JPH05218985A - Alarm detection circuit - Google Patents

Abstract

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.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output level alarm detection circuit in a communication device.

[0002]

2. Description of the Related Art In a conventional alarm detection circuit, an IF input signal is input to an amplifier 1 and an output of the amplifier 1 is input to a level variable attenuator 2 as shown in FIG. This level variable attenuator 2 was output to the outside as a final output after being controlled by the control signal from the outside so that the output level was kept constant. On the other hand, for the detection of the level alarm, the input side of the level variable attenuator 2 is branched and amplified by the amplifier 7 and then detected by the alarm detection circuit 8. That is, when the input to the level variable attenuator 2 drops below a certain level,
This limit level was monitored because the limit to be compensated by the variable attenuator 6 was reached.

[0003]

The above-mentioned conventional alarm detection circuit can detect only the level decrease before the input of the level variable attenuator, and cannot detect the level decrease of the final output of the level variable attenuator. It is possible. Therefore, the level variable attenuator becomes abnormal and there is a drawback that it cannot be detected even if the output level is lowered.

[0004]

SUMMARY OF THE INVENTION An alarm detection circuit of the present invention comprises an amplifier for amplifying an IF signal, and a level variable for maintaining the output level of the amplifier constant by varying an attenuation amount by an external control signal. An attenuator, a variable gain amplifier for inputting the branch signal of the final output of the level variable attenuator, and further changing the gain by the external control signal, and an alarm detection circuit for monitoring the output level of the variable gain amplifier The input level in the normal state of the alarm detection circuit is a constant level by controlling the output of the variable gain amplifier so that the gain increase amount of the same value as the attenuation amount of the level variable attenuator is obtained.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the invention will be described with reference to the drawings.
FIG. 1 is a block diagram of an embodiment of the present invention. In FIG. 1, the IF input signal is amplified by an amplifier 1, further attenuated by a level variable attenuator 2 according to a control signal from the outside, and then output from an output terminal as a final output. The output of the level variable attenuator 2 is branched and one of them is input to the variable gain amplifier 3. The control signal of the variable gain amplifier 3 is common to the control signal of the level variable attenuator 2, and for example, the increase amount of attenuation of the level variable attenuator 2 and the increase amount of the variable gain amplifier 3 correspond to the same value. Set. As a result, the output of the variable gain amplifier 3 always has a constant output level even if the amount of attenuation of the level variable attenuator 2 changes, so that the detection level of the alarm detection circuit 4 at the next stage is a fixed value. However, the normal condition can be monitored. In case of an abnormality, alarm detection is activated when the level drops below this fixed value.

[0006]

In the alarm detection circuit of the present invention, the level detection point is used as the output of the level variable attenuator, the amplifier in the preceding stage of the alarm detection circuit is used as the variable gain amplifier, and the variable gain is adjusted according to the attenuation amount of the level variable attenuator. The detection point of the alarm detection circuit can be set to the same level by changing the gain of the amplifier and keeping the input level to the alarm detection circuit constant during normal operation. Therefore, there is an effect that it is possible to detect a stable level drop of the final output.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional alarm detection circuit.

[Explanation of symbols]

 1,7 Amplifier 2 Level variable attenuator 3 Variable gain amplifier 4,8 Alarm detection circuit

Claims (1)

[Claims] 1. An amplifier for amplifying an IF signal, a level variable attenuator for maintaining a constant output level by varying the amount of attenuation of the output level of the amplifier with an external control signal, and a final output of the level variable attenuator. A variable gain amplifier that inputs a branch signal and that further varies the gain by the external control signal,
An alarm detection circuit for monitoring the output level of the variable gain amplifier is provided, and the output of the variable gain amplifier is controlled so that the gain increase amount of the same value as the attenuation amount of the level variable attenuator is obtained, thereby performing the alarm detection. An alarm detection circuit characterized in that the input level in a normal state of the circuit is a constant level.