| 摘要 |
PURPOSE:To enable to fix the gain of the amplifier to the gain at the alarm production or the center value of the amplifier gain with a simple constitution, by detecting the change in the pilot level and disconnecting the automatic gain control circuit through the use of alarm circuit. CONSTITUTION:The pilot signal PL at the output terminal OUT is picked up as the DC signal via BPF2, amplifier 3, rectifier 4, and DC amplifier 5, and the PL signal level is judged with the output of the amplifier 5. When the level of PL signal is lowered, the comparator 9-1 produces the low level alarm when it is equal to the voltage V-, the driving circuit 8 switches the switching-over switch 6 to the contact S1, disconnects the automatic gain control loop, applies the voltage v- to the variable gain amplifier 1, and fixes the amplifier 1 to a level immediately before the PL signal level decrease. If the PL level rapidly decreases to the zero level, the comparator 9-3 detects the level drop, the circuit 8 switches the switch 6 to the contact S3 position, the voltage VN is fed to the amplifier 1, and the gain is fixed to the center value of the amplifier gain. |
