JPH0124444B2 - - Google Patents

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. 1. In FIG. 1, 11 is a signal input terminal, 13 is a rectifier circuit, and 14 is a signal input terminal.
1 is a smoothing circuit, 12 is a variable gain circuit whose gain is controlled by the output of 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).

Figure 2 shows the signal waveforms in each part of the AGC circuit in Figure 1, Figure 2a shows the input signal waveform at the signal input terminal 11, Figure 2b shows the signal waveform at the output of the smoothing circuit 14, and Figure 2c is the AGC of output terminal 15
Shows the output signal waveform. 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 increased, and the variable gain circuit 1
The envelope component was prevented from being wrapped in the negative feedback control signal to 2. 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. explain.

(Structure of the Invention) The present invention includes a variable gain circuit that makes the input/output gain variable by external control, an absolute value circuit that converts the output of the variable gain circuit into an absolute value, and a switch that changes the first time constant and the second time constant. a variable time constant integrating circuit that can be switched to a time constant and smoothes the output of the absolute value circuit to control the variable gain circuit; and an integrating circuit that has a third time constant and smoothes the output of the absolute value 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, and the variable time constant integrator circuit and an exclusive OR circuit that controls a switch, the second time constant and the third time constant being smaller than the first time constant, and switching the switch when the input signal changes stepwise. The AGC circuit is characterized in that the time constant of the variable time constant integrating circuit is switched from the first time constant to the second time constant.

(First Embodiment) FIG. 3 shows an AGC circuit according to an embodiment of the present invention, in which 31 is a signal input terminal, 32 is a variable gain circuit,
33 is a rectifier circuit, 34 is a smoothing circuit, 35 and 36 are voltage comparison circuits, 37 is an exclusive OR circuit, 38 is a variable time constant smoothing circuit, and 39 is an AGC signal output terminal. In Fig. 3, the part surrounded by a dashed line is a circuit part newly added to the conventional AGC circuit shown in Fig. 1, and the integrating circuit consisting of resistor R2 and capacitor C2 is the smoothing circuit shown in Fig. 1. corresponds to Resistor R2 and capacitor C2, resistor R2 and R
3 parallel resistance value R _p , capacitor C2, and resistor R1
_{The time} constants _of each CR integrator circuit consisting of _the
1 and the time constant of the stepwise level change of the input is τ ₁ , then it is set as τ ₁ ≈τ ₃ <τ ₄ ≪τ ₂ .

FIG. 4 is based on the embodiment of the invention shown in FIG.
The signal waveforms in each part of the AGC circuit are shown, and the fourth
Figure a shows the input signal at the signal input terminal 31, the fourth
4b shows the output signal of the smoothing circuit 34, FIG. 4c shows the output signal of the voltage comparison circuit 35, FIG. 4d shows the output signal of the voltage comparison circuit 36, and FIG. 4e shows the output signal of the exclusive OR circuit 37. , FIG. 4f shows the output signal of the time constant variable smoothing circuit 38, and FIG. 4g shows the signal waveforms of the output signal at the output terminal 39.

Next, the operation of the present invention will be explained using FIGS. 3 and 4. Let the input signal be the signal shown in FIG. 4a.
The input signal passes through the variable gain circuit 32 and the rectifier circuit 33, and is discharged in the smoothing circuit 34 with a time constant τ ₄ at S1 in FIG. Figure b S11
It changes like this. Next, it is compared with the comparison voltage V _L in the voltage comparison circuit 36, and becomes a high level as shown at d S12 in FIG. 4. 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 becomes the inverted version of the output of the voltage comparison circuit 36, as shown in FIG. 4 e S13. 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 variable time constant smoothing circuit 38 is τ ₃ =C2R _p (≪
C2R2), and as a result, the output of the variable time constant smoothing circuit 38 is discharged with a time constant of τ ₃ and becomes a waveform as shown in FIG. The output signal of the signal output terminal 39 is also gain controlled with a time constant of τ ₃ (FIG. 4g). As a result of the gain control, the output of the rectifier circuit 33 recovers from the lowered level, the level of the output of the smoothing circuit 34 also increases and becomes higher than the comparison voltage V _L again, and the output of the voltage comparison circuit 36 becomes low level. 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 variable time constant smoothing circuit 3
The time constant of 8 returns to τ ₂ .

Next, when the input level of the input signal increases stepwise (Fig. 4a S2), the output of the smoothing circuit 34 becomes higher than the comparison voltage V _H in the voltage comparator circuit 35 (Fig. 4b S21), and the voltage comparison The output of the circuit 35 becomes low level (c S22 in FIG. 4). At this time, since the output of the voltage comparator circuit 36 is at low level, the output of the exclusive OR circuit 37 is at low level (e S23 in FIG. 4), and the time constant is changed by switching the switch SW of the variable time constant type smoothing circuit 38. The time constant of the type smoothing circuit 38 is switched from τ ₂ to τ ₃ (τ ₂ >>τ ₃ ), and the transient response of the AGC output signal is restored with the time constant of τ ₃ (=C2R _p ). After that, as in the case described above, the output of the smoothing circuit 34 becomes lower than the comparison voltage _VH , so the output of the voltage comparison circuit 35 becomes high level.
Since the output of the voltage comparison circuit 36 becomes low level,
The output of the exclusive OR circuit 37 becomes high level, and as a result, the time constant of the time constant smoothing circuit 38 is switched from τ ₃ to τ ₂ as the switch SW is turned off. Therefore,
The AGC circuit shown in FIG. 3 can obtain the AGC output shown in FIG. 4g for the input signal shown in FIG. 4a.

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, and the rectifier circuit 3 is used as a multiplier circuit.
3 as an absolute value circuit, smoothing circuit 34 as an accumulator (integration circuit), voltage comparison circuits 35 and 36 as a magnitude comparator, and variable time constant smoothing circuit 38 as a variable input gain accumulator. By replacing it with a digital signal processing circuit, it is also possible to construct an AGC circuit having the same function as that of the first embodiment.

In this case, it is possible to provide the same effects as the first embodiment, and also to have the advantage that LSI implementation is possible.

(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 it is configured with an integrator circuit instead of a differentiating circuit to detect step-like changes in level, it has an advantage in terms of margin for malfunction due to noise.
Since this is done on the AGC output side, it has the advantage that the step change detection output can be obtained over a wide level range.

[Brief explanation of drawings]

Figure 1 is an AGC circuit diagram of a conventional example, Figure 2 is a signal waveform diagram at each part of the AGC circuit shown in Figure 1, Figure 3 is an AGC circuit diagram of an embodiment of the present invention, and Figure 4 is a diagram of the AGC circuit shown in Figure 3. FIG. 3 is a signal waveform diagram at each part of the AGC circuit shown in FIG. 31... Signal input terminal, 32... Variable gain circuit, 33... Rectifier circuit, 34... Smoothing circuit, 3
5, 36... Voltage comparison circuit, 37... Exclusive OR circuit, 38... Variable time constant smoothing circuit, 39...
...output terminal, R1, R2, R3...resistance, C1,
C2...Capacitor, SW...Switch.

Claims (1)

[Claims] 1. 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 the above-mentioned circuit which can be switched between a first time constant and a second time constant by switching a switch. a variable time constant type integrator circuit that smoothes the output of the absolute value circuit to control the variable gain circuit; an integrator circuit having a third time constant that smoothes the output of the absolute value circuit; and an output of the integrator circuit. two voltage comparator circuits that compare the voltages with reference voltages of different levels, and an exclusive OR circuit that performs an exclusive OR of the outputs of the two voltage comparator circuits and controls a switch of the variable time constant integrator circuit by that signal. The second time constant and the third time constant are made smaller than the first time constant, and the switch is switched when the input signal changes stepwise to produce the variable time constant type. An AGC circuit characterized in that the time constant of the integrating circuit is switched from the first time constant to the second time constant.