| 摘要 |
PURPOSE:To detect the interruption of an optical signal without fail in a circuit not to have an AGC loop by amplifying only the output frequency component of a pilot signal oscillator to a large amplitude when an input signal is interrupted. CONSTITUTION:The output of a pilot signal oscillator 250, which generates the signal of a frequency to be higher than the frequency of the input signal by prescribed quantity, is added to a synthesizing means 270 and superimposed to the electric signal of the output of a photo-detecting element 100. When the input signal is normal, a variable attenuating/amplifying means 320 is operated in an area and the output frequency component of the pilot signal oscillator 250 to be superimposed to the input signal is suppressed to the small amplitude. When the input signal is interrupted, the variable attenuating/amplifying means 320 goes to be operated in a linear area and only the output frequency component of the pilot signal oscillator 250 is amplified to the large amplitude and detected through a filter 340. |
