| 摘要 |
<p>PROBLEM TO BE SOLVED: To enable automatic sensitivity control by detecting the relative size of a peak value and a reference value in a voltage comparing circuit of a receiving level control circuit. SOLUTION: A voltage comparing circuit 41 in a receiving level control part 40 compares a peak value of an already amplified received pulse output from a receiving and amplifying circuit 22 with a reference value V0 supplied from a reference voltage supply circuit 42. The circuit 41 transfers the output to the high state if the peak value is above the reference value V0 , and keeps the low state in the other cases. When the output of the circuit 41 becomes high, the output of an AND gate 44 becomes high, and the count value of an up/down counter 45 is counted up only by one. In the case of the low state, the count value is counted down by only one. The count value of the counter 45 is converted to the analog voltage to be synthesized with the d.c. voltage of a constant voltage generator 49b and supplied as bias voltage of an avalanche photo diode of a light receiving circuit 21.</p> |
