off to prevent the charging/discharging to capacitor 20 and thus to hold the tuning voltage. Accordingly, the AFC voltage can keep about 6V although no broadcasting wave exists to secure the same state as the case in which the optimum tuning point is obtained. Thus the steady tuning voltage of the channel can be kept although the electric field may change under reception to eliminate the second selection of the channel. |
