JPH0326009A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To effectively compensate the loss of a transmission line by providing a variable loss amplifier to an output stage of a variable gain amplifier so as to suppress the inter-code interference to a small value regardless of the quantity of the transmission loss. CONSTITUTION:When the gain of a variable gain amplifier 1 whose gain is controlled with a control circuit 3 is increased attended with the increase in the transmission line loss, the characteristic change making the band width narrow (compressed) is shown. On the other hand, a variable gain amplifier 2 whose gain is controlled with the control circuit 3 shown a characteristic change to decrease the loss at the band limited by the variable gain amplifier 1 when the gain of the variable gain amplifier 1 is increased attended with the increase in the loss of the transmission line. Since the overall frequency characteristic is made nearly constant regardless of the change in the loss of the transmission line, the loss of the transmission line is effectively compensated while minimizing the inter-code interference.

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention provides an automatic gain that can compensate for transmission path loss of digitally communicated signals without causing problems such as intersymbol interference. Regarding control circuits.

(Prior Art) With the development of digital signal processing technology, digital communication of information signals has become popular. This type of digital communication involves transmitting digital signals obtained by encoding various information signals using telecommunication cables, optical fiber cables, etc. as transmission paths (transmission media).

By the way, when receiving and reproducing a digitally communicated signal, generally a variable gain amplifier that amplifies the received digital signal and a control circuit that variably sets the gain of the variable gain amplifier according to the amplified output of the variable gain amplifier are used. By providing an automatic gain control circuit (AGC circuit) in the receiving input stage, the transmission path loss of the received digital signal can be compensated for.

AG that compensates for transmission line loss of such digital signals
Focusing on the overall frequency characteristics in the C circuit, generally the band of the overall frequency characteristics is the narrowest when the transmission line loss is maximum, and the band becomes wider as the transmission line loss decreases. However, if the overall frequency characteristic has a wide band, intersymbol interference is likely to occur in the output of the AGC circuit described above. For this reason, conventionally, waveform equalization processing is performed by limiting the overall frequency band so as to reduce intersymbol interference.

Note that this waveform equalization processing is generally performed using techniques such as full cosine roll-off equalization (FCRO equalization) and Gaussian equalization.

However, in order to perform uniform waveform equalization in all loss ranges of the transmission path that should be considered, the band of the AGC circuit described above should be set sufficiently wider than the required band, and then,
It is necessary to perform the above-mentioned waveform equalization process using a filter or the like. However, in order to ensure a sufficiently wide band for the AGC circuit, it is necessary to use, for example, a plurality of variable gain amplifiers with suppressed gain per stage, and a further increase in the number of stages of variable gain amplifiers poses a problem. Moreover, when processing signals that are digitally communicated via telecommunications cables, the transmission line loss in the telecommunications cable has the so-called J] characteristic, so if you try to compensate for the transmission line loss over a wide band, Furthermore, it is necessary to suppress the gain of the variable gain amplifier in each stage. For this reason, it is undeniable that the number of stages of the variable gain amplifier will further increase, and the scale of the AGC circuit will become larger.

On the other hand, in order to reduce the number of stages in a variable gain amplifier, for example, when the transmission line loss to be compensated is maximum, AG
What is necessary is to set the band of the C circuit. However, when the band of the AGC circuit is set in this way, as the amount of transmission line loss to be compensated decreases, the band of the overall frequency characteristic becomes longer, resulting in the above-mentioned problem of increased intersymbol interference. Furthermore, the εAGC circuit itself, whose overall frequency characteristic band is extended more than necessary, becomes more likely to oscillate, and new problems such as increased noise occur.

(Problem to be Solved by the Invention) Conventionally, when attempting to compensate for the transmission path loss of a digitally communicated signal and equalize the waveform so that the intersymbol interference is minimized, the transmission Since it is necessary to set a wide band for the AGC circuit to compensate for path loss,
There were problems such as the AGC circuit having to be constructed on a large scale.

In addition, in order to reduce the configuration scale of the AGC circuit, if the band of the AGC circuit is set so that the overall frequency characteristic becomes FCRO equalization or Gaussian equalization when the transmission line loss to be compensated is maximum, the above transmission line When the loss decreased, intersymbol interference and noise increased, and the circuit itself became more likely to oscillate, causing other problems.

The present invention has been made in consideration of these circumstances, and its purpose is to effectively compensate for the transmission line loss by suppressing the intersymbol interference to a small level, regardless of the amount of transmission line loss. An object of the present invention is to provide an automatic gain control circuit with a small circuit configuration that can perform

[Structure of the Invention] (Means for Solving the Problems) The automatic gain control circuit according to the present invention includes a signal system amplified by a variable gain amplifier for compensating transmission line loss of a digitally communicated signal, For example, a variable loss amplifier is provided at the output stage of a variable gain amplifier, and a control circuit that variably sets the gain of the variable gain amplifier simultaneously reduces the amount of loss in the variable loss amplifier when increasing the gain of the variable gain amplifier. Further, the present invention is characterized in that when the gain of the variable gain amplifier is lowered, the amount of loss in the variable loss amplifier is simultaneously increased.

(Function) According to the circuit of the present invention configured in this manner, when increasing the gain of the variable gain amplifier to compensate for transmission line loss, the loss amount in the variable loss amplifier is simultaneously controlled to decrease, and the loss amount in the variable loss amplifier is controlled to decrease. When reducing the gain of the variable gain amplifier, the amount of loss in the variable loss amplifier is controlled to increase at the same time.

As a result, if the band of the overall frequency characteristic is extended by gain control of the variable loss amplifier, control is applied to limit the band by the amount of extension by the variable loss amplifier, and conversely the band of the overall frequency characteristic becomes narrower. In such a case, since the variable loss amplifier controls the amount of band limitation for that band to be reduced, it is possible to keep the overall frequency characteristic almost constant regardless of changes in transmission line loss. can. Therefore, by setting the constant-controlled overall frequency characteristic to approximate the FCRO equalization or Gaussian equalization described above, transmission path loss can be effectively compensated for while minimizing intersymbol interference. It becomes possible to do so.

(Embodiment) An automatic gain control circuit (AGC circuit) according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the basic schematic configuration of the embodiment circuit.
2 is a variable gain amplifier that amplifies and processes digitally communicated signals, 2 is a variable loss amplifier that applies a predetermined loss to the output of the variable gain amplifier I to limit the band, and 3 is the output of the variable loss amplifier 2 ( Is there a system for controlling the gain of the variable gain amplifier 1 and the loss of the variable loss amplifier 2 according to the output of the present AGC circuit? This is an IrJ circuit.

This control circuit 3 is realized by, for example, a peak detection circuit.

Therefore, the variable gain amplifier l whose gain is controlled by the control circuit 3 is capable of transmitting digitally transmitted signals as shown in FIG. 2(a).
If the gain is increased as the transmission path loss increases, the frequency characteristic changes as shown by arrow A, such that the bandwidth becomes narrower (compressed). On the other hand, the variable loss amplifier 2 whose loss amount is controlled by the control circuit 3 has a frequency characteristic as shown in FIG. Accordingly, when the gain of the variable gain amplifier I is increased, the characteristics change as shown by arrow B such that the loss in the band limited by the variable gain amplifier I is reduced.

In other words, the control circuit 3 reduces the amount of loss in the variable loss amplifier 2 when increasing the gain of the variable gain amplifier l, and conversely reduces the amount of loss in the variable loss amplifier 2 when decreasing the gain of the variable gain amplifier 1. Correlate (correspond) the gain and loss amount so that the loss amount increases.
It's in control.

Therefore, the amount of loss in variable loss amplifier 2 is $11El,
This loss amount control is performed on frequency components in a band limited by the variable gain amplifier l, and this loss amount control suppresses signal loss in the band limited as the gain of the variable gain amplifier l increases. , if the band is extended as the gain decreases, by increasing the signal loss in that band, there is almost no change in the overall frequency characteristics as shown in Figure 2 (c). is set so that this does not occur.

According to the AGC circuit configured in this way, the frequency characteristics as shown in FIG. 2(a) of the variable gain amplifier 1 and the frequency characteristics as shown in FIG. 2(b) of the variable loss amplifier 2 can be synthesized. As shown in FIG. 2(e), the overall frequency characteristic can be made almost constant. Therefore, if this overall frequency characteristic is made to match (approximate) the FCRO equalization characteristic or Gaussian equalization characteristic of waveform equalization, the intersymbol interference can be minimized regardless of the magnitude (amount) of transmission path loss. It becomes possible to obtain an equalized waveform that compensates for the transmission line loss.

FIG. 3 shows an example of the configuration of a variable loss amplifier 2 for realizing an AGC circuit that is basically configured as shown in FIG. 1 and operates under control over frequency characteristics as shown in FIG. 2. FIG. Note that the variable gain amplifier l and the control circuit N. As for 3, various conventionally used ones are used here.

The variable loss amplifier 2 shown in FIG.
npn} transistor) The transistor amplifier circuit is basically constructed by connecting a load resistor RL for regulating DC operation to the collector of TR and connecting an emitter resistor RE to its emitter.

Note that RB, RB2 are base bias resistors that define the operating point of the transistor TR.

However, the load resistance RL has a variable capacitance diode C.
A series circuit consisting of V and a resistor R is connected in parallel via a decoupling capacitor DC, and constitutes a load impedance circuit section for the transistor TR. The variable capacitance diode Cv in this load impedance circuit section has a control voltage (
The capacitance value is variably set by the control signal CONT, and the impedance of the transistor TR as an alternating current load is variably set by the variably set capacitance value.

The amplification gain G of the variable loss amplifier 2 configured in this way is determined by multiplying the resistance value of the load resistor RL by a times the resistance R (
aR), the signal frequency ω is given as follows.

As shown in this amplification gain G, the gain for the input signal in the variable loss amplifier 2 has frequency dependence, and the gain decreases in a band where the signal frequency ω is high. The frequency characteristic (amount of decrease in gain) changes depending on the capacitance value of the variable capacitance diode Cv. As a result, the aforementioned control voltage (control signal CONT) is
1 to change its capacitance value, and by moving the position of the ball to change the frequency characteristics of the variable loss amplifier 2, the amount of loss in the frequency limited band can be adjusted as shown in Figure 2 (b). can be varied.

If the gain is insufficient with only one stage of transistor TR, the variable loss amplifier 2 shown in FIG. 3 may be configured in multiple stages to realize a variable loss amplifier with the desired gain. .

Thus, an AGC circuit is constructed using the variable loss amplifier 2 whose loss amount is controlled in this way and the variable gain amplifier i, and its overall frequency characteristic matches (approximates) the FCRO equalization characteristic or the Gaussian equalization characteristic. By doing so, the frequency characteristics can be made constant regardless of the magnitude of the transmission line loss (f1), so it is possible to obtain an equalized waveform that minimizes intersymbol interference and compensates for the transmission line loss. becomes possible.

By the way, the above explanation is about an embodiment in which the variable gain amplifier 1 and the variable loss amplifier 2 are configured separately, but when a current addition type circuit is used as the variable gain amplifier l, for example, the fourth It is also possible to construct an AGC circuit by incorporating the function of the variable loss amplifier 2 therein, as shown in FIG. 5 and FIG. 5, respectively.

FIG. 4 is a diagram showing a schematic configuration of an embodiment circuit using a single-configuration current addition type circuit, and TR1 and TR2 are transistors for signal amplification whose bases are commonly connected. Current distribution transistors TR3 and TR6 are connected to the collectors of these transistors TRI and TR2, respectively.
is the control voltage applied to its base (control signal CON
T), the current flowing through the load resistor RL is distributed to the transistors TRI and TR2 as [x: (1-x)]. However, the above X satisfies [0≦X≦l].

The input signal v1 of the transistors TRI and TR2 is determined by the current distribution ratio of these transistors TR3 and TR6.
The amplification gain is variably set.

Also, loss control networks LC. connected to the emitters of the transistors TRI and TR2, respectively. and the gain control network GC includes transistors TRI,
This determines the frequency characteristics of the amplification gain of TR2. This gain control network GC is realized as a resistor when the frequency characteristics of the digitally communicated signal are flat in terms of frequency, such as signals communicated via an optical fiber cable, and is implemented as a resistance. In the case of a signal having f-characteristics with respect to frequency, such as a signal communicated by a network, it is realized as a network composed of capacitance and resistance and peaking at a predetermined frequency. Now, to simplify the explanation, the gain control network GC is composed of only resistors, and the resistance value is RB.
If the gain control network GC is connected to the emitter of the transistor TR,
The emitter current I2 flowing through the input signal V, is as follows.

? In contrast, the loss control network LC includes emitter resistors R E, r connected in series as illustrated in FIG.
It is composed of R E■ and an inductor L connected in parallel to the emitter resistor REI. The impedance Z of the loss control network LC configured in this way is determined by the emitter resistance R. ,, If the values of RFl■ are R and aR, then when the current distribution ratio for the transistors TRI and TR2i distributed by the transistors TR3 and TR6 described above is represented by [x: (1-x)], The load current flowing through the load resistor RL is 1-x* 11
+ (1-x) ●Denoted by 12.

Therefore, the emitter current I1 flowing through the transistor TRI, which is formed by connecting the loss control network LC having the impedance 2 described above to the emitter, is as follows with respect to the input signal Vl.

becomes. A voltage V. generated across the load resistance RL due to this current l. becomes the waveform-equalized output signal.

However, as shown in the equation, the transistor TRI
, when the current ratio [control value x] distributed to TR2 is changed, the gain changes (the first stage of the above equation It can be seen that a change in the frequency characteristics (the latter part of the above equation) occurs. That is, if the above equation is transformed, depending on [control value It can be seen that the gain characteristics of the circuit gradually change from a gain that exhibits a flat frequency characteristic over a wide band to a gain that is band-limited.

More specifically, when the value of [control value Further, there is almost no increase in gain depending on the difference between the DC resistance aR of the loss control network LC and the DC resistance R of the gain control network GC. Therefore, at this time, the gain of the circuit can be kept low and the frequency characteristics can be band-limited.

However, as the value of the above [control value x] gradually increases,
Amplification function of transistor TRI and transistor TR2
The amplification functions on the transistor TRI side begin to function to the same extent, and the band-limiting effect on the transistor TRI side begins to decrease. At the same time, a gain increasing effect that depends on the difference between the DC resistance aR of the loss control network LC and the DC resistance RB of the gain control network GC occurs, so the gain gradually increases. In other words, as the amplification frequency band becomes limited as the gain increases, the band-limiting effect in that limited band decreases, so that the amplification frequency band limiting effect that accompanies the increase in gain can be effectively suppressed. can be compensated for (cancelled).

When the value of the above-mentioned [control value The gain increases depending largely on the difference between the DC resistance aR of the loss control network LC and the DC resistance R of the gain control network GC.

As a result, even if the above-mentioned [control value Become.

Thus, according to an AGC circuit configured using such a current addition type amplifier, when the gain is increased according to the transmission path loss of a digitally communicated signal, the frequency at which the band can be limited as the gain increases When the amount of loss for the frequency component is reduced and the gain is lowered, the amount of loss for the frequency component that has been band-limited is increased, so that the overall frequency characteristic depends on the amount of transmission line loss described above. It can be a constant characteristic that never occurs.

As a result, if the circuit constants are set so that the frequency characteristics match (approximate) the above-mentioned FCRO equalization characteristics or Gaussian equalization characteristics, the frequency characteristics will not be affected by the magnitude (amount) of transmission line loss. Therefore, it is possible to minimize intersymbol interference and obtain an equalized waveform that compensates for the transmission path loss.

In addition, by adopting this current addition type circuit system, the AGC
When realizing the circuit, it is also possible to employ a circuit with a differential amplification configuration as shown in FIG. That is, the circuit shown in FIG.
A pair of transistors (TRia.TR1b) (TR2a, TR2b) perform the amplification function achieved by R2.
), and essentially operates in the same way as the circuit shown in FIG. 4 described above. Therefore, the same effects as in the previous embodiment can be expected.

Note that the present invention is not limited to the embodiments described above. For example, in the embodiment, an example was shown in which a signal is amplified using one stage of transistors, but it goes without saying that if the amplification gain is insufficient, it is also possible to configure multiple stages of transistors to obtain the amplification gain. Nor. Further, as the circuit structure or system of the amplifier, various circuit systems that have been proposed in the past can be appropriately adopted, and the method for variably setting the amount of loss is not particularly limited. In short, the present invention sets the amplification gain variably according to the amount of transmission path loss, and compensates for the loss of the signal component whose band is limited due to the change in gain when controlling the output level to be constant. Various modifications and variations can be made without departing from the spirit of the invention.

[Effects of the Invention] As explained above, according to the present invention, when a variable gain amplifier and a variable loss amplifier are used and the gain of the variable gain amplifier is increased to compensate for transmission line loss, the gain of the variable gain amplifier is increased in order to compensate for transmission line loss. When the gain of the variable gain amplifier is lowered, the loss amount in the variable loss amplifier is simultaneously increased. Therefore, when the band of the overall frequency characteristic is extended by the gain control of the variable loss amplifier, If the band of the overall frequency characteristic becomes narrower, the amount of band restriction for that band is reduced, so the overall overall frequency remains the same regardless of changes in transmission path loss. The characteristics can be made almost constant.

Therefore, by setting the overall frequency characteristic that is controlled to be constant so as to approximate the aforementioned FCRO equalization or Gaussian equalization, transmission path loss can be effectively compensated for while minimizing intersymbol interference. This has great practical effects, such as making it possible to Moreover, effects such as being able to effectively compensate for transmission line loss with a simple circuit configuration and under simple control can be achieved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a basic configuration example of an automatic gain control circuit according to an embodiment of the present invention, FIG. 2 is a diagram showing amplification frequency characteristics for explaining the principle of operation of the present invention, and FIG. The figure shows an example of the configuration of a variable loss amplifier in the embodiment circuit, and FIGS. 4 and 5 each show an example of the configuration of a variable gain/loss amplifier realized using a current addition type amplifier circuit. be. 1... Variable gain amplifier, 2... Variable loss amplifier, 3
...control circuit, TR, TRI, ~TR6...transistor, RL...load resistance, RE...emitter resistance, C,...variable capacitance diode, R...resistance, G
C...gain control network, LC...loss control network.

Claims (1)

[Claims] A variable gain amplifier for compensating transmission path loss of digitally communicated signals, a variable loss amplifier provided in a signal path amplified through the variable gain amplifier, and a variable loss amplifier for increasing the gain of the variable gain amplifier. An automatic gain control circuit comprising: a control circuit that reduces the amount of loss in the amplifier, and increases the amount of loss in the variable loss amplifier when the gain of the variable gain amplifier is lowered.