NO-SIGNAL DETECTING CIRCUIT

摘要

PURPOSE:To detect easily no-signal with a good S/N by applying a frequency on the outside of a transmission signal band onto a reception signal to amplify the signal and detecting a ratio of a frequency component included in this amplified waveform. CONSTITUTION:An avalanche diode (APD) 1 receives an optical signal and converts it to an electric signal. A specific frequency signal is inputted to the control part of a high voltage generating circuit 8 through an oscillator 14 and is added to the transmission signal. This frequency signal is amplified together with the transmission signal and is obtained as a frequency component inputted from the oscillator 14 through a band pass filter 9. The output of the first peak detecting circuit 5 and that of the second peak detecting circuit 11 which detects the peak value of the output of the filter 9 are compared with each other by a comparing circuit 12 to detect the S/N of the specific frequency component to the transmission signal. Thus, the reception optical level is discriminated to detect no-signal well.