| 摘要 |
PURPOSE:To stably detect a level decrease in a final output by using an output of a variable attenuator for a level detection point and revising a gain of a variable gain amplifier of a pre-stage of the alarm detection circuit so as to make always the input to the alarm detection circuit constant in the normal state. CONSTITUTION:An IF input is amplified by an amplifier 1 and attenuated at a level variable attenuator 2 by the attenuation set by, an external control signal and the attenuated signal is outputted as a final output. Moreover, an output of the attenuator 2 is branched and the one is inputted to a variable gain amplifier 3. A control signal for the amplifier 3 is used in common for the control signal of the attenuator 2 and the increment of the attenuation of the attenuator 2 is set equal to the increment of the gain of the amplifier 3. Since the output of the amplifier 3 is always a constant output level even when the attenuation of the attenuator 2 is changed, even when a detection level of an alarm detection circuit 4 is a prescribed fixed level, the normal state is monitored, When the level is decreased from the fixed level in the abnormal state, alarm detection is activated. |
