PURPOSE: To improve S/N in a weak optical signal and BER (error rate) by providing 1st and 2nd PLLs composed of a phase comparator circuit, a loop filter and a voltage controlled oscillator circuit respectively.
CONSTITUTION: When an optical signal from a transmission section is made incident on one photodetector 1 or above in a light receiving section, the signal is converted into a weak electric signal corresponding to the intensity of the optical signal and inputted to a 1st PLL 8 via a preamplifying circuit 2, a limiter circuit 3 and a BPF 4. The PLL 8 is constituted of a closed loop composed of a phase comparator 5, a loop filter 6 and a voltage controlled oscillator 7 and obtains a signal synchronously with the phase of the input signal from the circuit 7. Then the output signal from the circuit 7 is demodulated into an organic signal by a closed loop of a 2nd PLL 12 (composed of a phase comparator 9, a loop filter 10 and a voltage controlled oscillator 11). Then the signal is amplified by an amplifier circuit 13, the waveform is shaped via a Schmitt trigger 14 and the resulting signal is outputted. Thus, the S/N of the minute optical signal is improved to improve the error rate.
SUGINO NOBUO
KURATA NOBORU
