JPH05347523A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To prevent generation of a transient excessive output level by detecting a continuous maximum gain state of a variable gain circuit, and setting forcibly this circuit to the minimum gain state in response thereto. CONSTITUTION:An output level S2 of a gain variable circuit 1 for executing variable gain amplification of an input signal S1 such as a high frequency signal, etc., is subjected to level detection by a level detecting circuit 2, and its detected level S3 is compared with a reference level R1 by a comparing circuit 3. A detecting circuit 4 detects a fact that a control output level S4 of the comparing circuit 3 controls the gain of the gain variable circuit 1 to the maximum value, and generates the maximum value signal S5. A control circuit 5 sets forcibly a level of a control signal S6 applied to the control end of the gain variable circuit 1 to the minimum value set by a reference level R3 from the original output level S4 of the comparing circuit 3 in accordance with the maximum value signal S5. In such a way, even at the time of applying an input signal from an input signal disconnected state, it does not occur that an excessive output level is sent out of the variable gain circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an automatic gain control circuit (hereinafter abbreviated as AGC circuit) which is widely used in receivers for high frequency signals.

[0002]

2. Description of the Related Art In various communication systems, the signal level of a high frequency signal or the like input to a receiving device changes due to factors related to a transmission line, a transmitting device and the like. In order to stabilize the signal output level of the receiving device against such a change in the signal input level, the AGC circuit is widely used in the receiving device.

FIG. 2 shows a block diagram of a conventional AGC circuit. The output level S2 of the gain variable circuit 1 that performs variable gain amplification of the input signal S1 such as a high frequency signal is level-detected by the level detection circuit 2, and the detection level S3 is compared with the reference level R1 by the comparison circuit 3. The comparator circuit 3 controls the gain of the gain variable circuit 1 by the control signal S6 so that these levels S3 and R1 match. In the level detection circuit 2, a method for detecting the total amplitude of the output signal level S2 or AG
A method of extracting the pilot signal provided in the input signal S1 provided for C and detecting the pilot signal level is generally used.

In the above AGC circuit, when the input signal S1 is cut off, the gain of the variable gain circuit 1 is set to the maximum value as the circuit operation. When the input signal S1 is applied in this setting state, an excessive signal output level S2 is sent at that time,
After that, the AGC circuit functions with the lapse of time to perform an operation of converging to a predetermined output level. The outline of this situation is shown by the solid line in the figure showing the outline of the time response of the output signal level in FIG.

[0005]

DISCLOSURE OF THE INVENTION The above-mentioned conventional AGC
In the circuit, if the input signal is suddenly applied from the disconnection state,
For example, when the power of the transmitter is turned on, an excessive signal output level is transmitted for a while according to the operation response time of the AGC circuit, which hinders the operation of the device connected in the subsequent stage. There was a problem.

Therefore, an object of the present invention is to prevent the generation of a transient excessive output level, which is a drawback of the conventional AGC circuit.
It is to provide a GC circuit.

[0007]

The AGC circuit of the present invention comprises:
Means is provided for controlling the gain of the variable gain circuit when the input signal is cut off to the minimum value.

That is, the AGC circuit of the present invention comprises an input terminal for an input signal, an output terminal for an output signal, and a control terminal for inputting a control signal for controlling the output signal level to a constant value even when the input signal changes. Gain control means for controlling the amplification gain by the control signal, first detection means for detecting the level of the output signal from the output end, and output levels of the first detection means and a first reference level. And a first comparing means for generating the control signal, the automatic gain control circuit further monitoring the control signal output from the first comparing means, and controlling the level of the control signal. Is a level that maximizes the gain of the gain varying means, a second detecting means for producing a maximum value signal, and the control signal in response to the maximum value signal, the amplification gain of the gain varying means. And a control means for supplying to said control terminal in place of the control signal for setting a minimum value.

[0009]

The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of an AGC circuit according to the present invention. In addition to the conventional AGC circuit of FIG. 2, the AGC circuit of FIG. 1 includes a detection circuit 4 connected to the output end of the comparison circuit 3, an output end of the comparison circuit 3 and a control end of the gain variable circuit 1. And a control circuit 5 connected between the two.

The detection circuit 4 detects that the control output level S4 of the comparison circuit 3 controls the gain of the variable gain circuit 1 to the maximum value and generates the maximum value signal S5. The control circuit 5 sets the level of the control signal S6 applied to the control end of the variable gain circuit 1 in accordance with the maximum value signal S5 output from the detection circuit 4 so that the gain of the circuit 1 is minimized.

When the input signal S1 to the variable gain circuit 1 is disconnected, the output level S4 of the comparison circuit 3 becomes a level that maximizes the gain of the variable gain circuit 1. Each level (S3,
The relationship between the large and small directions and polarities of (S4, S5, and S6) is not related to the principle of the present invention, but here, for convenience of description, the variable gain circuit 1 has a maximum value of the output level S4 of the comparison circuit 3. Assuming that the maximum gain is obtained in the circuit 1 with respect to the minimum value of the output level S4, the output level S4 of the comparison circuit 3 becomes the maximum value in the above state. Then, in the detection circuit 4, by comparing the output level S4 of the comparison circuit 3 with the second description level R2 in the built-in comparison circuit 7, it is confirmed that the output level S4 of the comparison circuit 3 becomes the maximum value. To detect. The integrating circuit 6 connected between the comparing circuits 3 and 7 can generate the maximum value of the output level S4 of the comparing circuit 3 which can occur when the input signal S1 is not broken, for example, the input signal level S1 from large to small. The comparison circuit 7 compares the transient maximum value that occurs in response to a sudden change.
Is a protection circuit provided so as not to perform the detection operation.

As described above, the detection circuit 4 detects the maximum non-transient gain state from the maximum output level S4 of the comparison circuit 3 and controls the control circuit 5 to minimize the gain of the amplification gain variable circuit 1. Based on the level of the control signal S6, the output level S4 of the comparison circuit 3 is forcibly changed to the third reference level R.
Set to the minimum value set in 3.

As described above, this AGC circuit has a function of forcibly setting the gain minimum state to the minimum gain state after the time corresponding to the set value of the integrating circuit 6 has passed in the maximum gain state, and the state of disconnection of the input signal S1. When the input signal S1 is applied to the output signal S1 from the above, the operation of gradually converging the level of the output signal S2 from a low level to a predetermined level as shown by the curve of the broken line in FIG. 3 can be achieved. it can.

[0015]

As described above, the present invention detects the continuous maximum gain state of the variable gain circuit and, in response thereto, forcibly sets the circuit to the minimum gain state, thereby eliminating the input signal interruption. Even when the input signal is applied from the state, the variable gain circuit does not send an excessive output level.

[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 AGC circuit.

FIG. 3 is a diagram showing an outline of a time response of an output signal level of an AGC circuit according to the related art and the present invention.

[Explanation of symbols]

 1 gain variable circuit 2 level detection circuit 3 comparison circuit 4 detection circuit 5 control circuit 6 integration circuit 7 comparison circuit

Claims (3)

[Claims] 1. An input signal input terminal, an output signal output terminal, and a control terminal for inputting a control signal for controlling the output signal level to a constant value with respect to changes in the input signal are provided, and the amplification gain is controlled by the control signal. Gain control means for controlling, first detecting means for detecting the level of the output signal from the output end, and the control signal is generated corresponding to the output level of the first detecting means and the first reference level. In the automatic gain control circuit including a first comparing means, the automatic gain control circuit further monitors the control signal output from the first comparing means, and the level of the control signal is the gain of the gain varying means. And a control signal for setting the amplification gain of the gain varying means to the minimum value, in response to the maximum value signal, the second detection means generating a maximum value signal. To An automatic gain control circuit, further comprising: a control unit that supplies the control end to the control end. 2. The second reference means is substantially equal to the level of the control signal from the first comparing means and the level of the control signal that maximizes the gain of the gain varying means. 2. The automatic gain control circuit according to claim 1, wherein the maximum value signal is generated when the level of the control signal is higher than or equal to the second reference level by comparing with the level. 3. The automatic gain control circuit according to claim 1, wherein the control means is arranged between an output end of the first comparing means and a control end of the gain varying means.