Abstract:
A circuit and method for equalization of a communication signal received over a communication system transmission line using switched filter characteristics. Equalization for frequency-independent and frequency-dependent attenuation of the communication signal is accomplished with a linear equalization channel which includes an input biasing circuit which provides a common input signal to two parallel amplifier paths. One path includes a wideband, fixed-gain, frequency-independent amplifier stage. The other path is a wideband multiplier amplifier stage in series with a wideband, frequency-dependent amplifier stage having a switchable high-pass characteristic. The outputs of the fixed-gain wideband frequency-independent amplifier stage and wideband, frequency-dependent amplifier stage having a switchable high-pass characteristic are both tied in common to the input of a wideband gain buffer amplifier stage, which has a switchable high-frequency boost frequency response characteristic. Filter characteristic for the frequency-dependent amplifier stages are selected to accommodate predetermined ranges of cable lengths. The amplifier stages are formed as simple n-MOS transconductance differential amplifiers with restive loads and with switchable filter components connected between the legs of the differential amplifiers.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/087,605, filed Jun. 1, 1998. 
     This application incorporates by reference the following U.S. patent applications that have been assigned to Enable Semiconductor, Inc., a California corporation: Ser. No. 09/061,937, filed Apr. 17, 1998, now U.S. Pat. No. 6,188,721, titled “System and Method for Adaptive Equalization of a Waveform Independent of Absolute Waveform Peak Value,” by Shirani et al.; and Ser. No. 09/062,342, now U.S. Pat. No. 6,148,025, filed Apr. 17, 1998, titled “System and Method for Compensating for Baseline Wander,” by Shirani et al. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to receivers connected to transmission lines and more specifically to a receiver circuit for compensating for signal loss caused by a transmission line. 
     2. Prior Art 
     Coaxial cables exhibit a skin effect which attenuates transmitted signals based on their frequency. The amount of attenuation in decibels (db) increases in proportion to the square root of the frequency f of the transmitted signal. Additionally, the coaxial cable length and diameter, the ambient temperature, and other factors affect attenuation of signals. 
     Equalization amplifiers are used to compensate for signal attenuation. Ideally, the equalization amplifier transfer function is the inverse of the transfer function representing the degree of signal attenuation in the transmission line. This enables recovery of data signal components (e.g., high frequency components) which have been attenuated by the transmission line. 
     Conventional equalization circuits are described in U.S. Pat. No. 5,115,213 to T. Eguchi; U.S. Pat. No. 4,187,479 to K. Ishizuka; U.S. Pat. No. 4,689,805 to S. Pyhalammi et al.; U.S. Pat. No. 5,036,525 to H. Wong; U.S. Pat. No. 4,275,358 to W. A. Winget; U.S. Pat. No. 4,378,535 to R. F. Chiu et al.; U.S. Pat. No. 4,768,205 to K. Nakayama; U.S. Pat. No. 5,337,025 to G. D. Polhemus; U.S. Pat. No. 5,293,405 to J. E. Gersbach et al.; U.S. Pat. No. 4,459,698 to O. Yumoto et al.; U.S. Pat. No. 4,583,235 to J. Domer et al.; U.S. Pat. No. 4,243,956 to M. Lemoussu et al.; U.S. Pat. No. 4,961,057 to S. Ibukuro; and in Giacoletto, L. J. (editor), Electronics Designersí Handbook (2nd ed.), McGraw-Hill Book Company, New York, N.Y. (1977). 
     A conventional equalization linear channel provides a single filter having a fixed transfer function which provides optimal equalization for a limited range of cable lengths. A filter whose transfer function is optimized for shorter cables will not be as optimal for longer cables. Similarly, a filter whose transfer function is optimized for long cables will not be as optimal for shorter cables. 
     Typically, an equalization linear channel, for short cables can be coupled in series with another short cable equalization linear channel to provide equalization for a longer cable. However, equalization imperfections occurring in the first stage are magnified by the second stage, resulting in non-optimal performance for a long cable. Also, each equalization stage consists of several amplifiers so coupling equalization stages in series puts numerous amplifier stages in series. Each amplifier stage loses some of the signal bandwidth, particularly at high frequencies, which reduces the bandwidth of the entire linear channel, thereby reducing the signal-to-noise ratio and the optimization for long cable equalization. 
     The problem of amplifier bandwidth loss is aggravated in amplifiers built using commercially available Complementary-Metal Oxide Semiconductor (CMOS) processes due to the lower tranconductances and higher offsets of Metal Oxide Semiconductor (MOS) devices compared to bipolar devices. The gain bandwidth product of CMOS amplifier structures is typically lower than that of bipolar amplifier structures. 
     A transmission cable typically has a frequency independent (or DC) loss which is a linear function of the cable length. An equalization architecture should restore the DC component lost in the cable and output a signal having an amplitude close to that of the originally transmitted signal. An adaptive equalization algorithm which is peak-dependent demands that the equalized output signal peak be very close to that of the originally transmitted signal before cable attenuation. Algorithms which are peak-independent are preferable but it remains important to restore as much as possible of the lost DC component to maximize the signal-to-noise ratio. 
     Conventional adaptive equalization systems also suffer from a problem known as “baseline wander” which occurs when using differential signal transmission (i.e., MLT-3 coding) over a twisted pair medium. MLT-3 coding systems use three voltage levels (i.e., +1V, 0V, and −1V) relative to a return voltage, referred to as the baseline or baseline reference which is typically set to ground. In practice, the baseline does not remain at ground, but instead wanders up and down, which is referred to as “baseline wander.” 
     What is needed is an equalization linear channel architecture which can be implemented in a commercially available CMOS process, which can provide optimum filter transfer characteristics over a wide range of cable lengths and which has sufficient bandwidth over a wide frequency range even when operating with a low (3 volt) supply voltage. This architecture should also restore most of the originally transmitted signal amplitude in order to maximize the signal-to-noise ratio. 
     SUMMARY OF THE INVENTION 
     The present invention provides equalization of signals received over a communication system transmission line. The invention includes an equalization linear channel having an input biasing stage, a linearity amplifier stage, a first DC gain stage, a second DC gain stage, a DC multiplier stage, an AC/DC gain stage, and a gain buffer stage. 
     The present invention is advantageous in that it provides an equalization linear channel which, over an exceptionally wide range of cable lengths, can optimally equalize waveforms to provide equalized waveforms close in amplitude to the transmitted waveforms independent of DC cable loss, thus maintaining a good signal-to-noise ratio. The circuit architecture comprises only a few amplifier stages in series and can be fabricated using well-known CMOS processes. The stages are transconductance amplifiers which are simple in construction, leading to a high bandwidth in each stage and improved immunity to both offset and power supply noise effects. The circuit architecture performs consistently over wide variations in temperature, power supply, and process, and can be implemented readily with either 3 volt or 5 volt power supplies. The 3 volt power supply architecture provides recognized power and system advantages over a 5 volt power supply. 
    
    
     These and other aspects of the invention will be appreciated by those skilled in the art upon review of the detailed description, drawings and claims set forth below. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an adaptive equalization system; 
     FIG. 2 is a block diagram of the equalization linear channel of FIG. 1; 
     FIG. 3 shows a preferred circuit schematic of input biasing stage of FIG. 2; 
     FIG. 4 shows a preferred circuit schematic of a linearizer amplifier of FIG. 2; 
     FIG. 5 shows a preferred circuit schematic of DC multiplier stage of FIG. 2; 
     FIG. 6 shows a preferred circuit schematic of first DC gain stage of FIG. 2; 
     FIG. 7 shows a preferred circuit schematic of second DC gain stage of FIG. 2; 
     FIG. 8 shows a preferred circuit schematic of AC/DC gain stage of FIG. 2; 
     FIG. 9 shows a preferred circuit schematic of a gain buffer of FIG. 2; 
     FIG. 10 is a graph showing contributions to the gain for a short cable; 
     FIG. 11 shows the summed gain from FIG. 10, the summed gain multiplied by a constant factor and the inverse of cable attenuation versus frequency for a category five twisted pair cable 80 meters long; 
     FIG. 12 shows contributions to the gain for a long cable; 
     FIG. 13 shows the summed gain from FIG. 12, the summed gain multiplied by a frequency independent constant factor and the inverse of cable attenuation versus frequency for a category five twisted pair cable 140 meters long; 
     FIG. 14 shows the gain through the first and second DC gain stages and the gain through the DC multiplier stage and the AC/DC gain stage as a function of the ratio Vcontrol/Vcontrol(Max) as this ratio goes from 0.1 to 1.0 (in mode  1 ). 
     FIG. 15 shows the summation of the gains of the gain curves from FIG. 14 multiplied by 7.6 db for several values of the Vcontrol/Vcontrol(Max) ratio and the cable attenuation curves for cable lengths of 9 meters and 75 meters (in mode  1 ); 
     FIG. 16 shows the gain through the first and second DC gain stages and the gain through the DC multiplier stage and the AC/DC gain stage as a function of the Vcontrol/Vcontrol(Max) ratio as it goes from 0.1 to 1.0 (in mode  2 ); 
     FIG. 17 shows a plot the summation of the gains of the gain curves from FIG. 16 multiplied by 7.6 db for several values of the Vcontrol/Vcontrol(Max) ratio and the cable attenuation curves for cable lengths of 10 meters and 85 meters (in mode  2 ); 
     FIG. 18 shows the gain through first and second DC gain stages and the gain through the DC multiplier stage and the AC/DC gain stage as a function of the Vcontrol/Vcontrol(Max) ratio as it goes from 0.4 to 1.0 (in mode  3 ); 
     FIG. 19 shows the summation of the gains of the gain curves from FIG. 18 multiplied by 7.6 db for several values of the Vcontrol/Vcontrol(Max) ratio and the cable attenuation curves for 80 meters and 140 meters (in mode  3 ); 
     FIG. 20 shows the total gain through equalization linear channel for the maximum value of Vcontrol/Vcontrol(max)=1 for the two cases of boost select  409 =0 and boost select  409 =1; and 
     FIG. 21 is a table of values for the 4 modes of operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram of an adaptive equalization system  150  used in a larger system  115 . Within adaptive equalization system  150 , equalization linear channel  310  receives differential data signals on input lines  315  and  317  and transmits an equalized differential data signal on output lines  325  and  330 . Digital logic control stage  405  receives a control input signal on line  422  from management port  420 , a delay line bias signal on line  382  from delay line calibration circuit  380 , and High Peak Hit HF signal on line  362 , High Peak Hit LF signal on line  364 , Slicer High signal on line  366 , Slicer Low  368 , Low Peak Hit signal on line  370 , and Low Peak Hit signal on line  372  from waveform analyzer stage  350 . Digital logic control stage  405  provides input signals Boost Select on line  409 , Gain Attenuator on line  411 , Filter Select on line  415 , an input signal on line  408  to equalizer DAC  410 , an input signal on line  413  to Base Line Wander (BLW) DAC  417 , and provides a High Peak Offset Control signal on line  340 , High Peak Digital Control signal on line  342 , Low Peak Digital Control signal on line  344  and Low Peak Offset Control signal on line  346  to waveform analyzer stage  350 . 
     Equalization linear channel circuit  310  selectively amplifies the input signal  105  received over and attenuated by transmission line  110  from signal source  102 . Transmission line  110  ranges in length typically from a few meters to over 140 meters. A signal propagating through transmission line  110  experiences attenuation or a line loss which varies in proportion to the square root of the frequency (f) and the length (L) of the transmission line, as expressed by equation (1): 
     
       
         line-loss=exp[−( Af+Bf )/ L ]+line-lossDC( L )  (1) 
       
     
     where A and B are constants and line-lossDC(L) is the DC loss of the signal propagating through transmission line  110 . Line-lossDC(L) is substantially due to resistance in transmission line  110  and increases with L. Equation (1) can be approximated by equation (2): 
     
       
         line-loss=exp[− Af/L ]+line-lossDC( L )  (2) 
       
     
     When connected to a transmission line of a known length and having a particular line-loss characteristic, equalization linear channel  310  generates and applies a gain versus frequency curve which compensates for the DC and frequency dependent attenuation experienced by signals traveling through the transmission line. Equalization linear channel  310  can generate a plurality of gain curves which can mirror the line-loss characteristics of many different transmission lines to equalize input signal  105 . The shape of these gain curves is determined by various transfer function characteristics within equalization linear channel  310 . For shorter cables (i.e., less than 80 meters), equalization linear channel  310  preferably employs a three-pole/zero transfer function filter. For longer cables (i.e., 80 to 140 meters) equalization linear channel  310  preferably employs a two-pole/zero transfer function filter. An equalized output signal  335  is then presented on an output line  325  of the equalization linear channel circuit  310 . 
     For cables longer than 140 meters, equalization linear channel  310  employs the above-mentioned two-pole filter and a fixed AC boost filter activated to further boost the higher frequency components of the input waveform which are heavily attenuated at such long cable lengths. 
     FIG. 2 is a block diagram of equalization linear channel  310  in accordance with the present invention for equalizing signals received through transmission line  110  (FIG.  1 ). Equalization linear channel  310  includes input biasing stage  132 , linearity amplifier stage  148 , first DC gain stage  130 , second DC gain stage  115 , DC multiplier stage  120 , AC/DC gain stage  125 , and gain buffer stage  135 . Input biasing stage  132  receives equalization gain control signals GainControl_p and GainControl_n on lines  260  and  261  respectively from linearity amplifier stage  148 , and provides signals data_p on line  200 , data_n on line  201 , data_p_shifted on line  202  and data_n_shifted on line  203  to first DC gain stage  130  and to DC multiplier stage  120 . First DC gain stage  130  provides input signals dc_p on line  210  and dc_n on line  211  to second DC gain stage  115 . DC multiplier stage  120  provides input signals mul_p on line  212  and mul_n on line  213  to AC/DC gain stage  125 . The second DC gain stage  115  and the AC/DC gain stage  125  output signals Vsump on line  170  and Vsumn on line  171  are the input signals to gain buffer stage  135 . Gain buffer stage  135  outputs the equalization signals eqoutp on line  330  and eqoutn on line  325  which are coupled to waveform analyzer stage  350  of FIG.  1 . 
     Input Biasing Stage  132   
     FIG. 3 is a schematic of preferred input biasing stage  132 , which receives the input differential signals In on line  315  and Inb on line  317 . It also receives two analog signals iblwn and iblwp on control bus  419  from baseline wander (BLW) digital-to-analog converter (DAC)  410  (FIG.  1 ). The two signals iblwp and iblwn are zero current when there is no “baseline wander” event. When a baseline wander event is detected, one signal on the analog bus will be either a positive or a negative current depending on the magnitude and direction of the baseline wander event. The other signal will be a current equal in amplitude but opposite in polarity to that of the first signal. 
     Signals GainControl_p  260  and GainControl_n  261  are used to select the gain curves to be applied to input waveform  320 . Input biasing stage  132  receives on line  400  a band gap-derived biasing voltage signal Vbias to establish the primary bias current of the input biasing stage. An Ibias signal on line  401  is a band gap-derived current of about 150 μa which is sourced across R 112 , providing a process and temperature independent voltage at the gates of transistors Q 105  and Q 107 . Since the gate-to-source voltage drops of transistors Q 105  and Q 107  are both process and temperature dependent, the current through Q 105 , Q 107 , R 109  and R 110  will exhibit both a process and temperature dependency. For fast process corners, the currents through Q 105  and Q 107  will be higher than for slow process corners. The amount of current through Q 105  and Q 107  and the amount of process and temperature skew to this current will be determined by the w/l ratios of Q 105  and Q 107  and the values of R 109  and R 110 . For fast corner circuits produced by an IC fabrication process the common mode of the data In on line  315  and Inb on line  317  relative to ground is reduced in voltage, and for slow corner circuits produced by an IC fabrication process the common mode of the data In on line  315  and Inb on line  317  relative to ground is increased. This biasing is important in ensuring that the next amplifiers in first DC gain stage  130  and in DC multiplier stage  120  (FIG. 1) will always operate as linear amplifiers despite variations in fabrication process and operating temperature. 
     Input biasing stage  132  outputs two pairs of differential signals. The first pair is Data_p  200  and Data_n  201  and the second pair is Data_p_shifted  202  and Data_n_shifted  203 . The two differential signal pairs are equal in differential peak-to-peak amplitude. With the common mode voltage of Data_p and Data_n defined as VCM(data), the common mode voltage of Data_p_shifted and Data_n_shifted defined as VCM(data_shifted); and the voltage difference between GainControl_p and GainContol_n define as Vcontrol, there is a relationship: 
     
       
         VCM(data)−VCM(data_shifted)=(Vcontrol/Vcontrol(Max))*0.4 Volts  (3) 
       
     
     where Vcontrol(MAX) is the maximum excursion of Vcontrol. 
     FIG. 3 shows input biasing stage  132  in a preferred embodiment in which transistors Q 104  and Q 105  set the biasing current at approximately 1.5 ma to establish the common mode voltage In on line  315  equal to (VDD−1.5 ma*(R 101  ohms)). Transistor Q 104  conducts a fixed current of approximately 1.3 ma and transistor Q 105  conducts a process and temperature dependent current. Transistors Q 106  and Q 107  set the biasing current at approximately 1.5 ma to establish the common mode voltage Inb on line  317  equal to (VDD−1.5 ma*(R 103  ohms)). Transistor Q 106  conducts a fixed current of approximately 1.3 ma and transistor Q 107  conducts a process and temperature dependent current. Using bias currents which are the summation of a fixed current and a temperature and process dependent current, enables setting the common mode voltage of the differential signals In and Inb optimally for any given process and temperature. If no baseline wander event is present, the currents through and the voltage drops across resistors R 102  and R 104  are equal. If a baseline wander event occurs, a time varying current of programmable polarity and of amplitude proportional to the magnitude of the baseline wander event will be sourced from iblwp line  266  and a time varying current of equal amplitude but opposite polarity will be sourced from iblwn line  267 . Since these currents are equal in amplitude but opposite in polarity, differential voltage swings which are equal in amplitude but opposite in polarity are developed across resistors R 102  and R 104 . Through this process, signals on lines  266  and  267  can maintain the same common mode voltage even as the baseline wander event generates a DC shift of the common mode voltage at In line  315  versus the common mode voltage at Inb line  317 . A differential voltage Vcontrol equal to GainControl_p−GainControl_n is applied across the differential transistor pair Q 100  and Q 101 . This differential voltage will lead to a differential current through Q 100  and Q 101 , which will lead to a differential voltage drop across R 105  and R 106  and thus a differential voltage (Data_p−Data_p_shifted) between Data_p and Data_p_shifted. The differential voltage Vcontrol applied across transistors Q 102  and Q 103  will lead to a differential current through Q 102  and Q 103  which will lead to a differential voltage drop across R 107  and R 108  and thus a differential voltage (Data_n−Data_n_shifted) between Data_n and Data_n_shifted. The common mode voltage VCM(data) will be higher than the common mode voltage VCM(data_shifted) by the amount of equation (3) 
     
       
         VCM(data)−VCM(data_shifted)=(Vcontrol/Vcontrol(Max))*0.4 Volts  (3) 
       
     
     This linearly varying dependency of the quantity VCM(data)−VCM(data_shifted) with Vcontrol is used to control the DC multiplier stage  120  (FIG.  1 ). 
     Linearity Amplifier  148   
     FIG. 4 is a schematic of a preferred embodiment of linearity amplifier  148 , which receives the differential current information represented by signals icontrol_p and icontol_n on two bit bus  412  from equalizer DAC  410  (FIG.  1 ), and outputs a linear voltage differential signal Vcontrol equal to GainControl_p−GainControl_n. As the differential current from DAC  410  goes from 0 ma to I(max), Vcontrol goes from 0 volts to Vcontrol(MAX). 
     The sum of icontrol_n and icontrol_p is a constant current set by the equalizer DAC  410  (FIG.  1 ). When icontrol_p and icontrol_n are equal, the currents through Q 300  and Q 301  are equal and thus the voltages GainControl_p and GainControl_n are equal. When icontrol_n is close to zero then icontrol_p is equal to its maximum value. The differential voltage Vcontrol is at its maximum value Vcontrol(max). In general Vcontrol=K(icontrol_p−icontrol_n). K is a process and temperature dependent coefficient. The approximate linearity and maximum voltage excursion of the circuit is determined by the sum of icontrol_p and icontrol_n and the w/l ratios of Q 300  and Q 301 , which are equal. The function of Q 302  is to offset GainControl_p and GainControl_n so as to assure the linearity of the Input bias stage. R 301 , C 302  and R 302 , C 301  and C 300  form a low pass filter, so that high frequency noise in icontrol_p and icontrol_n does not get through to the input bias stage. 
     DC Multiplier Stage  120   
     FIG. 5 is a schematic of a preferred embodiment of DC multiplier stage  120 . DC multiplier stage  120 , linearity amplifier  148  and input biasing stage  132  together perform an analog multiplication function on the input data signal In on line  315  and signal Inb on line  317  (FIG.  2 ). Stage  120  receives two pairs of differential input signals: Data_p on line  200  and Data_n on line  201 , and Data_p_shifted on line  202  and Data_n_shifted on line  203 ; and transmits one pair of differential output signals: mul_p on line  212  and mul_n on line  213 . Stage  120  receives signal Gain Attenuator on line  411  and receives signal Vbias on line  400  which provides a band gap derived biasing voltage to establish the primary bias current of stage  120 . If voltage Vin_diff is equal to Data_p−Data_n, voltage Vout_diff is equal to mulp−muln, VCM(data) is the common mode voltage of the differential pair Data_p and Data_n, and VCM(data_shifted) is the common mode voltage of Data_p_shifted and Data_n_shifted, then 
     
       
         Vout_diff=1.4*(Vin_diff(VCM(data)−VCM(data_shifted))/0.4 volts  (5) 
       
     
     The quantity VCM(data)−VCM(data_shifted) is related to the output of the linearity amplifier  148 , Vcontrol=(GainControl_p−GainControl_n), by the equation 
     
       
         VCM(diff)=(Vcontrol/Vcontrol(Max))*0.4 volts  (6) 
       
     
     where VCM(diff)=VCM(data)−VCM(data_shifted). Thus 
     
       
         Vout_diff=(1.4*Vin_diff*Vcontrol)/Vcontrol(Max)  (7) 
       
     
     which shows that Vout_diff is proportional to the product of Vin_diff times Vcontrol and varies from a minimum value of Vout_diff=0 when Vcontrol=0 to a maximum value of Vout_diff=1.4*Vin_diff when Vcontrol=Vcontrol(Max). In decibels, the gain is 20*Log(1.4*(Vcontrol/Vcontrol(Max))). 
     DC multiplier circuit  120  is implemented as a cross-coupled transconductance amplifier. When VCM(data) is equal to VCM(data_shifted) the currents through transistors Q 401  and Q 403  are equal and the currents through transistors Q 404  and Q 402  are equal. Thus, due to the cross-coupled arrangement of these transistors, no differential current flows through resistors R 402  and R 403  and thus no differential voltage builds up between mul_p and mul_n. When VCM(data) is higher than VCM(data_shifted) by the maximum of 0.4 Volts, transistors Q 403  and Q 404  are totally off, and the buffer functions as a transconductance buffer with a gain in decibels of 20*Log(1.4)=2.9 db. The maximum gain of 2.9 db is set primarily by the ratio of 2*R 402 /R 406 , the transconductances of transistors Q 401  and Q 402  and the bias currents through transistors Q 401  and Q 402 . Because multiplier circuit  120  has only one level of differential structures, as compared to a Gilbert multiplier having a stacked differential structure, circuit  120  can operate at a high bias current of approximately 1 ma without experiencing voltage headroom problems. The high biasing current and low resistance load resistors R 402  and R 403  allow for a high bandwidth circuit with a 3 db roll-off frequency greater than 200 Megahertz. In normal operation, transistor Q 407  is “Off” and no current flows through resistors R 404  and R 405 . In the process of adaptive equalization system  150  (FIG. 1) going through its training cycle, the Gain Attenuator pin  411  puts transistor Q 407  in its “On” state. When Gain Attenuator is high, transistor Q 407  is “On” and the gain of DC multiplier stage  120  is reduced to 20*Log(1.19*(Vcontrol/Vcontrol(Max))). 
     T 
     The reduction in gain is determined primarily by the resistance ratio of resistors R 404  to R 402 . The entire biasing current of the DC Multiplier stage  120  flows across R 401 . The generated constant voltage drop is used to ideally set the common mode voltages of mul_p and mul_n, so as to assure the linearity of both the DC multiplier stage  120  and the AC/DC gain stage  125 . For the 3 volt application, R 401  would typically be equal to zero ohms. 
     First Gain Stage  130   
     FIG. 6 is a schematic of a preferred embodiment of first DC gain stage  130 , which has two pairs of differential input signals Data_p on line  200  and Data_n on line  201 , and Data_p_shifted on line  202  and Data_n_shifted on line  203 ; and one pair of differential output signals DC_p on line  212  and DC_n on line  213 . Stage  130  also receives a signal Vbias on line  400  which provides a band gap derived biasing voltage to establish the primary bias current of stage  130 . If Data_p−Data_n is equal to Vin_diff and Vout_diff is equal to DC_p−DC_n, then 20*Log(Vout_diff/Vin_diff)=2.9 db. The gain of first DC gain stage  130  matches that of DC multiplier stage  120  in its highest gain condition. 
     First DC gain stage  130  is preferably implemented as a transconductance amplifier with a gain determined by the ratio 2*R 202 /R 206 , the transconductances of transistors Q 200 , Q 203 , Q 204 , and Q 205 , and the bias currents through transistors Q 200 , Q 203 , Q 204  and Q 205 . Because amplifier  130  has only one level of differential structures, it can operate at a high bias current of approximately 1 ma per leg without experiencing voltage headroom problems. The high biasing current and low load resistors allow for a high bandwidth circuit with a 3 db roll-off frequency greater than 200 Megahertz. Because both first DC gain stage  130  and DC multiplier stage  120  are implemented as differential structures with output resistors tied to the power supply, these circuits exhibit good power supply noise immunity. 
     The entire biasing current of the first DC gain stage  130  flows across R 201 . The generated constant voltage drop is used to ideally set the common mode voltages of DC_p and DC_n so as to assure the linearity of both the first DC gain stage  130  and the second DC gain stage  115 . For the 3 volt application, R 201  would typically be equal to zero ohms. 
     Second DC Gain Stage  115   
     FIG. 7 is a schematic of a preferred embodiment of second DC gain stage  115 , which is a transconductance amplifier stage. Stage  115  receives one pair of differential input signals DC_p on line  210  and DC_n on line  211 , and transmits one pair of differential output signals Vsump on line  170  and Vsumn on line  171 . These output signals are common with the output signals of AC/DC gain stage  125 . Stage  115  also receives the signal Vbias on line  400 , which provides a band gap derived biasing voltage used to establish the primary bias current of stage  115 . If Vin_diff is equal to DC_p−DC_n and Vout_diff is equal to Vsump−Vsumn, then 20*Log(Vout_diff/Vin_diff)=−10.45 db. The gain is primarily determined by the ratio of 2*Rsum/R 602  and the transconductances of transistors Q 600  and Q 601  and the bias currents through transistors Q 600  and Q 601 . Because amplifier  115  has only one level of differential structures, it can operate at a high bias current of approximately 1 ma per leg without experiencing voltage headroom problems. The high biasing current and low load resistors allow for a high bandwidth circuit with a 3 db roll-off frequency greater than 200 Megahertz. Because second DC gain stage  115  is implemented as a differential structure with output resistors referenced to the power rail, this circuit exhibits good power supply noise immunity. 
     AC/DC Gain Stage  125   
     FIG. 8 is a schematic of a preferred embodiment of AC/DC gain stage  125 , which is a transconductance amplifier stage with a dual state programmable filter integrated as the differential source impedance. The filter state is selected by the binary signal Filter Select on line  415 . For state one (short cable) with Filter Select=0 volts, the filter is implemented as a three pole/zero high pass filter in parallel with a DC gain component. For state two (long cable), the filter is implemented as a two pole/zero high pass filter in parallel with a different DC gain component. AC/DC gain stage  125  receives input signals mul_p on line  212  and mul_n on line  213  and transmits output signals Vsump on line  170  and Vsumn on line  171 . The output signals are shared with second DC gain stage  115 . 
     When the Filter Select signal on line  415  is low, the first pole/zero of the filter is set by capacitor C 1 . Since the Filter Select signal is low, transistor Q 3  is “Off” and there is a differential capacitance from capacitor C 2  in series with the small source-to-drain capacitance of transistor Q 3  in its “Off” state. This series capacitance is small relative to the other capacitances in the circuit. Since Filter the Select signal is low, transistor Q 4  is also “Off” and there is a differential capacitance from capacitor C 3  in series with the small source-to-drain capacitance of transistor Q 4  in its “Off” state. This series capacitance is small relative to the other capacitances in the circuit. Since the Filter Select_n signal on line  416  is high, transistors Q 5  and Q 12  behave as switches in the low impedance “On” state. The second pole/zero is formed by capacitor C 4  in series with resistors R 6  and R 7 . The third pole/zero is formed by capacitor C 5  in series with resistors R 8  and R 9 . Since the Filter Select signal is low, transistors Q 7  and Q 8  are “Off” and therefore capacitor C 6  in series with resistors R 10  and R 11  has very little effect on the frequency dependent (AC) characteristics of the filter in the low cable state. The DC gain is largely determined by the ratio (Rsum*2)/(R 3 +R 4 +R 5 ), and is also affected by the transconductances of the differential transistor pair Q 1  and Q 2 . 
     In the low Filter Select state, transistor Q 11  is “Off.” When the Filter Select signal is high, the first pole/zero of the differential filter between  418  and  417  is determined by capacitors C 1 , C 2  and C 3  in parallel. Since the Filter Select signal is high, transistors Q 3  and Q 4  behave as switches in the very low impedance “On” state. The second pole/zero of the filter is determined by capacitor C 6  in series with resistors R 10  and R 4 . Since the Filter Select signal is high, both transistors Q 7  and Q 8  are “On” and have low impedance relative to resistors R 10  and R 11 . Since the Filter Select_n signal on line  416  is low, transistors Q 5  and Q 12  are “Off” and the filters formed by resistors R 6 , R 7 , R 8  and R 9  and capacitors C 4  and C 5  have very little effect on the AC characteristics. The DC gain is largely determined by the ratio (Rsum*2)/(R 3 +R 5 ). In the Filter Select high state, transistor Q 11  is “On” and effectively shorts resistor R 4 . The DC gain is also affected by the transconductances of the differential transistor pair Q 1  and Q 2 . Transistors Q 9  and Q 10  each provide a biasing current of approximately 1 ma. 
     Because AC/DC gain stage  125  has only one level of differential structures, it can to operate at a high bias current of approximately 1 ma per leg without experiencing voltage headroom problems. The high biasing current and low load resistors allow for a high bandwidth circuit with a 3 db roll-off frequency greater than 200 Megahertz. Because this amplifier is implemented as differential structures with its output resistor, Rsum, tied to the power supply, this circuit exhibits immunity to power supply noise. The purpose of AC/DC gain stage  125  is to provide both a frequency independent (DC) and a frequency dependent (AC) boost to the differential input data signal mul_p on line  212  and signal mul_n on line  213 . When the Filter Select signal is low, the DC and AC boost characteristics of AC/DC gain stage  125  provide both the DC and AC boost required to restore the DC and frequency dependent AC loss of data as it is transmitted down a category five twisted pair cable 80 meters long, except for a frequency independent gain factor. Category 5 is the recommended grade of unshielded twisted pair copper wire to be used for fast (100 Mbits) Ethernet installations. When the Filter Select signal is low and data is received through a category five twisted pair cable 80 meters long, adaptive equalization system  150  (FIG. 1) will typically drive DC multiplier stage  120  to its highest gain value. When DC multiplier stage  120  is at its highest gain of 2.9 db, its output signals, mul_p and mul_n, are in phase and equal in amplitude to the output signals DC_p and DC_n of first DC gain stage  130 . The gain seen on Vsump line  170  and Vsumn line  171  due to second DC gain stage  115  and AC/DC gain stage  125  is the summation of the gains through each of these two buffers. Second DC gain stage  115  has a frequency independent gain of approximately −10.45 db and the AC/DC gain stage  125  has both a DC gain, component of approximately −26 db and a frequency dependent gain component due to the three pole/zero high pass filter. When the two DC gains are summed together the resultant DC gain is −9.1 db which is approximately 1.35 db higher than the DC gain of second DC gain stage  115  alone. After the signal travels along 80 meters of category five cable, the DC component of the signal is −1.4 db down from its originally transmitted value. The invention is able to restore this lost DC content by the 1.35 db DC gain added by the DC gain component of AC/DC gain stage  125 . 
     FIG. 10 shows four curves on a plot with the y-axis measuring gain in decibels (db) and the x-axis measuring frequency on a log scale. This plot shows each of the contributions to the gain for short cable due to (1) the AC filter of AC/DC gain stage  125 , (2) the DC component of AC/DC gain stage  125 , (3) the gain of second DC Gain stage  115  and (4) the summation of these three gain contributions. 
     FIG. 11 is a plot of (1) the summed gain from FIG. 10, (2) the summed gain multiplied by a constant factor of 10 db and (3) the inverse of cable attenuation versus frequency for a category five twisted pair cable 80 meters long. The similarity of curves (2) and (3) shows that the summation of the gains of second DC gain stage  115  and of AC/DC gain stage  125  multiplied by a frequency independent gain factor can compensate for the 80 meter AC and DC attenuation. 
     When the Filter Select signal on line  415  is high, the DC and AC boost characteristics of AC/DC gain stage  125  provide both the DC and AC boost required to restore the DC and AC loss of data as it is transmitted a category five twisted pair cable 140 meters long, except for a frequency independent gain factor. When Filter Select is high, and if data is sent through a category five twisted pair cable 140 meters long to adaptive equalization system  150 , adaptive equalization system  150  typically drives DC multiplier stage  120  to its highest gain value of 2.9 db, at which its outputs, mul_p and mul_n, are in phase and equal in amplitude to the outputs DC_p and DC_n of first DC gain stage  130 . The gain at Vsump  170  and Vsumn  171  due to second DC gain stage  115  and AC/DC gain stage  125  is the summation of the gains through these two buffers. The second DC gain stage has a frequency-independent gain of approximately −10.45 db and the AC/DC gain stage  125  has both a DC gain component of approximately −22 db and a frequency-dependent gain component due to the two pole/zero high pass filter. The sum of the two DC gains is −8.4 db, which is approximately 2.05 db higher than the DC gain of the second DC gain stage  115  only. After traveling along 140 meters of category five twisted pair cable, the DC component of the transmitted signal is −2.5 db down from its original transmitted value. Most of this lost DC content can be restored by the 2.05 db DC gain added by the DC gain component of AC/DC gain stage  125 . 
     FIG. 12 is a plot of four curves with the y-axis measuring gain in db and the x-axis measuring frequency on a log scale. This plot shows each of the contributions to the gain for long cable due to (1) the two pole AC filter of AC/DC gain stage  125 , (2) the DC component of the AC/DC gain stage  125 , (3) the DC gain of second DC gain stage  115 , and (4) the summation of these three gain contributions. 
     FIG. 13 is a plot of (1) the summed gain from FIG. 12, (2) the summed gain multiplied by a frequency independent constant factor of 10 db and (3) the inverse of cable attenuation versus frequency for a category five twisted pair cable 140 meters long. The similarity of curves (2) and (3) shows that the summation of the gains of second DC gain stage  115  and of AC/DC gain stage  125  multiplied by a frequency independent gain factor can compensate for the AC and DC attenuation from 140 meters of cable. 
     Of particular importance was the placing of the AC/DC Gain stage  125  after the DC multiplier stage  120  rather than vice-versa. If the AC/DC Gain stage  125  were placed before the DC multiplier stage, then even short cable signals with very little high frequency attenuation would have their high frequency components heavily amplified by the AC/DC Gain stage. The resultant heavily distorted signal would be presented to the DC multiplier stage. It would have been possible for there to be unwanted differential and/or common mode noise on the output of the DC multiplier stage which would lead to errors in the equalization algorithm. By placing the DC multiplier stage before the AC/DC Gain stage, only the necessary diffenential signal is presented by the DC multiplier stage to the AC/DC Gain stage. 
     Gain Buffer  135   
     FIG. 9 is a schematic of a preferred embodiment of gain buffer  135 , which is a transconductance buffer with a dual state programmable filter integrated as the differential source impedance. The filter state is selected by the binary signal Boost Select on line  409 . Gain buffer  135  receives differential input signals Vsump on line  171  and Vsumn on line  170 , and transmits differential output signals eqoutn on line  330  and eqoutp on line  325 . Gain buffer  135  also receives a band gap derived biasing voltage signal Vbias on line  400  to establish the primary bias current of gain buffer  135 . When the Boost Select signal on line  409  is low, gain buffer  135  acts as a transconductance buffer with a constant gain equal to approximately 7.6 db up to frequencies greater than 200 Megahertz. When the Boost Select signal is high, gain buffer  135  has a gain of 7.6 db for low frequencies, but high frequencies are boosted an additional amount by a high pass filter which is turned “On” by Boost Select being high. This additional AC boost is typically that required to restore the high frequency AC loss when a signal is sent through a twenty meter category five twisted pair cable. The Boost Select signal typically goes high when adaptive equalization system  150 , of which equalization linear channel  310  is part (FIG.  1 ), determines that the category five twisted pair cable to which it is attached is longer than 140 meters. By adding additional high frequency boost, equalization linear channel  310  is able to properly equalize signals transmitted along up to approximately 160 meters of category five twisted pair cable. 
     When the Boost Select signal is low, the gain is determined primarily by the ratio (2*R 500 /R 502 ) and also by the transconductances of transistors Q 500  and Q 501  and bias currents through transistors Q 500  and Q 501 . The gain, which is equal to as 20*Log((eqoutp−eqoutn)/(Vsump−Vsumn)), is approximately equal to 7.6 db up to frequencies greater than 200 Megahertz when Boost Select is low. When Boost Select is low, transistors Q 504  and Q 505  are in their high impedance “Off” state and thus the gain versus frequency response of the buffer is unaffected by the two pole high pass filter comprised of capacitor C 500  in parallel with the series combination of resistor R 503 , capacitor C 501 , and resistor R 504 . When Boost Select is high, transistors Q 504  and Q 505  are in their low impedance “On” state. The differential source impedance of the buffer now includes the two pole/zero high pass filter comprised of a first pole/zero determined primarily by capacitor C 500  and a second pole/zero determined primarily by capacitor C 501  in series with resistors R 503  and R 504 . For low frequencies, the gain of buffer  135  is still approximately 7.6 db, but for higher frequencies the gain is increased. Because gain buffer  135  is implemented with only one level of differential transistor structures, it can operate at a high bias current of approximately 1 ma without experiencing voltage headroom problems. The high biasing current and low resistance output resistors R 500  and R 501  allow for a circuit with a 3 db roll-off frequency greater than 200 Megahertz. The differential structure with output resistors tied to the power rail has excellent power supply noise immunity. 
     Operation 
     Four modes of operation will be described with reference to the table of FIG.  21 . These modes are controlled by the signals Gain Attenuator on line  411  (FIG.  5 ), Filter Select on line  415  (FIG.  8 ), and Boost Select on line  409  (FIG.  9 ), which are sourced from digital logic control stage  405  (FIG. 1) to equalization linear channel  310 . Once adaptive equalization system  150  determines which mode of operation equalization linear channel  310  is to operate in, the equalizer DAC  410  output on two bit bus  412  (FIG. 1) controls how much AC and DC boost is added to input waveform  320 . 
     In mode  1  the Gain Attenuator signal on line  411  is high (=1), the Filter Select signal on line  415  is low (=0), and the Boost Select signal on line  409  is low (=0). The differential input waveform  320  is carried on lines In  315  and Inb  317  to input biasing stage  132 . Input biasing stage  132  sets the common mode voltage of signals In on line  315  and Inb on line  317  relative to the power rail, and outputs two pairs of differential signals Data_p  200  and Data_n  201 , and Data_p_shifted  202  and Data_n_shifted  203 . Each of these two pairs of signals is equal in amplitude and in phase with the input data pair of signals In on line  315  and Inb on line  317 . The difference in the common mode voltages of these two pairs of signals is linearly dependent on the magnitude of the difference between the current carried on icontol_p and icontrol_n of the two bit bus  412  (FIG.  4 ). The two pairs of signals (Data_p, Data_n and Data_p_shifted, Data_n_shifted) are received by first DC Gain stage  130 , which has a gain of 2.9 db. The differential signals DC_p and DC_n are received by the second DC gain stage  115  (FIG.  1 ), which outputs the differential signals Vsump on line  170  and Vsumn on line  171 . The differential output of second DC gain stage  115 , (Vsump−Vsumn), is formed as the arithmetic sum of the −10.45 db gain due to second DC gain stage  115  and the gain due to AC/DC gain stage  125 . Thus as a result of going through first and second DC gain stages  130  and  115 , the differential voltage Vsump−Vsumn has one contribution equal to 2.9 db*(−10.45 db)=−7.55 db. The two pairs of signals (Data_p, Data_n and Data_p_shifted, Data_n_shifted) are also received by DC multiplier stage  120 . The output signals of DC multiplier stage  120  are mul_p on line  212  and mul_n on line  213 . The differential output Vout_diff is equal to (mul_p−mul_n). If the difference in the common mode voltage of Data_p, Data_n and the common mode voltage of Data_p_shifted, Data_n_shifted is defined as VCM(diff) and if Gain Attenuator  411  is high then 
     
       
         20*Log(Vout_diff/Vin_diff)=20*Log(1.19*(Vcontrol/Vcontrol(Max)))  (8) 
       
     
     where Vin_diff is defined as (Data_p−Data_n). When the delta current defined as (icontrol_p−icontrol_n) equals zero (FIG.  4 ), then Vcontrol defined as (GainControl_p GainControl_n) equals zero and Vout_diff equals zero. When the delta current defined as (icontrol_p−icontrol_n) equals I(Max), then Vcontrol equals Vcontrol(Max) and the gain is a maximum value of 1.51 db. The DC multiplier stage output is received by AC/DC gain stage  125 . In the Filter Select=0 mode, AC/DC gain stage  125  has a gain versus frequency response characterized by a DC gain of −26 db and an AC gain determined by a three pole high pass filter. The gain through DC multiplier stage  120  and through AC/DC gain stage  125  is 20*Log(1.19*(Vcontrol/Vcontrol(max)))*(gain of AC/DC gain stage). This gain is summed with the −7.55 db gain due to first and second DC gain stages  130  and  115 . The differential signals Vsump and Vsumn are received by gain buffer  135 . With Boost Select  409  equal zero, the gain of gain buffer  135  is 7.6 db. 
     FIG. 14 is a plot showing both the gain through first and second DC gain stages  130  and  115  and the gain through DC multiplier stage  120  and AC/DC gain stage  125  as a function of the ratio (=Vcontrol/Vcontrol(Max) as this ratio (goes from 0.1 to 1.0. Curves  1 - 6  represent (at 1.0, 0.9, 0.7, 0.5, 0.3 and 0.1 respectively. Curve  7  represents the gain of first DC gain stage  130  multiplied by the gain of second DC gain stage  115 . 
     FIG. 15 is a plot showing the summation of the gains of the gain curves from FIG. 14 multiplied by 7.6 db for several values of ratio (, and the cable attenuation curves for 9 meter and 75 meter length cables. In the FIG. 15 plot curves  1 - 6  represent the gain of the DC multiplier stage multiplied by the gain of the AC/DC gain stage summed with the gain of the DC gain stage  1  multiplied by the gain of the DC gain stage  2 , then multiplied by 7.6 db for different values of ((1.0, 0.9, 0.7, 0.3 and 0.1). Curves  7  and  8  are the inverse of the cable attenuation losses for 75 meter and 9 meter UTP cables, respectively. 
     The plot illustrates that by varying Vcontrol it is possible to match the gain curve to compensate for both DC and AC loss in the various cables. 
     Adaptive equalization system  150  (FIG. 1) initially starts in mode  1 , which is a power up training mode whose outcome determines whether adaptive equalization system  150  operates permanently in a short or long cable mode. Vcontrol initially starts at zero, and the equalization linear channel  310  output waveform  335  carried by signal eqoutp on line  330  is sent to waveform analyzer stage  350 . If adaptive equalization system  150  determines that the proper equalization has not been reached and that more AC and DC gain is required, then the output of equalizer DAC  410  (FIG. 1) will be increased and thus Vcontrol will be increased. As Vcontrol is increased a different AC and DC gain, determined by the output of DC multiplier stage  120  multiplied by AC/DC gain stage  125 , will be summed with the DC gain determined by the output of first DC gain stage  130  multiplied by second DC gain stage  115 . When adaptive equalization system  150  determines that the proper equalization has been reached, the value of Vcontrol will stay within a small voltage window. In mode  1 , for cables somewhat longer than 70 meters, adaptive equalization system  150  will not be able to properly equalize the incoming waveform even when maximum gain has been introduced with α=1. 
     If adaptive equalization system  150  is able to properly equalize in mode  1 , then it will move to mode  2 , which is similar to mode  1  in that it has Filter Select on line  415  and Boost Select on line  409  both low. However, since Gain Attenuator  411  is a low, the gain of DC multiplier stage  120  is 20*Log(1.4*(Vcontrol/Vcontrol(Max))) and therefore adaptive equalization system  150  will be able to equalize for cables up to 90 meters long. Mode  2  is the short cable non-training mode of operation. By having less gain capabilities in mode  1  than in mode  2 , if the adaptive equalization goes to mode  2  there is a guaranteed 15% additional gain to account for system environmental changes which might affect the boost requirements for a given cable length. 
     If adaptive equalization system  150  is not able to properly equalize in mode  1 , then it will move to mode  3  which is the long cable setting with the Filter Select signal on line  415 =1. Mode  3  is used for cables which are longer than 80 meters and shorter than 140 meters. Since mode  3  is a long cable setting, it is not necessary to have Vcontrol start at 0. If in mode  3 , adaptive equalization system  150  approaches close to the maximum value of Vcontrol, Vcontrol is approximately Vcontrol(Max), then adaptive equalization system  150  puts equalization linear channel  310  in mode  4 . Mode  4  is used for cables which are longer than 140 meters. Mode  4  is similar to mode  3  in that Filter Select is 1. However, Boost Select in now 1 and as a result an additional 2 pole/zero high pass filter embedded in the gain buffer  135  is activated which additionally boosts higher frequency components. Typically, this additional boost is that required to approximately recover the AC frequency attenuation for a category five twisted pair cable 20 meters long. By multiplying the summed gain at Vsump and Vsumn by the gain of gain buffer  135 , equalization linear channel  310  has sufficient high frequency boost to equalize signals attenuated by up to 170 meters of category five twisted pair cable. 
     FIG. 20 is a plot 2000 which shows the total gain through equalization linear channel  310  for the maximum value of (=1 for the two cases of (1) Boost Select  409 =0 and (2) Boost Select  409 =1. It can be seen that there is greater gain with Boost Select  409 =1. Also plotted are the inverse of the cable attenuation curves for (3) 140 meters and (4) 160 meters. It can be see that with Boost Select  409 =1, equalization linear channel  310  is able to compensate well for the cable attenuation at 160 meters. 
     While the present invention has been described with reference to a preferred embodiment, those skilled in the art will recognize that various modifications may be made. The invention is limited only by the following claims.