JPS59128806A - Agc circuit - Google Patents

Abstract

PURPOSE:To shorten the transient response time of an output and to reduce the error time by using a time constant variable type integration circuit which controls a variable gain circuit and switching the time constant of the integration circuit to a small value when an input signal has a stepped change. CONSTITUTION:An input signal is supplied to a smoothing circuit 34 through a variable gain circuit 32 and a rectifying circuit 33. The output of the circuit 34 is supplied to voltage comparators 35 and 36 to be compared with comparing voltages VH and VL and then supplied to an exclusive AND circuit 37. The switch SW of a time constant variable type smoothing circuit 38 is switched with the output of the circuit 37. Then an AGC signal is delivered to an output terminal 39 through control of the circuit 32. The time constants of a resistance R2 and a capacitor C2, a parallel resistance RP of resistances R2 and R3 and a CR integration circuit consisting of the C2, R1 and C1 are defined as tau2=C2R2, tau3=C2RP and tau4=C1R1 respectively. Then the time constant is defined at tau1 for the stepped level change of an input. Here tau1 tau3<tau4<<tau2 is satisfied. Thus the transient response time of an output is shortened, and the error time is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to an automatic gain control circuit that has low cross-modulation due to signal envelope components and has a fast response speed to signal level fluctuations.

(Prior Art) A conventional AGC circuit is shown in FIG. 11 in Figure 1
is a signal input terminal, 13 is a rectifier circuit, 14 is a smoothing circuit, 1
2 is a variable gain circuit whose output gain is controlled by the smoothing circuit 14, and 15 is an output terminal for obtaining the DC component of the output of the variable gain circuit 12. The output of the variable gain circuit 12 is returned to the control input of the variable gain circuit 12 itself via the rectifier circuit 13 and the smoothing circuit 14, thereby forming a negative feedback control loop to form an automatic gain control circuit (AGC). .

FIG. 2 shows signal waveforms in each part of the AGC circuit shown in FIG. 1, FIG. 2 (,) shows the input signal waveform at the signal input terminal 11, and FIG. , FIG. 2(c) shows the AGC output signal waveform of the output terminal 15. In the case of the conventional embodiment, in order to reduce cross-modulation due to the envelope component of the input signal, the time constant of the smoothing circuit 14 is made thicker so that the envelope component is not wrapped in the negative feedback control signal to the variable gain circuit 12. I was doing it. Therefore, when a step level change occurs in the input signal with an extremely fast time constant compared to the time constant of the smoothing circuit, the AGC output will change over a long period of time due to the time constant of the smoothing circuit 14, as shown in Figure 2. This has the disadvantage that a transient response occurs and the time required for an error to occur becomes longer.

(Object of the Invention) In order to eliminate the above-mentioned drawbacks, the present invention automatically switches and controls the time constant of the smoothing circuit when the input level changes stepwise to minimize the transient response.The details are as follows. Explain.

(Structure of the Invention) The present invention includes a variable gain circuit whose input/output gain is made variable by external control, an absolute value circuit which converts the output of the variable gain circuit into an absolute value, and a switch that controls a first time constant and a first time constant. a variable time constant integrator circuit that can be switched to a second time constant and smoothes the output of the absolute value circuit to control the variable gain circuit; and an integrator circuit that has a third time constant and smoothes the output of the absolute value circuit. a circuit, two voltage comparator circuits that compare the outputs of the integrator circuits with reference voltages of different levels, and an exclusive OR of the outputs of the two voltage comparator circuits; an exclusive OR circuit for controlling the switches of the circuit, and the second time constant and the third time constant are made smaller than the first time constant.1. The AGC circuit is characterized in that the time constant of the variable time constant type integrating circuit is changed from the first time constant to the second time constant by switching the switch when the input signal changes stepwise.

(First Embodiment) FIG. 3 shows an AGC circuit according to an embodiment of the present invention.
is a signal input terminal, 32 is a variable gain circuit, 33 is a rectifier circuit, 34 is a smoothing circuit, 36 is a voltage comparison circuit, 3
7 is an exclusive OR circuit, 38 is a variable time constant smoothing circuit,
39 is an AGC signal output terminal. In Fig. 3, the part surrounded by the dashed line is the AGC of the conventional example shown in Fig. 1.
The integrating circuit, which is a new circuit part added to the circuit and is composed of a resistor R2 and a capacitor c2, is comparable to the smoothing circuit shown in FIG. Resistor R2 and capacitor C2, parallel resistance value Rp of resistors R2 and R3 and capacitor C2, resistor R1 and capacitor C
The time constant of each CR integrator circuit consisting of τ2=C2R
2, τ3 = C2Rp, τ, = ClR1, and if the time constant of the stepwise level change of the input is τ1, then τ1 dance τ
3<τ4 × τ2.

FIG. 4 shows signal waveforms at various parts of the AGC circuit according to the embodiment of the present invention shown in FIG. 3, FIG. 4(,) shows the input signal at the signal input terminal 31, and FIG. 34, FIG. 4(c) is the output signal of the voltage comparison circuit 35, FIG. 4(d) is the output signal of the voltage comparison circuit 36, and FIG. 4(,) is the output of the exclusive OR circuit 37. 4(f) is the output signal of the variable time constant smoothing circuit 38, and FIG. 4(0) is the signal waveform of the output signal at the output terminal 39.

Next, the present invention will be explained in detail using FIGS. 3 and 4.

Let the input signal be the signal shown in FIG. 4(,).

The input signal is sent to the variable gain circuit 32. Since the input signal passes through the rectifying circuit 33 and enters the smoothing circuit 34 in a step manner, the output of the smoothing circuit 34 changes as shown in FIG. 4(b) O. Next, it is compared with the comparison voltage VL in the voltage comparison circuit 36,
The high level shown in Fig. 4(a) @ is achieved. At this time, since the output of the voltage comparison circuit 35 is held at a high level, the output of the exclusive OR circuit 37- is the 41kth (e
) As shown in @, the output of the voltage comparison circuit 36 is inverted. The switch SW of the variable time constant smoothing circuit 38 is turned on when the output of the exclusive OR circuit 37 is at a low level. When the switch SW is turned on, the time constant of the time constant variable smoothing circuit 38 becomes τs = C2Rp (< C2R2
) As a result, the output of the variable time constant smoothing circuit 38 is discharged with a time constant of τ3 and has a waveform as shown in FIG. 4(f), and the output controls the variable gain circuit 32. ,A
The output signal of the GC signal output terminal 39 is also gain controlled with a time constant of τ3 (FIG. 4(g)). As a result of the gain control, the output of the rectifier circuit 33 returns to its lowered level, the level of the output of the smoothing circuit 34 rises again, the comparison voltage V becomes higher, and the output of the voltage comparison circuit 36 becomes a low level. Become.

Subsequently, the output of the exclusive OR circuit 37 becomes high level, and the switch SW of the variable time constant smoothing circuit 38 is turned off, so that the time constant of the variable time constant smoothing circuit 38 returns to τ2.

Next, when the input level of the input signal increases stepwise (
FIG. 4(a) [phase]), the comparison circuit 3 in which the output of the smoothing circuit 34 is higher than the comparison voltage VH in the voltage comparison circuit 35
Since the output of 6 is low level, exclusive OR circuit 3
The output of 7 becomes low level (Fig. 4(e) @ ),
Switch SW of the variable time constant smoothing circuit 38 is changed to change the time constant of the variable time constant smoothing circuit 38 from τ2 to τ3 (τ2
〉τ3), and the AGC output signal is τ3 (-〇2R
The transient response is restored with a time constant of , ). After that, as in the case described above, the output of the smoothing circuit 34 becomes lower than the comparison voltage V, so the output of the voltage comparison circuit 35 becomes high level, and the output of the voltage comparison circuit 36 becomes low level, so the exclusive OR The output of the circuit 37 is not at a high level, and as a result, the time constant of the time constant smoothing circuit 38 becomes τ3 when the switch SW is turned off.
to τ2. Therefore, in the AGC circuit shown in Fig. 3, for the input signal shown in Fig. 4(,), Fig. 4G)
The AGC output shown in can be obtained.

The AGC circuit of this embodiment has the advantage that the transient response time of the output to a stepwise change in the input level can be greatly shortened, and the error time can be shortened.

(Second Embodiment) Although the first embodiment describes the case of analog signal processing, similar effects can be obtained even if this is replaced with digital signal processing. That is, in FIG. 3 described in the first embodiment, the variable gain circuit 32 is used as a multiplier circuit, the rectifier circuit 33 is used as an absolute value circuit, the smoothing circuit 34 is used as an accumulator (integration circuit), and the voltage comparison circuit 35 . 3
An AGC circuit having the same function as the first embodiment can also be obtained by configuring a digital signal processing circuit by replacing 6 with a magniceed comparator and the variable time constant smoothing circuit 38 with a variable input gain accumulator. Can be configured.

In this case, it is possible to provide the same effects as the first embodiment, and also to have the advantage of being able to be integrated into an LSI.

(Effects of the Invention) The present invention can greatly shorten the transient response time of the output in response to stepwise changes in the input level in the AGC circuit, and therefore can be used in the receiving section of a modem. In addition, since an integrator circuit is used instead of a differentiating circuit to detect step-like changes in level, there is an advantage in terms of margin for malfunction due to noise. This has the advantage that the step change detection output can be obtained over a wide level range.

[Brief explanation of the drawing]

FIG. 1 is an AGC circuit diagram in a conventional example, FIG. 2 is a signal waveform diagram at each part of the AGC circuit shown in FIG. 1, FIG. 3 is an AGC circuit diagram according to an embodiment of the present invention, and FIG. FIG. 3 is a signal waveform diagram at each part of the AGC circuit shown in FIG. 3J...Signal input terminal, 32...Variable gain circuit, 3
3... Rectifier circuit, 34... Smoothing circuit, 35.36.
...Voltage comparison circuit, 32...Exclusive OR circuit, 38
... variable time constant smoothing circuit, 39... output terminal, R
1, R2゜R3...Resistor, C1, C2...Capacitor, sw...Switch. Patent applicant Oki Electric Industry Co., Ltd. Figure 3 Figure 4

Claims (1)

[Claims] a variable gain circuit whose input/output gain is variable by external control; an absolute value circuit which converts the output of the variable gain circuit into an absolute value;
a time constant variable type integrating circuit that can be switched between a first time constant and a second time constant by switching a switch and smoothes the output of the absolute value circuit to control the variable gain circuit; an integrator circuit that smoothes the output of the absolute value circuit; two voltage comparator circuits that compare the output of the integrator circuit with reference voltages of different levels; and an exclusive control circuit for the outputs of the two voltage comparator circuits. an exclusive OR circuit which performs a logical sum and controls a switch of the variable time constant type integrating circuit by the signal thereof, and the second time constant and the third time constant are set to be equal to the first time constant. The AGC circuit is characterized in that the time constant of the variable time constant integrating circuit is changed from the first time constant to the second time constant by switching the switch when the input signal changes stepwise. .