| 摘要 |
PURPOSE:To adjust the overall band characteristics from an antenna terminal to a detection circuit, by controlling the local oscillating frequency of a tuner, through the use of an automatic gain control voltage of a video intermediate frequency processing circuit. CONSTITUTION:A signal introduced to an antenna terminal 11 is selected for channel with a tuner 12 and inputted to a video intermediate frequency processing circuit 13, where the said signal via a preamplifier 14 and a BPF15 at first is inputted to an intermediate frequency amplification and a video detection circuit 16, and the video intermediate frequency is introduced to a ratio detection section 20. The composite video signal in video detection is introduced to an output terminal 17 and inputted to an AGC circuit 18. Further, when the antenna input electric field is lower, if it exceeds the threshold level of a DC voltage converting circuit 29, the terminal voltage of a varactor diode 23 is changed and the tuning point of the detection section 20 is shifted. Thus, since the band of an AFC circuit 21 is narrowed, antinoise performance at weak electric field can be increased. |
