JPS58141018A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To obtain an AGC circuit which has superior transient response characteristics to an instantaneous excessive input by performing gain control so that the output level of a variable gain amplifier is stabilized, and leading out the output of the variable gain amplifier or amplitude limiter as an output signal. CONSTITUTION:A negative feedback circuit is equipped with the amplitude limiter 13 to perform control so that the gain control voltage (output of gain control signal generating circuit 7) applied to the variable gain amplifier 4 does not exceed a prescribed value. Then, no excessive signal is inputted to the feedback circuit of the AGC circuit. Consequently, the output of the AGC circuit when a signal rising abruptly is inputted has a minimum level in rising less than when no amplitude limiter is provided and level suppression of time succeeding it is less. Therefore, a fast return to a stationary state is obtained in case of an instantaneous excessive input to improve transient response characteristics.

Description

[Detailed description of the invention] The present invention relates to an automatic gain control circuit (AGC circuit).

Syllable companders are often used to improve the signal-to-noise ratio (8/N) of communication lines. In addition, AG is used to limit the output signal level in response to excessively high input signals.
C circuits are also often used.

On the transmitting side of an overconfident line, both a syllable compander compressor and an AGC circuit are often used in combination.The compressor for 8/N improvement is placed in the front stage as much as possible to reduce the output level for excessive and large input signals. It is common that the AGC circuits for restriction are arranged as late as possible. Since the compressor is at its maximum gain when there is no input signal, the compression function does not work sufficiently for the steeply rising part of the input signal, resulting in an excessive transient output. AGC
The circuit works squarely to reduce the output level of the AGC circuit in order to try to compress this excessive transient output, but A
The time constant (recovery time) of the GC circuit is usually long, so even after the compressor recovers from a transient state to a steady state, the gain remains low for some time. ], there is a part where the -1 level is too low, and it is difficult to keep the output signal level constant.
This was contrary to the purpose of the GC circuit.

FIG. 1 is a block diagram showing a conventional AGC circuit when receiving a signal from a compressor, and FIG. 2 is a signal waveform diagram of each part thereof. A signal 21 input to the input terminal 1 passes through the compressor 2 and becomes a signal 22, which is further amplified by the variable gain amplifier 4 of the AGC circuit 3 and output from the output terminal 5 to an external device. A
The GC circuit 3 includes a gain amplifier 4, a level fluctuation detection circuit (detection 1)) and a gain control signal generation circuit 7.
A negative feedback circuit is provided. The level of the output signal 230 obtained from the output terminal 5 is excessive during the rising portion of the instantaneous excessive input, and becomes excessively low for a while after that, resulting in poor transient response characteristics.

The present invention provides an AG with excellent transient response characteristics against instantaneous excessive input.
The purpose is to provide C circuits.

The configuration lines of the automatic gain control circuit according to the present invention include a variable gain amplifier receiving an input signal, an amplitude limiter receiving the output of this variable gain amplifier, a level fluctuation detection circuit receiving this amplitude limited tibo output, and this level fluctuation detection a circuit receiving the circuit output and generating a signal for controlling the gain of the variable gain amplifier, the gain control being performed by the variable gain amplifier O.
The present invention is characterized in that the output level is stabilized in the direction shown in FIG.

Next, the present invention will be described in detail with reference to the drawings.

FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. 4 is an output waveform diagram thereof. This AGC circuit includes an amplitude limiter 13 in the negative feedback circuit, and controls the gain control voltage (output of the gain control signal generation circuit 7) applied to the variable gain amplifier 4 so that it does not exceed a predetermined value. This prevents excessive signals from being input to the feedback circuit Ka of the AGC circuit. Therefore, when a sharply rising signal is input, this A
The GC circuit O output has a smaller maximum level at startup than when there is no amplitude control @O, and there is less suppression of the 0 level during the following time, so it is better when there is an instantaneous excessive input.
It quickly returns to steady state and improves transient response. When input waveform 22 illustrated in FIG. 2 is applied to terminal 11, terminal 12
The output waveform 33 at ' is as shown in FIG. 1st
The transient response is significantly improved compared to waveform 23 in the conventional circuit shown in the figure.

FIG. 5 is a circuit diagram showing another embodiment of the present invention. The output terminal 12 is connected to the output terminal of the variable gain amplifier 4 in a nine-way system, and the rest is the same as the circuit shown in FIG.

In the above explanation, the case where the output signal of the compressor is received has been described, but the present invention can also be applied to an input signal having an instantaneous excessive waveform similar to the compressor output, such as a pulse waveform accompanied by ringing or surge. exhibits an effect.

As described above in detail, one advantage of the present invention is that it is possible to provide an AGC circuit with excellent transient response characteristics against instantaneous excessive input.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional automatic gain control circuit that receives the compressor output, Fig. 2 is a signal waveform diagram of each part of the same, Fig. 1 Ics is a circuit diagram showing an embodiment of the present invention, and Fig. 4 is a solid state output waveform. FIG. 5 is a circuit diagram showing another embodiment of the present invention. 2... Compressor, 3... Automatic gain control circuit, 4... Variable gain amplifier, 6... Level fluctuation detection circuit, 7... - Gain control signal generation circuit, 13...amplitude limiter. No. 1 Lock foil Z @ Sword 4 figure') 153 figure %5 figure - and

Claims (1)

[Claims] a variable gain amplifier that receives an input signal; an amplitude limiter that receives the output of the variable gain amplifier; a level fluctuation detection circuit that receives the output of the amplitude limiter; a circuit that generates a signal for controlling the gain, the gain control is performed in a direction to stabilize the output level of the variable gain amplifier, and the output of the variable gain amplifier or the amplitude limiter is sent to the outside as an output signal. An automatic gain control circuit characterized by: