JPS6365165B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in automatic gain control circuits for suppressing signal level fluctuations.

Generally, microwave communication devices require high-quality transmission, so an automatic gain control circuit is required to suppress level fluctuations due to fading, device drift, etc., and keep 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 has a function that monitors the signal level at the input side of the automatic gain control circuit and switches the gain to a fixed gain (standard gain) when the level falls below a specified value. Ta.
In addition to the original automatic gain control circuit, this method requires a filter, a high-gain amplifier, a detector, and a comparator to detect the signal level on the input side, which is expensive and requires a large structural dimension. The problem was that it became a big problem.

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, and to
The object of the present invention is to provide an automatic gain control circuit that is inexpensive and compact by adding a simple logic circuit that uses the output of the output side detector and the control voltage of the variable attenuation element as input information.

Another object of the present invention is to provide an automatic gain control circuit that switches to a fixed gain based only on output side information without using a monitoring function for the input signal level when the input signal level attenuates below a certain specified value. Our goal is to provide the following.

The automatic gain control circuit of the present invention has a configuration including a variable attenuator that outputs the signal level of an input signal varied by a control signal, an amplifier that amplifies the output of this variable attenuator, and a detector that detects the output of this amplifier. a control circuit that supplies the output voltage of the detector and outputs a control voltage that varies the amount of attenuation of the variable attenuator so as to keep the output signal level constant; a switch means that switches between a set constant voltage and a set constant voltage using a switching signal and supplies the control signal to the variable attenuator; When the output voltage of the detector exceeds the second set value, the input of the switch means is switched to the constant voltage side, and when the output voltage of the detector exceeds the second set value, the input of the switch means is switched to the output of the control circuit. and a logic circuit that outputs the switching signal for switching to the voltage side, and is characterized in that it automatically switches between fixed gain control and automatic gain control based on the output of the amplifier.

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 detector 3 with a time constant τ for detecting the level; a control circuit 4 for inputting and inverting the detected voltage Vd to control the attenuator 1; and a switch 5 for switching the amplifier 2 to fixed gain or automatic gain control, and the output voltage Vo of the control circuit 4 and the detector 3.
The logic circuit 6 receives the detected voltage Vd as input. This logic circuit 6 includes a comparator 7,
8, 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 4 and the gain of the amplifier circuit consisting of the attenuator 1 and the amplifier 2, where Va is the voltage that gives the amplifier circuit a fixed gain K. , Vref1 is a voltage that provides the gain A 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. 3a is a characteristic diagram showing the relationship between the input level of this amplifier circuit and the detected voltage Vd of the detector 3.
During automatic gain control, the output level, that is, the detection voltage Vd, is approximately constant as shown by the characteristic line D of the voltage Vd.
However, in the case of a fixed gain K, the detected voltage Vd changes according to the input level as a characteristic line C. Here, Vref2 is the input level (M+H)
It shows the voltage that switches from fixed gain K to automatic gain control when . However, H indicates hysteresis at the time of switching.

FIG. 4a is a characteristic diagram showing the relationship between the input level of this amplifier circuit and the control voltage Vo. During automatic gain control operation, as shown in characteristic line E, when the input level increases, the control voltage Vo is reduced and the attenuator 1
When the input level becomes low, the control voltage Vo is increased to reduce the attenuation of attenuator 2.
The output level is controlled to be 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 Vref1, and the output of the comparator 7 becomes low level. As a result, the output of the AND gate 9 becomes low level, the switch 5 is switched to the Va side, and the gain of this amplifier circuit is set to the fixed gain K. Due to this switching to fixed gain, the control voltage momentarily becomes Va, but
The time constant τ+α(α
is passed through the delay circuit 10 (time margin), so the signal is sent to the comparator 7 with a delay of (τ+α).
It is driven to a high level where Va is supplied. Therefore, when the input level is decreasing, the output of the comparator 7 outputs a signal whose high level momentarily becomes a low level, as shown in FIG. 4b. On the other hand, the comparator 8 detects the detector 3 at the moment when the switch 5 switches from automatic gain control to fixed gain.
Since the output voltage Vd of the gate becomes lower than Vref2 and the output of the comparator 8 becomes a low level as shown in FIG. 3b, the output of the gate 9 maintains a low level. Therefore, the output of the comparator 7 instantaneously becomes a low level, causing the switch 5 to maintain the Va side.

Next, when the input level increases from a low level to a high level and becomes greater than M+H, the output of the comparator 8 becomes high level as shown in FIG. 3c. On the other hand, the output of comparator 7 maintains a high level as shown in FIG. 4c, so when the output of comparator 8 becomes high level, the output of gate 9 becomes high level and switch 5 is It is switched connected 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 an automatic gain control circuit without having a special signal level monitoring function, the automatic gain control circuit can be made inexpensive and compact, and has the function of switching to a fixed gain. A gain control circuit can be realized.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of an embodiment of the present invention, Fig. 2 is a characteristic diagram showing the relationship between the control circuit output voltage and the amplifier gain, and Fig. 3a shows the relationship between the signal input level and the detected voltage of the detector. Figures 3b and 3c are output level diagrams of the comparator 7 when the input level is lowered and raised, Figure 4a is a characteristic diagram showing the relationship between signal input level and control voltage, and Figure 4b is a characteristic diagram showing the relationship between signal input level and control voltage. , c are output diagrams 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, 7, 8... Comparator, 9... AND gate, 10... Delay circuit.

Claims (1)

[Claims] 1. A variable attenuator that varies the signal level of an input signal using a control signal and outputs it, an amplifier that amplifies the output of this variable attenuator, a detector that detects the output of this amplifier, and a detector that detects the output voltage of this detector. a control circuit that outputs a control voltage that supplies and varies the amount of attenuation of the variable attenuator so as to keep the output signal level constant; and a control circuit that switches the output voltage of the control circuit and a constant voltage set to a predetermined value by a switching signal. A switch means that switches and supplies the control signal to the variable attenuator, a switching output of the switch means and an output voltage of the detector are input, and when the output voltage of the switch means becomes equal to or less than a first set value; The input of the switching means is switched to the constant voltage side, and when the output voltage of the detector exceeds the second set value, the switching signal is output for switching the input of the switching means to the output voltage side of the control circuit. An automatic gain control circuit comprising: a logic circuit that automatically switches between fixed gain control and automatic gain control according to the output of the amplifier.