JPS5851609A - Automatic gain controlling circuit - Google Patents

Abstract

PURPOSE:To obtain an automatic gain controlling circuit inexpensively with small size, by providing a logical circuit using an output of an output side detector of the automatic gain controlling circuit and a control voltage of a variable attenuation element as input information. CONSTITUTION:An input signal is outputted from an amplifier 2 coupled with a variable attenuator 1, the output signal is detected at a detector 3 with a time constant, and an output voltage vd is inputted to a control circuit 4 and a logical circuit 6. The voltage vd controls the attenuator 1 via a switch SW5 with an output voltage vo amplifying the voltage vd to control the output of the amplifier 2 to be a constant level. An output of the SW5 is inputted to a comparator CMP7 via a delay circuit 10 of the circuit 6. When the input signal of an AGC circuit is a threshold value or less, the voltage VO inputted to the CMP7 is increased than a voltage vref1, the output of the CMP7 switches the SW5 to a constant voltage va via a gate 9 to constitute the amplifier 2 as a fixed gain. When the input level is increased, the voltage vd is higher, the output of the CMP8 of the circuit 6 switches the SW5 to the circuit 4 via the gate 9 to attain AGC operation.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved automatic gain control circuit for suppressing signal level fluctuations.

In general, microwave communication equipment requires an automatic gain control circuit that suppresses level fluctuations caused by rounding, fading, equipment drift, etc., which require high quality transmission, and keeps the level constant. In this case, when the input level of the automatic gain control circuit attenuates below a certain specified value, the gain increases significantly and high-level noise is transmitted, degrading the quality of the line.

To prevent this, conventional technology is equipped with a function that monitors the signal level on the input side of the automatic gain control circuit and switches the gain to a fixed gain (III quasi-gain) when the level falls below a specified value. was.

In addition to the automatic gain control circuit inherent in this method, a wave generator, high gain amplifier, wave detector, and comparator are required to detect the signal level on the input side, and it is expensive and structurally difficult. 9. The disadvantage is that the dimensions of the item are large.

The purpose of this invention is to eliminate the function of monitoring the signal level on the input side of the conventional automatic gain control circuit.

By adding a simple bottle logic circuit that uses the output-side detection WhoO output and the control voltage of the variable attenuation element as input information, the input signal level of the automatic gain control circuit, which is inexpensive and compact, can be adjusted to the level when the input signal level is attenuated below a fixed value. An object of the present invention is to provide an automatic gain control circuit that switches only information on the output side to a fixed gain from K without using the monitoring function.

According to this invention, there is provided an amplifier for amplifying an input signal via a variable attenuator, a detector for detecting the output of the amplifier, and a detector for supplying the output voltage of the detector and outputting the amount of attenuation of the variable attenuator. a control circuit that outputs a variable control voltage to keep the signal level constant; a switch that switches between the output voltage of the control circuit and a constant voltage set to a predetermined value;
means for connecting the switching output of the switching means to the variable attenuator; and when the switching output of the switching means remains below a predetermined value, the switching means is switched to the constant voltage until the output voltage of the detector reaches a predetermined value or higher. and a logic circuit for switching the switch means to the output of the control circuit when the gain is reached.

The present invention will be explained in detail below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the invention.

An attenuator 1 connected between input terminals and consisting of a variable resistance element controlled by voltage or current; a fixed gain amplifier 2 connected to this attenuator 1 and taking out the output to its output terminal; and a branch connection at the output terminal to output a signal. A detection voltage Wh3 with a time constant of 1 to detect the level, a rounded control circuit 4 which manually inverts the detected voltage Vdt and controls the attenuator 1, and a control circuit 4 connected to the output of the control circuit 4 and connected to the attenuator 1 and and a switch 5 for switching the amplifier 2 to fixed gain or automatic gain control.

The output voltage Vo of the control circuit 4 and the detected voltage v of the detector 3
dt-human-powered logic circuit 6. This logic circuit 6 includes comparators 7.8. ! :.

It includes an AND gate 9 and a (τ+α) delay circuit 11.

Next, the logic circuit 6 will be explained using the characteristic diagrams shown in FIGS. 2 to 4.

FIG. 2 is a characteristic diagram showing the relationship between the control voltage Vo of the control circuit 40 and the gain of the amplifier circuit consisting of the attenuator 1 and the amplifier a2. Lva is the voltage that gives the amplifier circuit a fixed gain K, p, Vrefl is a voltage that gives the gain value immediately before switching to the fixed gain K when the signal input level of this amplifier circuit becomes below a certain threshold value M. - FIG. 3 is a characteristic diagram showing the relationship between the input level of this amplifier circuit and the detected voltage Vd of the detector 3.

While automatic gain control is in effect, the output level, that is, the detected voltage Vd, follows a nearly constant voltage vb' characteristic line, but in the case of fixed gain KO, the detected voltage Vd changes in response to the input level. Characteristic line C is shown. Here Vre
f2 indicates a voltage at which the fixed gain K is switched to automatic gain control when the input level (M+H) is reached; however, H indicates hysteresis at the time of switching.

Figure 4(a) shows the input level and control voltage v of this amplifier circuit.
FIG. 2 is a characteristic diagram showing the relationship with O. During automatic gain control operation, as shown in characteristic IIE, when the input level increases, the control voltage vO is decreased to increase the amount of attenuation of the attenuator 1, and when the input level is low, the control voltage Vo is increased to increase the attenuation amount. The amount of attenuation of the device 2 is reduced, and the output level is controlled to remain constant. When the input level of this automatic gain control circuit becomes smaller than the threshold value M, the control voltage Vo becomes larger than Vrefl*<p, and the output of the comparator 7 becomes lower. As a result, the output of the AND gate 9 becomes low level, and the switch 5f is switched to the Va side to set the gain of this amplifier circuit to a fixed gain. Due to this switching to the fixed gain, the control voltage momentarily becomes Va, but the time constant τ of the detection Ws3
Since it passes through the delay circuit 10 with a larger time constant τ+α (α is the time margin), the comparator 7 has (τ+α).
α) Va is supplied with a time delay and driven to high level. Therefore, when the input level is decreasing, the output of the converter tough outputs a signal in which the NO 1 level momentarily becomes the LOW level, as shown in FIG. 4(b).

On the other hand, the comparator 8 detects the output voltage V of the detector 3 at the moment when the switch 5 switches from automatic gain control to fixed gain.
When d becomes less than Vref2, the comparator 80 output becomes sr
Since the output of the gate 9 is kept at a low level as shown in FIG. s (b) Ic, the output of the gate 9 maintains a low level. Therefore, since the output of the converter tough becomes low level momentarily, the switch 5 is held at the 1 side.

Next, when the input level rises from low level to high level and becomes greater than M+H, comparator 8
The output becomes high level as shown in FIG. 3(C). On the other hand, since the output of comparator 7 maintains a high level as shown in FIG. 4(C), comparator 8
When the output of the gate 90 becomes high level, the switch 5 is switched to the output Vo of the control circuit 4, and this circuit is switched to automatic gain control.

As explained above, by adding a simple logic circuit to the input side of the automatic gain control circuit without having a special signal level tube monitoring function, it can be made inexpensive and compact, and the function of switching to a fixed gain can be achieved. An automatic gain control circuit can be realized.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of an embodiment of the present invention, Figure 2 is a characteristic diagram showing the relationship between control circuit output voltage and amplifier gain, and Figure 3 (a
) is a characteristic diagram showing the relationship between the signal input level and the detected voltage of the detector, and Figures 3(b) and (C) are converter tough output level diagrams when decreasing and increasing the input level.Figure 4(a) ) is a characteristic diagram sHa showing the relationship between the signal input level and the control voltage, and (C) is an output level diagram of the comparator 7 when lowering and increasing the input level. In the figure, 1...attenuator, 2...
...Amplifier, 3...Detector. 4...Control circuit, 5...Switch, 6
......Logic circuit L7, 8...Comparator, 9...And gate, 10...Delay circuit, f Fig. 2 + (/ 1 Figure 3 Figure 4

Claims (1)

[Claims] an amplifier to which an input signal is amplified via a variable attenuator; a detector to detect the amplified sO output; and an output voltage of the detector to supply the attenuation amount of the variable attenuator to a constant output signal level. a control circuit that outputs a variable control voltage, a switch means that switches between the output voltage of the control circuit and a constant voltage set to a predetermined value, and means that connects the switching output of the switch means to the variable attenuator. Then, when the output voltage of the 1,000 means is below a predetermined value, the switch means is switched to the constant voltage, and when the output voltage of the detector becomes equal to or higher than the predetermined value, the switch means is controlled as described above. 1. An automatic gain control circuit, comprising: a logic circuit for switching between fixed gain control and automatic gain control based on the output of the amplifier.