OPTICAL RECEIVING CIRCUIT FOR ANGLE MODULATING SIGNAL

摘要

PURPOSE:To reduce the influence of heat noise and to improve a CN ratio by setting up the amplification factor of an avalanche photodiode to a low value in a large photodetecting power area so that the amplitude of a photodetecting output signal does not exceed the dynamic range of an optical receiving circuit and increasing the amplification factor of the diode in a small photodetecting power area so that the amplitude is increased. CONSTITUTION:A photodetecting output signal outputted from an amplitude limiter 4 is led into a peak value detector 8 to detect the peak value. A differential amplifier 9 compares the detecting output of the detector 8 with a previously set reference voltage VR and supplies the difference voltage to a voltage variable type bias power supply 10 consisting of a DC/DC converter to variably control the bias voltage of an APD 1. The 2nd amplitude limiter 7 is connected between the limiter 4 and a demodulator 5 and limits the amplitude of a hotodetecting output signal obtained at the time of small photodetecting power which can not be limited by the limiter 4 by setting up the limit level less than that of the limiter 4 arranged on the prestage to remove the noise of an AM component.