| 摘要 |
PURPOSE:To decrease the time until the AGC operation is made stable from the reception of a pilot signal by holding a bias voltage fed to a gain variable element to a holding circuit. CONSTITUTION:When a pilot output level is normal, an output of an AGC control circuit 3 is fed to an avalanche photo diode (PD)1 as a gain variable element, and when an output level of a reception amplifier is at the outside of the normal range, an output of the holding circuit 5 is fed to the PD1, through a changeover switch 6 at the stable state and inputted to a memory circuit 5 through a delay circuit 4 and a changeover switch 7. Thus, the output of the circuit 3 at the steady-state is held in the memory circuit 5. On the other hand, a level decision circuit 8 throws the switches 6, 7 as shown in dotted lines when the pilot level is at the outside of the prescribed range. Thus, when the pilot level is in the normal range, the output of the circuit 3 gives a bias voltage of the PD1 and when the level is at the outside of the prescribed range, the voltage held in the circuit 5 gives the bias voltage of the PD1.
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