JPH05291986A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To compensate the delay of AGC signals and to correspond to the rapid fluctuation of input signal level due to phasing by providing a delay circuit to delay the input signals to a level control circuit. CONSTITUTION:A demodulation main signal A is mixed with the output of a 2nd local oscillator 9 by means of a 2nd intermediate frequency amplifier 7 and a 2nd mixer 8. An FSK signal is taken out through a 2nd intermediate frequency filter 10. On the other hand, an AGC level measurement signal B becomes one input of a differential amplifier 14 through a 1st intermediate frequency amplifier 11, a detector 12, and a low-pass filter 13. The other input outputs an AGC signal from a reference voltage source 15. The output of the 2nd intermediate frequency filter 10 is inputted to the 2nd intermediate frequency amplifier 17 through a delay circuit 16. The 2nd intermediate frequency amplifier 17 amplifies the output signal from the delay circuit 16 with the gain based on the AGC signal of the differential amplifier 14 to be inputted to a demodulator 18 and outputs the demodulation signal of an FSK signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an automatic gain control (AG
C) It relates to a circuit.

[0002]

2. Description of the Related Art Conventionally, frequency modulation (FM) signals, phase modulation (PM) signals (including frequency shift keying (FSK) signals, phase shift keying (PSK) signals, and other digital modulation methods) are received. In machines, automatic gain control circuits are used to keep the input signal level to the demodulator constant. The automatic gain control circuit stabilizes the signal level by the following procedure.

(1) A high frequency (RF) signal or an intermediate frequency (IF) signal is partially branched at a predetermined signal level measurement point,
The branch signal is subjected to envelope detection by an envelope detector (AM detector), and the envelope detection output signal is band-limited using a low pass filter (LPF).

(2) After that, the LPF output signal and the reference voltage are compared by a comparator, and an AGC signal for controlling the gain of the variable gain amplifier or the attenuation rate of the variable attenuator is generated from the difference between them. To do.

(3) The gain of the variable gain amplifier or the attenuation rate of the variable attenuator is controlled by this AGC signal to keep the level of the signal input to the demodulator constant.

As such an automatic gain control circuit, a feedback type is known in which a variable gain amplifier or a variable attenuator for gain adjustment is arranged in front of a signal level measuring point.

[0007]

In such a conventional gain control circuit, the AGC signal changes after a certain time τ has elapsed since the level of the received signal changed. In the feedback type automatic gain control circuit, the delay time τ is the time t required for the received signal to propagate from the gain adjusting amplifier (variable gain amplifier) to the level measurement point.
It is impossible in principle to make it smaller than 0.

It is also conceivable to arrange a variable gain amplifier or variable attenuator for gain adjustment after the signal level measuring point to form a feedforward type automatic gain control circuit. In such an automatic gain control circuit, the time t1 from the level measurement point to the gain adjusting amplifier (variable gain amplifier) at the subsequent stage and the time t2 to the appearance of the AGC signal output from the comparator are almost the same. Since they can be the same, such an automatic gain control circuit is suitable for high speed data communication and the like. However, in the automatic gain control circuit, generally, t2> t because the time delay in the envelope detector in the signal generation circuit for generating the AGC signal is large.
There is a relation of 1, and the time t3 is between the time t1 and the time t2.
It has been found that there is a difference of (= t2-t1). Therefore,
When a sudden change in the input signal level due to fading occurs in a time shorter than time t3, the automatic gain control is delayed with respect to the change in the signal level, and the level of the signal input to the demodulator is kept constant. It has been found that there is an inconvenience in that it cannot be maintained. Further, in the feedforward type automatic gain control circuit, in order to be able to control it, it is necessary that the dynamic characteristics of the controlled object be completely accurately grasped and not changed, but this is a practical problem. Is impossible.

An object of the present invention is to solve such a problem and to provide an automatic gain control circuit capable of effectively preventing the deterioration of the reception quality due to the level fluctuation of the received signal.

[0010]

In order to achieve the above object, in the automatic gain control circuit of the present invention as set forth in the first aspect of the present invention, the signal level for detecting the fluctuation of the input of the receiver is detected. A signal generation circuit that generates an AGC signal based on the signal at the measurement point and a gain control circuit that is arranged at a stage subsequent to the signal level measurement point so that the demodulator input becomes constant based on the AGC signal generated by the signal generation circuit. A level control circuit for performing control, and further has a delay circuit for delaying an input signal to the level control means so as to compensate for the delay of the AGC signal.

In the automatic gain control circuit of the present invention as set forth in claim 2, the feedforward AGC signal and the feedback signal are detected based on the signal at the signal level measuring point in order to detect the fluctuation of the receiver input. AGC
First and second signal generation circuits that generate signals, and feedforward AGCs that are arranged in the subsequent and preceding stages of the signal level measurement point and that are generated by the first and second signal generation circuits
The first and second level control circuits are provided to control the gain so that the demodulator input becomes constant based on the signal and the feedback AGC signal. Further, the feedforward AGC signal responds to a sudden change in the signal level. In order to include at least the high frequency component, feedback AGC
The signal contains its low frequency components to accommodate gradual changes in signal level.

[0012]

In the automatic gain control circuit configured as described above, the delay circuit is provided to delay the input signal to the level control circuit to compensate for the delay of the AGC signal. It corresponds to the fluctuation of the input signal level. In addition, both the feedforward type and the feedback type are used, and since the feedback AGC signal includes a low frequency component of the signal level fluctuation, feedback control responds to a gentle fluctuation of the signal level.

[0013]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a diagram showing an FS for an 800 MHz band mobile telephone using an automatic gain control circuit according to a first embodiment of the present invention.
The main part of the K signal receiver is shown in a block diagram.

The received FSK signal is supplied to the high frequency amplifier 1,
After passing through the high frequency band filter 2, high frequency amplified and filtered, it is mixed with the output of the first local oscillator 4 by the first mixer 3 used for suppressing the image frequency with respect to the second intermediate frequency, The FSK signal down-converted to the first intermediate frequency by the first intermediate frequency filter 5 is taken out, amplified by the first intermediate frequency amplifier 6, and then outputted to the signal level measuring point T. The first intermediate frequency signal at the signal level measurement point T is divided into a demodulation main signal A and an AGC level measurement signal B.

The demodulation main signal A is the second intermediate frequency amplifier 7
After being amplified by, the second mixer 8 mixes it with the output of the second local oscillator 9, and the second intermediate frequency filter 10 further extracts the FSK signal down-converted to the second intermediate frequency. On the other hand, the AGC level measurement signal B is
After being amplified by the first intermediate frequency amplifier 11, envelope detection is performed by the envelope detector (AM detector) 12 and then filtered by the low pass filter 13. Low pass filter 1
The output of 3 is applied to one input of the differential amplifier 14 which constitutes a comparator, and the predetermined DC voltage of the reference voltage source 15 is applied to the other input of the differential amplifier. The differential amplifier 15 is AG
Output C signal. The output of the second intermediate frequency filter 10 is delayed by the delay circuit 16 and then added to the input of the second intermediate frequency amplifier 17, which is a variable gain amplifier.
The second intermediate frequency amplifier 17 has a delay circuit 16 with a gain based on the AGC signal of the differential amplifier 14 which constitutes a comparator.
The output signal from the amplifier is amplified and the amplified output is output to the demodulator 18. The demodulator 18 outputs a demodulated signal of the FSK signal.

As the constituent elements of the above-mentioned receiver, conventionally known ones can be used. As an example, the center frequency of the pass band of the first intermediate frequency filter 5 is set near 100 MHz. The center frequency of the pass band of the second intermediate frequency filter 10 is set to 10 to 20 times the rate of the FSK signal. A diode detector is used as the AM detector 12. As the second intermediate frequency amplifier 17, an operational amplifier using a FET (field effect transistor) is used. The cutoff frequency of the low-pass filter 13 is set so that the signal level fluctuation component based on the fading of the FSK signal can be passed. As the delay circuit 16, a digital filter, a delay line using a surface acoustic wave element, or the like can be used.

As the delay time of the delay circuit 16, the time t1 required for the demodulation main signal A to reach the amplifier 17 from the level measuring point T and the envelope of the AGC level measuring signal B at the level measuring point T are shown. The value is set so that the time t2 until the fluctuation appears as a change in the AGC signal of the output of the differential amplifier 14 constituting the comparator coincides. In order to set the delay time, the amplitude-modulated F for the purpose of measurement is used.
The SK signal is injected into the input of the high frequency amplifier 1 so that the envelope signal of the input intermediate frequency signal to the second intermediate frequency amplifier 17 and the AGC signal added thereto are aligned in time.
It is performed while observing the waveforms of both with a two-dimensional oscilloscope.

According to the receiver shown in FIG. 1, the level of the signal input to the demodulator 18 can be kept constant even if a sudden change in the received input level occurs due to fading or the like. It is possible to prevent the quality of the demodulated wave from deteriorating.

FIG. 2 shows a receiver similar to that of FIG. 1 using an automatic gain control circuit according to a second embodiment of the present invention. The same elements as those in FIG. 1 have the same reference numerals.

The received FSK signal is supplied to the high frequency amplifier 1,
After passing through the high frequency band filter 2, high frequency amplified and filtered, it is mixed with the output of the first local oscillator 4 by the first mixer 3 and down-converted to the first intermediate frequency by the first intermediate frequency filter 5. FSK signal is taken out,
After being amplified by the first intermediate frequency amplifier 6, the signal is output to the level measurement point T. The intermediate frequency signal at the level measurement point T is
It is divided into a demodulation main signal A and an AGC level measurement signal B.

The demodulation main signal A is the first intermediate frequency amplifier 7
After being amplified by, the second mixer 8 mixes it with the output of the second local oscillator 9, and the second intermediate frequency filter 10 further extracts the FSK signal down-converted to the second intermediate frequency. On the other hand, the AGC level measurement signal B is
After being amplified by the second intermediate frequency amplifier 11, the envelope is detected by the envelope detector (AM detector) 12 and then filtered by the low-pass filter 13. Low pass filter 1
The output of 3 is applied to one input of the differential amplifier 14 which constitutes a comparator, and the predetermined DC voltage of the reference voltage source 15 is applied to the other input of the differential amplifier.

The output of the second intermediate frequency filter 10 is the second
After being amplified by the intermediate frequency amplifier 19, it is converted into a binary code by the A / D converter 20 through a process of sampling, quantization, and binary coding. A / D converter 2
The binary code of the output of 0 is applied to one input of the coefficient unit 22 after passing through the delay circuit 21. On the other hand, the differential amplifier 14, which is a comparator, outputs an AGC signal. Differential amplifier 1
The AGC signal output from the A / D converter 4 is also converted into a binary code by the A / D converter 25 through a process of sampling, quantization and binary encoding. The binary code of the output of the A / D converter 25 is applied to the other input of the coefficient unit 22. The coefficient unit 22 is a kind of digital multiplier, and is an A / D
Based on the binary code output by the converter 25, an operation is performed so that the value of the binary code output from the delay circuit 21 is increased or decreased so that the input signal level of the demodulator 18 described later becomes constant. The output of the coefficient unit 22 is the D / A converter 2
Join 3. The analog output of the D / A converter 23 is
After passing through the second intermediate frequency band filter 24, the demodulator 18
Join in. The demodulator 18 outputs a demodulated signal of the FSK signal.

As the A / D converters 20 and 25 and the D / A converter 23, conventionally known converter circuits can be used, but they may be performed by software using a microcomputer. As the coefficient unit 22,
A conventionally known digital multiplication circuit can be used, but software may be used. A known delay circuit using a shift register or the like can be used as the delay circuit 21, but a memory may be used to perform the reading and writing. The delay time of the delay circuit 21 is the time t1 required for the demodulation main signal A to reach the coefficient unit 22 from the level measurement point T and the envelope of the AGC level measurement signal B at the level measurement point T. Fluctuation is A /
The value is set so that the time t2 until it appears as a change in the output of the D converter 25 matches.

According to the receiver shown in FIG. 2, it is possible to cope with a sudden change in the received input level, the setting and adjustment of the delay time of the delay circuit 21 are simplified, the accuracy of the delay time is high, and the coefficient unit is used. 22 has the effect that the gain can be easily adjusted.

FIG. 3 shows a receiver similar to that of FIG. 1 using an automatic gain control circuit according to a third embodiment of the present invention. The same elements as those in FIG. 1 have the same reference numerals.

The received FSK signal is supplied to the high frequency amplifier 2
6. After passing through the high frequency band filter 2, high frequency amplified and filtered, the first mixer 3 mixes it with the output of the first local oscillator 4 and down-converts the FSK signal by the first intermediate frequency filter 5. After being taken out and amplified by the first intermediate frequency amplifier 6, it is output to the level measuring point T. The intermediate frequency signal at the level measurement point T is divided into a demodulation main signal A and an AGC level measurement signal B.

The demodulation main signal A is the first intermediate frequency amplifier 7
After being amplified by, the second mixer 8 mixes with the output of the second local oscillator 9, and the second intermediate frequency filter 10 further down-converts the first intermediate frequency FSK signal to be taken out. On the other hand, the AGC level measurement signal B is
After being amplified by the first intermediate frequency amplifier 11, the envelope is detected by the envelope detector (AM detector) 12 and output to the branch point U. The envelope detection output at the branch point U is further divided into a feedback AGC level measurement signal C and a feedforward AGC level measurement signal D. The feedback AGC level measurement signal C is filtered by the low-pass filter 29. The output of the low pass filter 29 is
A predetermined DC voltage of the reference voltage source 31 is applied to one input of the differential amplifier 30 which constitutes the comparator, and to the other input of the differential amplifier 30. The output of the differential amplifier 30 generates a feedback AGC signal L and is supplied to the high frequency amplifier 26. The feedforward AGC level measurement signal D is applied to the bandpass filter 27. Bandpass filter 27
Is added to the voltage of the reference power supply 28 to become a unipolar signal, and then added to one input of the differential amplifier 14 which is a comparator. The DC voltage of the reference power supply 15 is applied to the other input of the differential amplifier 14. The output of the differential amplifier 14 generates a feedforward AGC signal H and is supplied to the second intermediate frequency amplifier 17.

The cutoff frequency of the low-pass filter 29 may be set to be the same as the low-pass cutoff frequency of the bandpass filter 27, or may be set so that the passbands thereof partially overlap.
A low-pass filter may be used instead of the band-pass filter 27, and the cutoff frequency thereof may be set higher than that of the low-pass filter 29. The cut-off frequency of the low-pass filter 29 is set so as to prevent a sudden change component of the reception input level due to fading or the like. A dual-gate FET is used as the high-frequency amplifier 26, and the reception signal is supplied to one of the gates and the feedback AGC signal L is supplied to the other gate. As the delay time of the delay circuit 16, the time t1 required for the demodulation main signal A to reach the amplifier 17 from the level measuring point T and the level measuring point T
The value is set such that the fluctuation of the envelope of the AGC level measurement signal B at 1 appears at the time t2 until the variation of the AGC signal of the output of the differential amplifier 14 constituting the comparator appears.

According to the receiver shown in FIG. 3, the differential amplifier 1
The feedforward AGC signal which is the output signal of 4 is
It is supplied to the intermediate frequency amplifier 17 as an AGC signal corresponding to a rapid signal level change due to fading or the like.
On the other hand, the feedback AGC signal L, which is the output signal of the differential amplifier 30, corresponds to the gradual fluctuation of the reception level caused by the increase of the distance between the transmitting station and the receiving station.
It is supplied to the high frequency amplifier 26 as a C signal. Therefore,
Good gain control can be maintained even if the influence of an unexpected disturbance is added.

FIG. 4 shows a receiver similar to FIGS. 2 and 3 using an automatic gain control circuit according to a fourth embodiment of the present invention.
The same elements as those in FIGS. 2 and 3 are designated by the same reference numerals.

The received FSK signal is supplied to the high frequency amplifier 2
6. After passing through the high-frequency band filter 2, high-frequency amplified and filtered, mixed by the first mixer 3 with the output of the first local oscillator 4, and down-converted to the first intermediate frequency by the first intermediate-frequency filter 5. The FSK signal thus obtained is taken out, amplified by the first intermediate frequency amplifier 6, and then outputted to the level measuring point T. The intermediate frequency signal at the level measurement point T is divided into a demodulation main signal A and an AGC level measurement signal B.
Be divided.

The demodulation main signal A is the first intermediate frequency amplifier 7
After being amplified by, the second mixer 8 mixes it with the output of the second local oscillator 9, and the second intermediate frequency filter 10 further extracts the FSK signal down-converted to the second intermediate frequency. On the other hand, the AGC level measurement signal B is
After being amplified by the third intermediate frequency amplifier 11, the envelope is detected by the envelope detector (AM detector) 12 and output to the branch point U. The envelope detection output at the branch point U is further divided into a feedback AGC level measurement signal C and a feedforward AGC level measurement signal D. The feedback AGC level measurement signal C is filtered by the low-pass filter 29. The output of the low pass filter 29 is
A predetermined DC voltage of the reference voltage source 31 is applied to one input of the differential amplifier 30 which constitutes the comparator, and to the other input of the differential amplifier 30. The differential amplifier 30 generates the feedback AGC signal L, and the feedback AGC signal L is supplied to the high frequency amplifier 26. The feedforward AGC level measurement signal D is applied to the bandpass filter 27. The output of the band-pass filter 27 is added to the voltage of the reference power supply 28 to become a unipolar signal, and then applied to one input of the differential amplifier 14 which is a comparator. The DC voltage of another reference power supply 15 is applied to the other input of the differential amplifier 14. The output of the differential amplifier 14 is a feedforward AGC.
Generate the signal H.

The output of the second intermediate frequency filter 10 is the second
After being amplified by the intermediate frequency amplifier 19, it is converted into a binary code by the A / D converter 20 through a process of sampling, quantization, and binary coding. A / D converter 2
The binary code of the output of 0 is added to one input of the coefficient unit 22 after passing through the delay circuit 21. On the other hand, the output of the differential amplifier 14 which is a comparator is also converted into a binary code by the A / D converter 25 through a process of sampling, quantization and binary coding. The binary code of the output of the A / D converter 25 is applied to the other input of the coefficient unit 22. Coefficient unit 2
2 performs an operation based on the binary code of the A / D converter 25 so that the value of the binary code of the output of the delay circuit 21 increases or decreases, and the input signal level to the demodulator 18 described later becomes constant. To do so. The output of the coefficient unit 22 is applied to the D / A converter 23. The analog output of the D / A converter 23 passes through the second intermediate frequency band filter 24, and the demodulator 18
Join in. The demodulator 18 outputs a demodulated signal of the FSK signal.

The delay time of the delay circuit 21 is as follows.
The time t1 required for the demodulation main signal A to reach the coefficient unit 22 from the level measurement point T, and the fluctuation of the envelope of the AGC level measurement signal B at the level measurement point T depend on the A / D converter 25.
It is set to a value such that the time t2 until it appears as a change in the output of is in agreement.

The receiver shown in FIG. 4 can cope with a sudden change in the signal level due to fading or the like, and
Good gain control can be maintained even if the influence of an unexpected disturbance is added. In addition, the delay time of the delay circuit 21 can be easily set and adjusted, the accuracy of the delay time is high, and the coefficient unit 22 can easily adjust the gain.

[0037]

As described above, according to the automatic gain control circuit of the present invention, even when a sudden change in the input signal level occurs, or when an unexpected disturbance is applied,
It is possible to keep the level of the signal input to the demodulator of the receiver at a good level. Further, in a mobile communication system such as a car phone, the signal processing for generating the AGC signal is complicated and time-consuming, and the time difference between the AGC signal and the input signal of the level control circuit tends to be large. Is particularly useful for receivers in such communication systems.

[Brief description of drawings]

FIG. 1 is a block diagram showing a receiver using an automatic gain control circuit according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a receiver using an automatic gain control circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a receiver using an automatic gain control circuit according to a third embodiment of the present invention.

FIG. 4 is a block diagram showing a receiver using an automatic gain control circuit according to a fourth embodiment of the present invention.

[Explanation of symbols]

 1, 26 high frequency amplifier 2 high frequency band filter 3, 8 mixer 4, 9 local oscillator 5 first intermediate frequency band filter 6, 7, 11 first intermediate frequency amplifier 10, 24 second intermediate frequency filter 12 AM detector 13, 29 Low-pass filter 14,30 Comparator 15,28,31 Reference voltage source 16,21 Delay circuit 17,19 Second intermediate frequency amplifier 18 Demodulator 20,25 A / D converter 22 Coefficient unit 23 D / A converter 27 Band filter

Claims (2)

[Claims] 1. A signal generation circuit for generating an AGC signal based on a signal at a signal level measurement point to detect fluctuations in a receiver input, and a signal generation circuit disposed after the signal level measurement point,
A feedforward type automatic gain control circuit comprising a level control circuit for controlling gain so that a demodulator input becomes constant based on an AGC signal generated by a signal generation circuit. An automatic gain control circuit having a delay circuit for delaying an input signal to a level control circuit to compensate. 2. A feedforward AGC based on a signal at a signal level measuring point for detecting fluctuations in a receiver input.
Signal and feedback First and second signal generation circuits for generating AGC signals, and feedforward AGC signals and feedbacks generated by the first and second signal generation circuits, which are arranged at the latter stage and the former stage of the signal level measurement point. What is claimed is: 1. An automatic gain control circuit using both a feedforward type and a feedback type, comprising first and second level control circuits for controlling gain so that a demodulator input becomes constant based on an AGC signal. The forward AGC signal contains at least its high-frequency component in order to respond to a sudden change in signal level, and the feedback AGC signal
An automatic gain control circuit, characterized in that the signal includes its low frequency component in order to respond to the gradual fluctuation of the signal level.