| 摘要 |
PURPOSE:To simplify the AGC circuit in a multichannel receiving system, by refreshing the AGC control voltage of a variable attenuator at every constant time in the head end device for community receiving for TV broadcast and FM broadcast. CONSTITUTION:The output of an antenna 1 passes through band pass filter groups 3a-3c which permit only one receiving channel to pass through, and outputs have levels made uniform and are synthesized by a synthesizer 5 and are amplified and are transmitted from a terminal 8 to a main line. Meanwhile, a part of the output is taken out by a branch equipment 7 and is mixed with the output of a local oscillator 14 by a mixer 11 and is taken out from an AGC detector 13 to control variable attenuators 4a-4c of respective channels. The oscillation frequency of the local oscillator 14 is changed by the channel selecting signal from a channel signal generator 15, and destinations of voltage hold refresh circuits 20a-20c are switched by an AGC signal switching part 17. Consequently, since the AGC control voltage is refreshed at every constant time, the device copes with the fluctuation of the antenna receiving input in respect to time. |
