CN1409511A - Self adaptive stube equalizer - Google Patents

Abstract

The present invention proposes an adaptive short-line equalizer, which includes a common-mode operational amplifier: a first capacitor and a first resistor are connected in parallel between its forward input terminal and its forward output terminal; The second capacitor and the second resistor are connected in parallel, and also include: a first resistance-capacitance network, a second resistance-capacitance network, an input common-mode voltage generation circuit, a transmission attenuation comparison level generation circuit, a reference voltage source, a comparator, and a transmission line The long judging circuit obtains a group of control signals by judging the result output by the comparator, and simultaneously selects the variable capacitors in the first variable capacitor network or the second variable capacitor network through the group of control signals, To achieve the purpose of adaptive equalization. The present invention can automatically adjust the gain and frequency characteristics of the equalizer according to different transmission line lengths, and change the position of the zero pole of the equalizer, thereby achieving the purpose of automatically equalizing E1 signals passing through transmission lines of different lengths; the equalizer of the present invention is flexible in configuration , which can meet the requirements of short-line (less than 1 km) transmission distance.

Description

Adaptive Stub Equalizer

technical field

The invention relates to a synchronous digital transmission technology in the communication field, in particular to an E1 line equalizer applied in an SDH system.

Background technique

In the communication transmission system, E1 is a commonly used digital transmission standard. In the field of communication, signal distortion and attenuation is a very important problem. Because of the signal distortion caused by the transmission line, it is often necessary to use an equalizer in the E1 interface circuit for signal equalization. The equalizer is actually a compensation circuit that compensates the frequency and phase characteristics of the input signal, thereby filtering out the signal distortion caused by the transmission line, and amplifying the weakened signal to obtain a signal with less distortion.

The position of the equalizer in the E1 line of the SDH system is shown in Figure 1. After the external signal is attenuated by the transmission line, it is firstly equalized by the equalizer to obtain an undistorted signal. The signal processed by the equalizer is converted into a digital signal by the pattern judgment and recovery module, and the digital signal is then recovered by data clock and The debounce circuit is input into the decoding circuit, and the decoding circuit decodes the input HDB3 code or AMI code and sends it to the SDH system for processing. It can be seen from Figure 1 that after signals with different transmission distances and different attenuation levels enter the equalizer, how to ensure that the signal is output without distortion is a very important issue, and the equalization effect of the equalizer has a great impact on the system. . Specifically, after the input signal is attenuated by the transmission line, the signal distortion is mainly reflected in the following aspects: First, the waveform amplitude of the received signal becomes smaller, which is caused by the attenuation of the transmission line; second, the peak delay, which is due to The delay characteristic of the transmission line is caused; the third is that the pulse width is greatly increased, which is caused by the frequency characteristic of the transmission line.

Figure 2 shows the signal waveform of the input standard square wave signal attenuated by the transmission line. The longer the transmission line transmission distance, the greater the signal attenuation and the wider the pulse width. The role of the equalizer is to use waveform compensation to correct the distorted waveform. The waveform of the input signal after the equalizer is shown in Figure 3. It can be seen that the equalizer can compensate and reduce the distortion of the input signal to the greatest extent.

The traditional equalizer is composed of an operational amplifier and a fixed feedback equalization network. Since the equalization network is fixed and the transfer function is also fixed, it can only perform equalization compensation for the attenuation of a specific transmission distance, and cannot be flexibly based on the actual transmission. Adjust the distance accordingly. Once the transmission distance of the input signal is longer or shorter than the range that the equalizer can equalize, the input signal will be under-balanced and over-balanced after being output by the equalizer, so the ideal Balanced effect.

Contents of the invention

The technical problem to be solved by the present invention is to propose an E1 short-line equalizer capable of self-adaptive adjustment in order to overcome the disadvantage that the equalizer in the prior art cannot flexibly make corresponding adjustments according to the actual transmission distance.

The technical scheme of the present invention is such that an adaptive short-line equalizer includes a common-mode operational amplifier: used to provide high gain and low output impedance, and a first capacitor is connected in parallel between its positive input terminal and positive output terminal and the first resistor; between its inverting input terminal and inverting output terminal, a second capacitor and a second resistor are connected in parallel, and it is characterized in that it also includes:

The first resistance-capacitance network: connected in series to the positive input terminal, including the fourth resistor connected in series with the first variable capacitance network and then connected in parallel with the third resistor; the second resistance-capacitance network: connected in series to the negative input terminal, including the sixth The resistor is connected in parallel with the fifth resistor after being connected in series with the second variable capacitor network;

Input common-mode voltage generation circuit: the output of this circuit provides a common-mode voltage to the input of the equalizer;

Transmission attenuation comparison level generation circuit: provide the attenuation level of the input signal after passing through the transmission line with different distances;

Reference voltage source: obtain a stable voltage that is not affected by fluctuations in temperature and power supply voltage, provide it to the transmission attenuation comparison level generation circuit, and provide voltage to the input common-mode voltage generation circuit at the same time;

Comparator: provide a comparison between the input signal and the attenuation level from the transmission attenuation comparison level generation circuit, and input it to the transmission line length judgment circuit for judgment;

Transmission line length judging circuit: By judging the output of the comparator, a set of control signals is obtained, and the variable capacitors in the first variable capacitor network or the second variable capacitor network are selected simultaneously through the set of control signals , to achieve the purpose of adaptive equalization.

The equalizer described in the present invention is a line amplitude equalizer, which can automatically adjust the gain and frequency characteristics of the equalizer according to different transmission line lengths, and change the position of the zero pole of the equalizer, so as to achieve automatic equalization through transmission lines of different lengths. purpose of the E1 signal. The equalizer of the invention has flexible configuration and can meet the requirement of short-line (less than 1 km) transmission distance.

Description of drawings

The present invention will be further described in detail below in conjunction with the accompanying drawings.

Figure 1 is a schematic diagram of the position of an equalizer in an SDH system.

FIG. 2 is a schematic diagram of a waveform of a signal attenuated by a transmission line.

FIG. 3 is a schematic diagram of a waveform equalized by an equalizer.

Figure 4 is a graph showing the relationship between transmission line attenuation, equalizer amplification and total attenuation characteristics and E1 signal frequency.

Fig. 5 is a specific structural diagram of the stub equalizer of the present invention.

FIG. 6 is a schematic structural diagram of the operational amplifier 50 in FIG. 5 .

FIG. 7 is a schematic diagram of the reference voltage source in FIG. 5 .

Detailed ways

The design principle of the equalizer of the present invention is such that by the transmission line theory, the attenuation of the transmission line increases gradually with the increase of the signal frequency and the transmission distance, because the transmission function of the transmission line is approximate to a non-ideal low-pass filter with a single dominant pole. Therefore, in the transfer function of the equalizer of the present invention, there needs to be a main zero to eliminate the influence of the main pole in the transmission line function, and eliminate the signal attenuation caused by the limited bandwidth of the transmission line, so that the channel attenuation can be effectively compensated, especially at high frequencies. Attenuation at the end, so as to obtain ideal low-pass transmission characteristics. When the signal passes through the transmission network of this characteristic, the attenuation and distortion of the signal can be minimized.

Fig. 1 to Fig. 3 are prior art and theories, which have been described in detail above, and will not be repeated here.

In the relationship graph shown in FIG. 4 , the abscissa represents the frequency of the E1 signal, and the ordinate represents the amplitude, and the unit is dB. The attenuation characteristics of the transmission line are related to the length of the transmission line and the frequency of the input signal. If the E1 input signal is not compensated, the attenuation and distortion of the signal output directly from the transmission line will be very large. Therefore, an equalizer must be used to compensate the attenuation of the transmission line. The equalizer can compensate the different attenuation amplitudes of the input signal to the maximum extent, and set a gain peak at the point where the signal attenuation is maximum. The characteristic curve of the equalizer is shown in curve 3. The result of the joint action of the transmission line and the equalizer is an ideal low-pass characteristic curve, as shown in curve 1. The input signal passes through such a low-pass network, and the attenuation and distortion of the signal are very small.

For the curve (2), the ordinate represents the attenuation range of the transmission line. It can be seen that when the E1 signal frequency is 2.048 MHz, the attenuation of the transmission line is the largest.

In the curve (3), the ordinate represents the gain of the equalizer. For the E1 signal, the signal attenuation is the largest when it is close to 2.048MHz, so a gain increase is required here. In order to prevent crosstalk and noise interference in the high frequency band, The gain of the equalizer is reduced at the high frequency end above the E1 frequency point.

Curve (1) is the curve of the joint action of the transmission signal and the equalizer, which is the total attenuation characteristic when the E1 signal frequency is 2.048MHz, similar to an ideal low-pass filter.

The concrete structure of adaptive equalizer of the present invention is as shown in Figure 5, and this equalizer is by the operational amplifier with high gain wide bandwidth, input common-mode voltage generation circuit, reference voltage source, transmission attenuation comparison level generation circuit, comparator, The transmission line length judging circuit is composed of a resistor and a variable capacitor network.

The self-adaptive equalizer of the present invention forms a closed-loop feedback network through a resistance and variable capacitance network and an operational amplifier to jointly complete the equalization function. The adaptive adjustment circuit part is composed of an input common-mode voltage generation circuit, a reference voltage source, a transmission attenuation comparison level generation circuit, a comparator and a transmission line length judgment circuit to jointly complete the transmission distance judgment and automatically adjust the function of the variable capacitor in the equalizer , that is, it can output different control signals to the equalizer according to different transmission distances to automatically adjust the variable capacitance of the equalizer, thereby automatically balancing different transmission lines.

The working principle of the self-adaptive adjustment circuit part of the self-adaptive equalizer of the present invention is: the attenuation level generation circuit of the attenuation comparison level provides the attenuation level of the input signal after transmission lines with different distances, and a comparison level can be set every 100 meters; this is Since the transmission line attenuates 2.1dB for every 100 meters of transmission, the longer the signal transmission distance, the greater the signal attenuation, and the lower the obtained attenuation comparison level. Taking the transmission of the input signal via the coaxial line as an example, if the amplitude of the input signal is 2.37V, a set of attenuation comparison levels can be easily obtained through calculation. The function of the comparator is to compare the input signal attenuated by the transmission line with the attenuation comparison level to obtain a set of comparison results and send them to the transmission line length judgment circuit for judgment. This circuit can estimate the distance of signal transmission and output a A set of control signals is used to automatically select the value of the variable capacitor in the resistor-capacitor network of the equalizer. The greater the signal attenuation, the higher the selected value of the variable capacitor.

The common-mode operational amplifier 50 adopts a full CMOS circuit and adopts two-stage amplification to obtain high voltage gain; the resistance-capacitance network determines the transmission characteristics of the equalizer. The equalizer can respectively equalize the positive and negative AMI or HDB3 coded signals coming from the transmission line. The signals are input in differential form and output in differential form. The input signal is superimposed on the common mode level and input to the operational amplifier.

The equalization characteristics of the equalizer are realized by a closed-loop network composed of a resistor-capacitor network and an operational amplifier. The equalization characteristics of the equalizer can be obtained through the analysis of the transfer function of the equalizer.

Taking the positive signal as an example, the transfer function of the equalizer is (where Csel is the value of the variable capacitor): $\frac{\mathrm{Vout}+}{\mathrm{Vin}+}=\frac{\mathrm{SC}1R4+\mathrm{SC}3R1}{S^{2}C1\mathrm{CasR}1R3R4+\mathrm{SC}1R4R1+\mathrm{SC}3R1+R4}....\left(1\right)$ Take R1=R4 here;

but $\frac{\mathrm{Vout}+}{\mathrm{Vin}+}=\frac{\mathrm{SC}4+\mathrm{SC}3}{S^{2}C1\mathrm{CasR}3R4+\mathrm{SC}1R4+\mathrm{SC}3+1}....\left(2\right)$

It can be seen from the above formula that the equalizer has two zeros and two poles, Z1=CselR4, Z2=CselR3, and the poles are P1=C1R4, P2=CselR3. Since the effects of Z2=P2 cancel each other out, the frequency response characteristics of the equalizer depend on P1 and Z1. Csel should be greater than C1 to achieve BOOST. Increasing Csel can enhance the equalization effect of the equalizer. The position of the zero pole of the equalizer is determined by self-adaptation The control signal generated by the adjustment circuit is used to complete the adjustment. The control signal is related to the transmission distance. The longer the transmission distance, the greater the signal attenuation, and the greater the value of the variable capacitance Csel corresponding to the control signal. Therefore, different transmission attenuation conditions correspond to different Csel values, thereby changing the position of the zero pole of the equalizer to achieve different equalization effects.

This kind of self-adaptation can flexibly and automatically change the position of the zero pole of the equalizer according to the length of the transmission distance, and adjust the frequency response characteristics of the equalizer in real time. When the input signal passes through a long-distance transmission line, the high-frequency part attenuates greatly, and the equalizer increases the gain in its high-frequency part to compensate for the attenuation of the signal at high frequency; when the signal passes through a short transmission line, due to the high-frequency If the degree of attenuation is low, the high-frequency gain of the equalizer will be adjusted accordingly. The specific implementation is as follows: if the distance of the transmission line is short, select a small Csel value through the adaptive circuit; if the distance of the transmission line is long, select a larger Csel value through the adaptive circuit.

The main unit circuit inside the equalizer will be described in detail below with reference to FIG. 6 and FIG. 7 . FIG. 6 is a structural diagram of a common-mode operational amplifier 50 of the equalizer, and the common-mode operational amplifier 50 is composed of three stages of amplifying units. The transconductance differential input stage has high input impedance, low DC offset voltage and noise, and its function is to amplify the input signal. The amplified signal is further amplified by the second stage composed of a high-gain amplifier stage. Here, a gain amplifier stage composed of a common-source common-drain structure is used to achieve high-gain amplification. The common-source common-drain structure is also called the CASCODE structure. , The advantage of using this structure circuit is its high output impedance and large gain. Due to the reduction of the influence of the Miller effect, the -3DB bandwidth is widened, and at the same time, the voltage suppression of the entire circuit is better. The function of the output stage is to provide a relatively low output impedance and output the signal differentially. The bias circuit provides a stable and proper operating point for each stage of the amplifier, which ensures that each transistor in the operational amplifier is in the saturation region under static and specified dynamic conditions. The operating current provided by the bias circuit is determined by the gain of the operational amplifier, the output dynamic range, and so on.

Since the common-mode output signal of an operational amplifier with differential output may be uncertain, this will cause the operational amplifier to deviate from the high-gain region. In order to prevent this phenomenon and stabilize the common-mode output signal, a common-mode feedback is added to the operational amplifier. detection circuit. The circuit samples the common-mode output signal and compares it with the common-mode voltage of the input signal, and feeds back the comparison result to the current source load of the cascode circuit for adjustment, so that the output common-mode voltage is stable.

The generation of the reference voltage source, as shown in Figure 7, the reference voltage value is obtained by adding a voltage drop of V _BE generated by the diode and the voltage passing through the resistor R3. The function of the operational amplifier here is to provide amplification, and the negative feedback circuit Ensure that the voltages output to the two input terminals A5 and A2 of the operational amplifier are equal, thereby stabilizing the common-mode voltage output. The power supply sensitivity and temperature coefficient of this circuit are very small and the noise is very low, and the long-term stability is high. The analysis of the circuit is as follows,

When the circuit shown in Figure 7 is balanced, VA2=VA5, and the output terminal of the operational amplifier AMP1 plays the role of negative feedback by comparing the levels of A2 and A5, so that the output of Vref is more stable. $\mathrm{VBE}1=\frac{\mathrm{KT}}{q}\ln \left(\frac{\mathrm{IC}1}{\mathrm{iso}1}\right)$ $\mathrm{VBE}2=\frac{\mathrm{KT}}{q}\ln \left(\frac{\mathrm{IC}2}{\mathrm{iso}2}\right)$ $R4I4=\frac{\mathrm{KT}}{q}\ln \left(\frac{\mathrm{IC}1\×\mathrm{iso}2}{\mathrm{IC}2\×\mathrm{iso}1}\right)$ $\frac{\mathrm{IC}1}{\mathrm{ic}2}=6,\frac{\mathrm{iso}2}{\mathrm{iso}1}=8;$ Determined according to the width-to-length ratio of the tube and the area of the diode $R4I4=\frac{\mathrm{KT}}{q}\ln 48$ $\mathrm{Vref}=\mathrm{VBE}1+R1*5*\frac{\mathrm{KT}}{q}\ln \left(\frac{48}{R4}\right)$

Through the simulation of the Vref circuit, the reference voltage source is not sensitive to the power supply voltage and temperature changes, and the change of Vref does not exceed 3mV in the temperature range of -40°C to 85°C.

In summary, using the equalizer of the present invention can achieve better equalization effects for E1 signals transmitted at different distances through adaptive adjustment, and by changing the transmission characteristics of the equalizer, a similar principle can be applied to E1 signals with various rates. line equalization.

Claims (3)

1. An adaptive short-line equalizer, comprising a common-mode operational amplifier: used to provide high gain and low output impedance, between the positive input terminal and the positive output terminal, a first capacitor and a first resistor are connected in parallel; A second capacitor and a second resistor are connected in parallel between the input terminal and the reverse output terminal, and it is characterized in that it also includes: The first resistance-capacitance network: connected in series to the positive input terminal, including the fourth resistor connected in series with the first variable capacitance network and then connected in parallel with the third resistor; the second resistance-capacitance network: connected in series to the negative input terminal, including the sixth The resistor is connected in parallel with the fifth resistor after being connected in series with the second variable capacitor network; Input common-mode voltage generation circuit: the output of this circuit provides a common-mode voltage to the input of the equalizer; Transmission attenuation comparison level generation circuit: provide the attenuation level of the input signal after passing through the transmission line with different distances; Reference voltage source: obtain a stable voltage that is not affected by fluctuations in temperature and power supply voltage, provide it to the transmission attenuation comparison level generation circuit, and provide voltage to the input common-mode voltage generation circuit at the same time; Comparator: provide a comparison between the input signal and the attenuation level from the transmission attenuation comparison level generation circuit, and input it to the transmission line length judgment circuit for judgment; Transmission line length judging circuit: By judging the output of the comparator, a set of control signals is obtained, and the variable capacitors in the first variable capacitor network or the second variable capacitor network are selected simultaneously through the set of control signals , to achieve the purpose of adaptive equalization. 2. The adaptive stub equalizer according to claim 1, characterized in that, the first resistance-capacitance network and the second resistance-capacitance network have the same structure. 3. The adaptive stub equalizer according to claim 1, wherein the first resistor is equal to the fourth resistor, and the second resistor is equal to the sixth resistor.

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