| 摘要 |
PURPOSE:To hold the increment magnification M of an APD at a fixed value at the time of interrupting an optical signal and to obtain a noise signal suppressing effect and an AGC loop oscillation preventing effect. CONSTITUTION:When a received optical signal A is interrupted a clock interruption judging means constituted of a clock extracting circuit 7, a level detecting circuit 8 and a comparator 9 extracts a clock signal G from a digital receiving signal C supplied from an AGC amplifier circuit 2 and generates a control signal J for judging clock signal interruption, i.e., optical signal interruption, when the level H of the signal G is lower than an identification level Vth. At the time of receiving a control signal J, a selector circuit 10 selects an optional reference signal Vs out of a comparing signal E to be AGC voltage obtained at the time of inputting the optical signal A and the reference signal Vs and impresses a fixed bias voltage F0 to an APD 1 through a DC/DC converter 4. |
