JPS623930Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an AGC circuit used in communication equipment.

The AGC circuit automatically adjusts the gain in response to fluctuations in the input signal and keeps the output signal level constant.
Widely used in communication equipment such as FM receivers and data modems.

FIG. 1 shows the configuration of a conventionally used AGC circuit. This circuit consists of an attenuator 1 that adjusts the gain, an absolute value circuit (full wave rectifier circuit) 2 that converts the signal output from the attenuator 1 into a DC level signal, and a signal output from the absolute value circuit 2. A low-pass filter 3 removes pulsating components from the signal, and a comparator 4 that compares the level of the DC signal after passing through the low-pass filter 3 with a reference AGC setting level and generates a gain setting signal corresponding to the level difference between the two. It is configured. In this circuit, from input terminal 5 to attenuator 1
When a signal is supplied to the attenuator 1, the comparator 4
The attenuation rate is sequentially set in accordance with a gain setting signal supplied from the controller at predetermined time intervals. As a result, an output signal adjusted to a desired level is obtained at the output terminal 6 provided on the output side of the attenuator 1.

By the way, conventional AGC circuits like this used a low-pass filter in the feedback loop, so
There was a delay in the gain setting signal caused by the filter. For this reason, when the gain switching speed of the attenuator 1 is increased, the attenuation rate of the attenuator 1 no longer follows the level of the input signal, resulting in a problem that the AGC circuit oscillates. In other words, the conventional AGC circuit has the disadvantage that it cannot be used for high-speed modems and the like that require shortened response times.

This invention was made in view of the above circumstances, and is an AGC that can sufficiently shorten the response time.
The purpose is to provide circuits.

In the present invention, in addition to the conventional feedback loop that passes through a low-pass filter, a feedback loop that does not pass through a low-pass filter is provided, and the attenuator is controlled in two stages by these loops to achieve the above-mentioned object.

The present invention will be described in detail with reference to Examples below.

The same parts as in Fig. 1 are given the same reference numerals as in Fig. 2.
The figure shows an AGC circuit provided with a feedback loop for comparing pulsating current signals output from the absolute value circuit 2. This second feedback loop includes a second
A second comparator 7 with an AGC set level is provided.

The operation of this AGC circuit will be explained using FIG. 3. Assume that a carrier arrives as an AGC input at time _t1 , as shown in FIG. Before this time _t1 , the input terminal 5 is affected by noise appearing in the transmission system. Since the noise level is relatively small, the attenuator 1 is set to the minimum attenuation rate under the control of the comparators 4 and 7.

When the carrier arrives at time _t1 , the attenuator 1 attenuates the level of the carrier at the set minimum attenuation rate (FIG. 3b). As a result, the output level of the absolute value circuit 2 increases rapidly and exceeds the second AGC setting level _l2 of the comparator 7 at time _t2 sampled by the sampling clock e. The comparator 7 thereby supplies a gain setting signal to the attenuator 1 and rapidly increases its attenuation rate.

To explain this more specifically, when a digital attenuator is used as the attenuator 1, the second comparator 7 is activated from the stage when the output level of the absolute value circuit 2 exceeds the second AGC setting level _l2 . A signal is sent that causes an up-down counter (not shown) to significantly count up or count down. Then, the attenuation rate of the digital attenuator is significantly changed by a decoder (not shown) provided on the output side of the up-down counter.

When the level of the output signal of the absolute value circuit 2, in which the attenuation rate of the attenuator 1 is roughly set in this way, falls below the second AGC setting level _l2 at time _t2 , the second comparator 7 starts its operation. stop. After this, the output level of low-pass filter 3 and the first
The AGC setting level l ₁ (|l ₁ | < | l ₂ |) is the first
are compared by a comparator 4. That is, the first comparator 4 supplies a gain setting signal to the attenuator 1 and gradually increases or decreases its attenuation rate.
Keep AGC output at a constant level. Such a circuit operation can be realized by sending a signal from the first comparator 4 to cause the up-down counter to perform a small count-up or count-down.

In this way, according to the present invention, the gain is quickly set using a feedback loop that does not include a filter, so the delay caused by the feedback loop is very small, and oscillation does not occur even if the speed of the attenuator gain control is significantly increased. There isn't.

In the embodiment, the output signal of the absolute value circuit is used as the signal that does not pass through the low-pass filter, but instead of this, the alternating current signal output from the attenuator may be used directly. In this case, the result of instantaneous value sampling is supplied to a second comparator that compares levels in both positive and negative directions.

Furthermore, in the embodiment, there is no particular restriction on the operating time of the second comparator that compares signals that do not pass through the low-pass filter, but the operation of the second comparator is restricted after a predetermined time has elapsed after the signal arrives. It is also possible to do so. Such a circuit can be easily constructed using a circuit for detecting the arrival of a signal and a timer circuit.

Furthermore, in the embodiment, an attenuator (variable attenuator) is provided between the input terminal and the output terminal, but an amplifier with a fixed amplification factor may be used in conjunction with this, or a variable gain amplifier may be used in place of the attenuator. Of course, it is okay to do so.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional AGC circuit, Figure 2 is a block diagram of a conventional AGC circuit.
The figure is a block diagram of an AGC circuit according to an embodiment of the present invention, and FIG. 3 is a waveform diagram for explaining the operation of each part of the AGC circuit according to the embodiment. 1...Attenuator, 2...Absolute value circuit, 3...
...Low pass filter, 4...First comparator, 7...
...Second comparator.

Claims (1)

[Scope of utility model claims] a setting means for amplifying or attenuating the input signal and setting the level of the output signal; an absolute value circuit for rectifying the alternating current signal output from the setting means; and a low-pass for removing ripple components from the output signal of the absolute value circuit. a filter, a first comparator that compares the DC signal level after passing through the low-pass filter and a first AGC setting level serving as a reference; and a second AGC setting level and an AC signal output from the setting means or the first AGC setting level. a second comparator that compares the pulsating flow signal output from the absolute value circuit by instantaneous value sampling; and controlling the gain of the setting means based on gain setting signals output from the first and second comparators. An AGC circuit comprising gain control means for controlling the gain.