| 摘要 |
PURPOSE:To supply the same voltage as a voltage in AGC-on mode from a variable voltage source to the AFC terminal of a tuner even when an AGC circuit is off, by connecting the variable voltage source to the AFC output terminal of the AGC circuit. CONSTITUTION:The AFC circuit 1 enters AFC-off mode when a switch 6 is closed. At this time, the AFC voltage to be inputted to the tuner 2 should be the same as the output voltage of the AFC circuit when there is no input in AFC-on mode. Integrated circuits IC of AFC circuits, however, have variance in DC offset, every IC is different in input voltage and a frequency shift is caused with a fixed bias. For this purpose, the output voltage is measured when there is no input in AFC-on mode and the AFC mode is turned off to close a switch 20 associatively, so a variable resistance 231 is adjusted so that the output voltage at this time is equal to said measured value, thus completing a preliminary adjustment. Then, the same voltage is supplied to the tuner 2 in both AFC-on mode and AFC-off mode, so no local oscillation frequency shift is generated. |
