AUTOMATIC FREQUENCY CONTROL UNIT

摘要

PURPOSE:To prevent malfunctions when the first local oscillation frequency shifts upward, by fixing temporarily the second local oscillation frequency to a frequency lower than the reference tuning state at a power on time and a channel selection operation time. CONSTITUTION:The +B voltage of constant-voltage circuit 14 is divided by resistances 15 and 16 to obtain value VL lower than the AFC reference voltage (the AFC control voltage in a normal tuning state). FET 11a of switching circuit 11 is turned on by pulss from a power on detecting circuit and a channel selection detecting circuit to apply voltage VL to the varactor diode of the second local oscillator 7, and the capacity is increased more than the reference state (the normal tuning state) to obtain frequency fL lower than the reference frequency. This frequency fL is fixed temporarily, so that malfunctions are prevented when the first local oscillation frequency shifts from a low frequency to a high frequency at a power on time and a channel selection operation time in the separate carrier-system TV receiver.