JPS6231855B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an automatic gain control circuit used in a receiving section of a communication device.

In the receiving section of a communication device, it is desirable that the output signal level of the receiving section remains constant even with changes in the received input signal level. is being used. This consists of the circuit shown in Figure 1, where the input signal S ₁ passes through the gain control circuit 1 and outputs the output signal to the output terminal of the AGC circuit.
Output as S ₂ . The level of this output signal _S2 is simultaneously detected by a detection circuit 2, and this detected signal level and a preset reference value _S4 are compared in a comparator circuit 3, and an output _S3 corresponding to the difference is sent out. . In other words, the output S3 of the comparator circuit ₃ controls the gain of the gain control circuit ₁ as a control signal, and the receiver output signal
It operates so that the level of _S2 remains constant.

In the conventional AGC circuit explained above, if the level of the input signal _S1 becomes higher than the AGC possible range, the AGC circuit will not operate normally.
An excessive level of signal will be sent to the equipment placed next to the AGC circuit, which may have an adverse effect on the equipment placed next, such as damaging the electrical circuit. Also, the level of input signal S ₁ is
The same thing can happen if there is a sudden increase even within the range where AGC is possible. In other words, if the level of the input signal S1 of the gain control circuit ₁ increases within a short time less than the response time of the AGC loop, the output signal
The level of _S2 is not controlled to a constant value and is output to the next stage of the AGC circuit at an excessive level. Therefore, conventionally, as a countermeasure to this problem, a method has been considered in which the level of the signal is limited by using a nonlinear element such as a diode on the output side of the AGC circuit. However, this method causes nonlinear distortion in the output signal even when the AGC circuit is within its normal operating range, so it may not be an appropriate countermeasure when the input signal _S1 is an analog signal. It's hard to say.

The present invention eliminates these drawbacks of the conventional ones, and allows normal operation even when the input signal level increases beyond the AGC range or when the input signal level increases rapidly.
It is designed to perform AGC operation, and focusing on the point that the output signal of the comparator circuit, that is, the control signal of the gain control circuit, saturates to the maximum value limited by the circuit power supply voltage, etc., This saturation state is detected by comparing it with a set reference value.
The object of the present invention is achieved by forcibly lowering the signal level of the AGC circuit output.

The automatic gain control circuit of the present invention includes a gain control circuit whose output signal level is controlled with respect to an input signal level by a control signal applied from the outside, a first detection circuit which detects the output signal level, and the first detection circuit. a comparison circuit that compares the output signal from the detection circuit with a preset reference value and generates the control signal and supplies it to the gain control circuit so that the output signal level is constant with respect to changes in the input signal level; and monitor the output signal level of this comparison circuit,
a second detection circuit that detects whether the output signal level is within the gain controllable operating range; and a second detection circuit that detects whether the output signal level of the gain control circuit exceeds the gain controllable operating range. and a gate circuit that is controlled by the output of the detection circuit so as to stop outputting the output signal when the detection circuit detects that the detection circuit has detected the detection circuit.

FIG. 2 is a block diagram showing the basic configuration of an AGC circuit according to the present invention, in which parts corresponding to those in FIG. 1 are designated by the same reference numerals. The present invention has a configuration in which the second detection circuit 5 and the gate circuit 7 are connected. The normal operation of the AGC circuit shown in FIG. 2 is similar to the operation of the conventional AGC circuit shown in FIG. 1, and the gate circuit 7 does not operate. When the level of the input signal S ₁ is higher than the AGC possible range of the AGC circuit, or when the level of the input signal S ₁ increases rapidly, the control signal S ₃ of the gain control circuit 1 is limited by the circuit power supply voltage, etc. Since the signal is saturated to the maximum value, the detection circuit 5 detects that this saturated control signal is outside the range in which AGC is possible, and sends out a control signal _S6 that closes the gate circuit 7. That is, the output _S6 of the detection circuit 5 is used as a control signal to control the opening and closing of the gate circuit 7 to prevent an excessive output signal from entering the next stage of the AGC circuit. FIG. 3 shows an embodiment of the gate circuit 7.

In this embodiment, resistor 31 and transistor 32
A gate circuit is constructed, and the gate function is realized using the switching operation of the transistor 32 with the emitter-base voltage as the threshold for the output _S6 from the second detection circuit 5. . That is, when the control signal _S3 in FIG. 2 exceeds the reference value _S5 , the output _S6 of the second detection circuit 5 makes the transistor 32 conductive, and the gain control circuit 1
The object of the invention is achieved by attenuating the level of the output S ₂ of .

According to the present invention, even when the input signal level becomes higher than the AGC possible range, or even when the input signal level increases rapidly within the AGC possible range, the device connected to the output side of the AGC circuit is can be protected.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a conventional AGC circuit, FIG. 2 is a block diagram showing the basic configuration of an AGC circuit according to the present invention, and FIG. 3 is a diagram showing an embodiment of the gate circuit 7 in FIG. be. 1...gain control circuit, 2...detection circuit, 3...
Comparison circuit, 5... second detection circuit, 7... gate circuit.

Claims (1)

[Claims] 1. A gain control circuit whose output signal level relative to the input signal level is controlled by an externally applied control signal, a first detection circuit that detects the output signal level, and an output signal from the first detection circuit that is set in advance. a comparator circuit that generates the control signal and supplies it to the gain control circuit so that the output signal level is constant with respect to a change in the input signal level compared with a reference value determined by the input signal level; a second detection circuit that monitors and detects whether or not the output signal level is within the gain controllable operation range, and the second detection circuit adjusts the output signal of the gain control circuit to the gain controllable operation range and a gate circuit controlled by the output of the second detection circuit so as to stop the output signal when detecting that the range has been exceeded.