| 摘要 |
PURPOSE:To keep the detection gains at an approximately fixed level by turning two signals sent from a photodetector into a single signal through time division to be approximately averaged and compared with a reference level and controlling the detection gain through a closed loop in response to the difference obtained from said comparison. CONSTITUTION:An analog switch 15 set at the next stage of preamplifires 3 and 4 switches two input signals in time division by a switch changeover clock signal 20 having a frequency sufficiently high than a control band. The output signals of both amplifiers 3 and 4 are synthesized into a single signal in time division is supplied as a numerator signal of a divider 7. The output of this divider 7 is transmitted through an LPF 17 which cuts off the frequencies sufficiently lower than the control band and approximately averaged. This averaged signal is compared with a reference level, and a signal corresponding to the difference of comparison is supplied to the denominator of the divider 7. The divider 7 functions to reduce its output in case the averaged signal is higher than the reference level. While the output of the divider 7 is increased if the averaged signal is lower than the reference level. Thus the detection gain is controlled through a closed loop.
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