| 摘要 |
PURPOSE:To realize high S/N and low noise by controlling an attenuator in response to an output level of plural filters connected in parallel with an input terminal. CONSTITUTION:A signal fed to the input terminal is divided into two; one is given to a subtraction circuit 1b through an amplifier 1 and the other is fed to BPFs 3-N connected in parallel with the amplifier 1. When the applied signal does not include a frequency component over f5 (f3-fN are the center frequencies of each BPF), an output is obtained from the BPFs 3, 4 but no output is given from the BPFs 5-N. Thus, level detection circuits 3b, 4b discriminate that a signal is outputted to turn off attenuators 3c, 4c and bypass the signal and level detection circuits 5b-Nb discriminate that no signal exists to turn on attenuators 5c-Nc. Thus, the nose generated in the BPFs 5-N is not added by the addition/subtraction circuits 1a, 1b, the signal is free from noise and the noise at no signal is only the noise generated from the amplifiers 1, 2. |
