| 摘要 |
PURPOSE:To detect accurately a period when no signal exists, by detecting whether the signal is included in the light after converting the light into electric signals, and amplifying them, and actuating automatic gain control. CONSTITUTION:The optical signal given from an optical transmission line is coverted into an electric signal by an avalanche photodiode APD module 14. This electric signal is amplified by a preamplifier 1, and the gain of the signal is supplied to a variable amplifier 2. Then the signal which is equalized and amplified is reproduced and converted again into an optical signal to be relayed. A part of the output signals of the amplifier 1 is applied to a detecting circuit 17. The circuit 17 checks whether the received signal has a prescribed level. When no optical signal is received any more, the gain of an AGC amplifying circuit 2 is set constant. The gain of a receiving circuit is lowered in a period when no signal exists in order to prevent the deterioration of S/N for both a discriminating circuit and a timing reproducing circuit within a relaying circuit. |
