| 摘要 |
PURPOSE:To correctly detect the disconnection of an optical signal input by monitoring a change in a control signal for controlling a phase difference in a phase synchronizing loop circuit and detecting the disconnection of the optical signal input. CONSTITUTION:When an optical signal input is disconnected, the contents of an output from a clock extraction circuit 3 are made completely indistinct and the indistinct contents are inputted to a phase synchronizing loop circuit 2, so that a phase difference control signal is changed to a voltage shifted from a fixed voltage obtained when the optical input exists. The DC phase difference control signal is compared with a previously set reference value by an optical input disconnection deciding circuit 4 to decide the disconnection of the optical signal input. Thus, the disconnection of the optical signal input can correctly be detected. |
