JPH01154613A - Automatic gain control amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent the reduction in a transmission signal level by applying gain control based on a signal obtained from the correlation between an estimated signal and a transmission signal. CONSTITUTION:A reference signal is generated by a reference signal generating circuit 5 and fed to a correlation device 4 while its level is always kept constant. The correlation device 4 takes correlation between a reference signal and an output of an amplifier 3 and a low frequency signal corresponding to the level of the transmission signal itself is obtained from the output of the amplifier 3 without being affected by noise. The low frequency signal is detected by a detector 6 and fed to a gain variable element of the amplifier 3 through a loop filter 7. Since the control voltage fed to the gain variable element is a voltage corresponding to the transmission signal itself, the level of the transmission signal of the amplifier 3 is kept always constant and the substantial automatic gain control is attained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an automatic gain control amplifier circuit for a receiving device that receives very low level signals such as a line-of-sight communication system and keeps the output level constant. .

[Conventional technology]

The automatic gain control amplifier circuit of a conventional receiving device has the configuration shown in Figure 2, in which the signal received by the receiving antenna 1 is mixed with the local oscillation signal by the down converter 2 and converted to a low frequency generally called an intermediate frequency. After frequency conversion, the signal is amplified by a variable gain amplifier 3, and fluctuations in the input signal level are absorbed by this amplifier 3, and a constant output level is output from the output port.

Here, automatic gain control is applied to keep the output level constant, and its operation is to take out a part of the output of the amplifier 3, detect its level with the detection circuit 6, and filter the loop filter 7, which has a DC amplification function. This was achieved by negative feedback to the variable gain element, which is a component of the amplifier 3, after passing through the signal.

For example, when the input signal level increases, the output level of the amplifier 3 tries to increase, but this level is detected by the detection circuit 6, creates an error signal with respect to the reference level, and the error signal is amplified by the loop filter 7. and amplifier 3
By controlling the variable gain element of the amplifier 3 and operating it to lower the gain, as a result, the gain of the amplifier 3 decreases by the amount that the received input level increases, and the output level becomes constant.

[Problems to be Solved by the Invention] In the conventional automatic gain control amplifier circuit described above, automatic gain control is performed by detecting the output of the amplifier 3; However, the gain is controlled so that the level of the detected signal remains at a constant value, which causes the following problems.

That is, the output of a down converter in a receiving device generally consists of the transmitted signal and the noise generated at the receiver input, and the output level is given by the power phase of the transmitted signal and noise. will be detected.

FIG. 3 shows the detection voltage characteristics of the detection circuit 6. In this figure, characteristic a indicates a characteristic when noise is superimposed on a signal in a normal receiving state, or a detection characteristic when noise is removed from the conditions of characteristic a and only a pure transmission signal is left.

In characteristic a, in a region where the received signal level is high and the C/N (signal-to-noise ratio) is good, the noise can be ignored with respect to the transmitted signal level, so the detection voltage changes according to the transmitted signal level, but the C/N In the region where the voltage has deteriorated, the noise cannot be ignored, and when the voltage decreases further, the noise becomes dominant, and the detected voltage is almost determined by the noise power. Under this situation, amplifier 3 operates as an automatic gain control amplifier.
Since the total level of the output is kept constant, the transmission signal level decreases by the amount that the C/N ratio deteriorates, and the original function of keeping the signal level itself constant is impaired.

Originally, the detection characteristics of only the transmission signal are as shown in the characteristics, and if this characteristic is obtained, the transmission signal level will be kept constant at the output of the amplifier 3, but in the conventional method, the detection characteristics of the transmission signal and the noise superimposed on it will be kept constant. It is impossible to separate C
There was a problem in that the transmission signal level decreased in the region where /N deteriorated.

SUMMARY OF THE INVENTION An object of the present invention is to provide an automatic gain control amplifier circuit that can prevent a drop in transmission signal level.

[Means for solving problems]

The automatic gain control amplifier circuit of the present invention uses a reference signal from an estimated signal of a signal sent out on the transmitting side to an automatic gain control amplifier circuit that controls the gain of the amplifier to be constant according to the output level of the amplifier that amplifies the received signal. and a correlator that outputs a signal correlated with the transmission signal from the output level of the amplifier and the reference signal, and controls the gain of the amplifier based on the signal output from the correlator. It is configured to do this.

In other words, when we take the correlation between the amplifier output and the estimated signal, we find that there is a correlation corresponding to the level between the original transmission signal included in the amplifier output and the estimated signal, but there is a correlation between the noise and the estimated signal. This is based on the fact that there is no correlation, which allows the correlator to output a signal corresponding to the original signal level.

〔Example] Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In this figure, a signal received by a receiving antenna 1 is frequency-converted to a low frequency by a down converter 2, becomes an intermediate frequency signal, is amplified by an amplifier 3, and is output from an output port with a constant output signal level. .

This output level is detected by a detection circuit 6 through a correlator 4, and is negatively fed back to the variable gain element of the amplifier 3 through a loop filter 7 having a DC amplification function. Further, a reference signal generation circuit 5 is connected to the correlator 4, and these constitute an automatic gain control circuit.

Here, although communication signals are generally transmitted using various modulation methods, we will focus on digital modulation methods such as phase modulation methods as the target of this method. This is because, according to these systems, even when receiving signals at a considerably low level, it is possible to reproduce the signal (replica of the transmitted signal) on the receiving side that is considered to have been originally sent by the transmitting side.

These transmission estimation signals can be easily obtained from the output of the demodulator, and by modulating the carrier of the intermediate frequency with this reproduced signal, the signal to be originally transmitted in the intermediate frequency range, that is, the reference signal, can be obtained. This reference signal is generated by the reference signal generation circuit 5.
It is added to the correlator 4 with a constant level.

In the correlator 4, a correlation is taken between the reference signal and the output of the amplifier 3, but since the output of the amplifier 3 is given by a composite wave of the signal and noise transmitted on the transmitting side, the output of the correlator is It will be as follows.

First, regarding the correlation with the transmission signal, since the level of the reference signal is constant, the output level of the correlator is a voltage corresponding to the level fluctuation of the transmission signal, and that voltage is the difference between the reference signal and the transmission signal. It will appear with a frequency and phase difference between the two. Therefore, by passing the signal through a low-pass filter (integrator) that sufficiently passes phase 9 frequency fluctuations of the transmission signal, a low frequency signal having a voltage corresponding to the level fluctuation of the transmission signal can be extracted.

On the other hand, the correlation between the noise and the reference signal is that the noise itself is thermal noise generated at the receiver input terminal, and the amplitude is small.

Since the phase varies randomly, there is no correlation with the reference signal, and the average value after passing through the low-pass filter becomes zero, and as a result, the output voltage of the correlator becomes zero.

By the above-described operation of the correlator, a low frequency signal corresponding to the level of the transmission signal itself can be obtained from the output of the amplifier 3 without being affected by noise.

This low frequency signal is detected by a detector 6, and a loop filter 7
to the variable gain element of amplifier 3. Since the control voltage applied to the variable gain element is a voltage corresponding to the transmission signal itself, the level of the transmission signal of the amplifier 3 is always constant, and the original automatic gain control is performed.

〔Effect of the invention〕

As explained above, the present invention performs gain control based on the signal obtained from the correlation between the estimated signal and the transmitted signal, so it is possible to perform automatic gain control to keep the level of the signal to be transmitted constant. Even if the C/N deteriorates in the region of low reception level and the noise level cannot be ignored compared to the signal level, it is possible to prevent the transmission signal level from decreasing. In particular, in systems where communication is always performed at a low reception level, such as non-line-of-sight communication, keeping the transmission signal level constant is effective in smoothing the operation of the demodulation circuit following the automatic gain control amplifier circuit. It's very large.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a conventional block diagram, and FIG. 3 is a detection voltage characteristic diagram of a detection circuit. 1... Receiving antenna, 2... Down converter, 3
...Amplifier, 4...Correlator, 5...Reference signal generation circuit, 6...Detection circuit, 7...Loop filter. Figure 1

Claims (1)

[Claims] (1) In an automatic gain control amplifier circuit comprising an amplifier that amplifies a signal received by a receiving antenna, and a detector and a loop filter that control the gain of the amplifier to be constant according to the output level of the amplifier, on the transmitting side. a reference signal generation circuit that reproduces a reference signal from an estimated signal of the transmitted signal;
A correlator is provided that outputs a signal correlated with the transmission signal from the output level of the amplifier and the reference signal, and the detector and loop filter control the gain of the amplifier based on the signal output from the correlator. An automatic gain control amplifier circuit configured to perform the following steps.