| 摘要 |
PURPOSE:To realize a circuit which is IC-implemented without using any expensive crystal filter, by allowing a PLL circuit to operate as a band-pass filter. CONSTITUTION:A noise N and a pilot signal C which are branched by a pilot branching filter 7 from a communication circuit 5 are inputted to an AGC circuit 1 to hold the signal C at a specific level C0. At this time, the control voltage VC of a voltage-controlled attenuator circuit 6 coresponds to the difference between the signals C and C0. This signal C0 and a noise N0 with a specific adjacent frequency band are inputted to the PLL circuit 2 which functions as a band-pass filter circuit for the specific band to obtain a specific band noise 11 centering on the signal C0. This is applied to a level detecting circuit 3 to detect its level, obtaining a voltage VNO corresponding to the input noise level. Further, this is corrected by a correcting circuit 4 consisting of a differential circuit to obtain a voltage proportional to the input noise level N. |
