Abstract:
Transmit amplitude independent adaptive equalizers are provided that compensate for transmission losses in an input signal when the transmit signal amplitude is unknown. Several embodiments are provided, including a first embodiment having an equalizer core, a controllable-swing slicer and an amplitude control loop, a second embodiment having an equalizer core, a fixed-swing slicer and a control loop, a third embodiment having an equalizer core, a variable gain amplifier, and a variable gain amplifier control loop, and a fourth embodiment having an equalizer core, a fixed-swing slicer, a variable gain amplifier, and a variable gain amplifier control loop.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to the field of equalizers. More particularly, the invention provides a transmit amplitude independent adaptive equalizer that is capable of compensating for transmission losses in an input signal when the transmit signal amplitude is unknown. The invention is particularly well suited for use in digital communication components, such as receivers, equalizers, high-speed backplanes, Printed Circuit Board Trace equalizers, automatic gain control devices, and other types of digital communication components. 
     2. Description of the Related Art 
     The use of an equalizer to compensate for loss resulting from the non-idealities of a transmission medium is known.  FIG. 1  is a block diagram showing an equalizer  12  implemented in a typical digital communications system  10  in which an input signal  14  is transmitted through a transmission medium  16 . Typical transmission media  16  used for transmission of digital signals over relatively short distances include, for example, printed circuit board (PCB) traces and coaxial cables. These, and other known transmission media, typically cause significant frequency dependant losses in digital signals being transmitted over the media and consequently distort the digital data, often resulting in pulse spreading and interference between neighboring pulses (known as intersymbol interference). In addition, the input signal  14  is further corrupted during transmission by noise  18  induced by the transmission medium  16 . The equalizer  12  regenerates the transmitted signal  20  by providing gain to compensate for the frequency dependant losses caused by the transmission medium  16  (up to some maximum length) while preferably minimizing the effect of noise  18 . This function is typically achieved by applying a transfer function to the received signal  20  that approximates the inverse of the transmission losses. 
       FIG. 2  is a graph  30  showing the loss (in dB) incurred in the transmission medium  16 , plotted as a function of both the length (l) of the medium  16  and the frequency (f) of the signal. Generally, the loss over a transmission medium (such as a coaxial cable or PCB trace) may be approximated in the frequency domain by the following equation:
   L ( f )= e   −l(ks√{square root over (jf)}+kd|f|) ; 
where f is the frequency, l is the length of the transmission medium, j=√{square root over (−1)}, k s  is the skin effect loss constant of the transmission medium, and k d  is the dielectric loss constant of the transmission medium. The value of L(f) is plotted in  FIG. 2  for transmission media of two different lengths: Length  1  (shorter) and Length  2  (longer). As the length (l) of the transmission medium increases, the loss increases. In addition, as the frequency (f) increases, the loss increases.
 
     To counteract the transmission loss shown in  FIG. 2 , an equalizer  12  should have a frequency characteristic that is the inverse of the loss function of the transmission medium. The inverse loss function may be approximated as follows: 
           1     L   ⁢     (   f   )         =       G   ⁢     (   f   )       =     1   +     KH   ⁢     (   f   )             ;       
 
where K is a control variable that is proportional to the length (l) of the transmission medium. The value of K typically varies from zero to unity (or some other constant) as the transmission medium approaches its maximum length.
 
       FIG. 3  is a graph  40  showing the inverse loss function G(f), plotted in dB on the same axes as the loss function L(f). As shown in this figure, the inverse loss function G(f) provides a frequency dependant gain equivalent to the loss L(f) incurred in the transmission medium. The characteristics of the inverse loss function G(f) are explained in more detail in U.S. patent application Ser. No. 09/055,515 (hereinafter referred to as “the &#39;515 application”) which is owned by the Assignee of the present application, and which is hereby incorporated into the present application by reference. 
       FIG. 4  is a block diagram of an equalizer core  50  that implements the inverse loss function G(f). The equalizer core  50  includes a transfer function block  52  (H(f)), a multiplier  58 , and an adder  56 . This circuit  50  applies variable gain to an input signal  57  by applying the transfer function H(f) in order to generate a resultant signal and then by multiplying the resultant signal from the transfer function block  52  by a gain control signal  58  (K). The gain control signal  58  (K) preferably controls the amount of gain applied by the transfer function H(f) by multiplying the output of the transfer function block  52  by a factor typically varying from zero (0) to unity (1) depending upon the length (l) of the transmission medium  16 . For instance, when the transmission medium  16  is at a maximum length, the transfer function H(f) is generally multiplied by unity (1) to provide the maximum gain. The output of the multiplier is then summed with the input signal  57  by the adder  56  in order to produce an equalized output signal  59  corresponding to the inverse loss function (1+KH(f)). An exemplary circuit for implementing the transfer function block  52  is described in the above-referenced &#39;515 application. 
       FIG. 5  is a block diagram of an alternative equalizer core  60  that implements a bandwidth-limited inverse loss function. In this circuit  60 , a low-pass filter  62  is added to the equalizer core  50  shown in  FIG. 4  to reduce noise encountered in the transmission medium  16 . This alternative implementation  60  reduces the amplification of high frequency noise, and thus increases the signal-to-noise ratio (SNR) of the equalized output signal  64 . A graphical representation  70  of the bandwidth-limited inverse loss function  72 , plotted on the same axes as the loss function L(f) is shown in FIG.  6 . 
       FIG. 7  is a block diagram showing a multiple-stage equalizer core  80  having three equalizer stages  82 ,  84  and  86 , each of which implements the inverse loss function G(f). The three cascaded equalizer stages  82 ,  84  and  86  are preferably the same as the equalizer core  50  shown in FIG.  4 . Alternatively, the multiple-stage equalizer core  80  could include a plurality of bandwidth-limited stages as shown in  FIG. 5 , or other types of cores. In any case, each equalizer stage  82 ,  84  and  86  includes a gain control signal (K 1 , K 2  or K 3 ) that is used to control the gain implemented by the transfer function H(f) in proportion to the length of the transmission medium  16 . The advantages of utilizing a multiple-stage equalizer core are explained in detail in the &#39;515 application. 
     Operationally, each stage  82 ,  84  and  86  in the multiple-stage equalizer core  80  is configured to equalize signals transmitted over transmission media up to a percentage of the total maximum transmission medium length. For instance, if the multiple-stage equalizer core  80  is capable of equalizing losses incurred in a printed circuit board (“PCB”) trace of up to 30 inches, then each core stage  82 ,  84 , and  86  will typically be configured to equalize losses in PCB traces of up to 10 inches. The stages  82 ,  84  and  86  are then cascaded such that they operate sequentially to equalize PCB traces of up to 30 inches. 
       FIG. 8  is a graph  90  showing how the gain control signals K 1 , K 2  and K 3  in the multiple-stage equalizer core  80  are varied according to the length of the transmission medium. The value K, shown along the x-axis in  FIG. 8 , represents the percentage of the transfer function H(f) that needs to be applied to an input signal in order to supply the gain necessary to equalize a transmission medium of a given length. As the transmission medium length increases, the gain necessary to equalize the transmission losses in the medium also increases.  FIG. 8  shows that the gain control signals K 1 , K 2  and K 3  cause gain to be supplied sequentially by the equalizer stages  82 ,  84  and  86 . For instance, if each equalizer stage  82 ,  84  and  86  is capable of providing the necessary gain to equalize 10 inches of a PCB trace, then the gain control signal K 1  would typically control the gain necessary for PCB traces from 0 to 10 inches, the combined gain control signals K 1  (at unity) and K 2  would typically control the gain necessary for PCB traces from 10 to 20 inches, and the combined gain control signals K 1  (at unity), K 2  (at unity) and K 3  would typically provide the gain for PCB traces from 20 to 30 inches. For example, if the PCB trace were 15 inches in length and each equalizer stage  82 ,  84  and  86  can equalize 10 inches, then K 1  would be at its maximum value (unity), K 2  would be at the value necessary to cause the second equalizer stage  84  to equalize a 5 inch transmission medium, and K 3  would be zero. 
       FIG. 9  is a block diagram showing an exemplary equalizer system  100  such as described in the referenced &#39;515 application. This equalizer system  100  includes an equalizer core  102 , a slicer  104 , an automatic gain control circuit (AGC)  106 , a transmitter  108 , and a transmission medium  110 . The equalizer core  102  may be either a single-stage core as shown in  FIG. 4  or  5  or a multiple-stage core as shown in  FIG. 7 , and operates, as described above, to compensate for the losses incurred in the transmission medium  110 . The output  112  of the equalizer core  102  is coupled to the slicer  104 , which converts the output signal  112  from the core  102  to a digital output signal  114  having a known swing (A) that approximates the swing (B) of the data sent from the transmitter  108 . Since the swing (B) of the transmitted data is known and reproduced as the swing (A) of the digital output signal  114  from the slicer  104 , the difference in energy between the equalizer core output signal  112  and the digital output signal  114  approximates the energy lost in the transmission medium  110 , which is proportional to its length. The AGC  106  compares the energy of the equalizer core output signal  112  with the energy of the digital output signal  114  from the slicer  104  to generate the gain control signal K. 
     The AGC  106  includes a core-side band-pass filter  116 , a core-side envelope detector  118 , a slicer-side band-pass filter  120 , a slicer-side envelope detector  122 , an adder  124 , and a sequencer  126 . Operationally, the AGC  106  filters the core and digital outputs  112  and  114  to mid-band frequencies using the band-pass filters  116  and  120 . The advantage of filtering the core and digital outputs  112  and  114  to their mid-band frequencies is explained in detail in the &#39;515 Application. Following this filtering function, the AGC  106  then detects the signal energy of the two band-limited signals with the envelope detectors  118  and  122 . Finally, it determines the difference between the two signal energies with the adder  124 , which provides the gain control signal K. If the equalizer core  102  is single-stage, then the gain control signal K is typically coupled directly to the core  102  to control the variable gain as described above. If, however, the equalizer core  102  is of the multiple-stage type, then the sequencer  126  is used to convert the gain control signal K from the adder  124  into a plurality of multiple-stage gain control signals Ki, such as K 1 , K 2  and K 3  described above with reference to  FIGS. 7 and 8 . In either case, the gain control signal(s) K (or Ki) enable the equalizer core  102  to equalize the core output signal  112  by forcing it to the same energy level as the digital output signal  114  from the slicer  104 . A further description of the AGC  106  is provided in the above referenced &#39;515 application. 
     One skilled in the art will appreciate that the signal swing (B) at the transmitter  108  must be known a priori and accurately replicated by the slicer  104  if the equalizer system  100  shown in  FIG. 9  is to achieve optimal performance. Any significant difference between the signal swing (B) at the transmitter  108  and the signal swing (A) of the digital output signal  114  will directly result in a gain (equalization) error. For example, an increase in the swing (B) of the transmitted signal will force the AGC loop  106  to settle at a lower gain than necessary to compensate for the transmission loss (under-equalization). Similarly, a decrease in the swing (B) of the transmitted signal will result in over-equalization. Even if the swing (B) of the transmitted signal were tightly controlled, similar equalization errors may be caused by mismatch in the digital output swing (A) generated by the slicer  104 . Such mismatch errors may be caused, for example, by variations in temperature, power supply voltages, or manufacturing processes. 
     SUMMARY 
     A transmit amplitude independent adaptive equalizer is provided. One embodiment of the equalizer comprises an equalizer core, a controllable-swing slicer and an amplitude control loop. The equalizer core is coupled to an input signal from a transmission medium, and generates a core output signal by applying a frequency dependant gain to the input signal to compensate for losses incurred in the transmission medium, The controllable-swing slicer is coupled to the core output signal and a swing control input, and converts the core output signal into a digital output signal having a variable swing that is controlled by the swing control input. The amplitude control loop is coupled to the core output signal and the digital output signal, and generates the swing control input by comparing the core output signal with the digital output signal. 
     A second embodiment of the equalizer comprises an equalizer core, a fixed-swing slicer, and a control loop. The equalizer core is coupled to an input signal from a transmission medium and a gain control input, and generates a core output signal by applying a frequency dependant gain to the input signal. The frequency dependant gain is controlled by the gain control input and compensates for losses incurred in the transmission medium. The fixed-swing slicer is coupled to the core output signal and converts the core output signal to a digital output signal having a fixed digital output swing. The control loop is coupled to the core output signal and the digital output signal and normalizes the core and digital output signals with respect to their low-frequency energy levels, compares the normalized core output signal with the normalized digital output signal to approximate a normalized energy level difference, and generates the gain control input. 
     A third embodiment of the equalizer comprises a variable gain amplifier, a variable gain amplifier control loop, and an equalizer core. The variable gain amplifier is coupled to an input signal from a transmission medium and a variable gain control signal, and applies a variable gain to the input signal to generate an equalizer core input signal having a pre-determined signal swing. The variable gain amplifier control loop is coupled to the input signal and the equalizer core input signal, and compares the input signal with the equalizer core input signal in order to generate the variable gain control signal. The equalizer core is coupled to the equalizer core input signal, and applies a frequency dependant gain to the amplifier output in order to compensate for attenuation of the input signal caused by losses incurred in the transmission medium. 
     A fourth embodiment of the equalizer comprises a variable gain amplifier, an equalizer core, a fixed-swing slicer and a variable gain amplifier control loop. The variable gain amplifier is coupled to an input signal from a transmission medium and is also coupled to a variable gain control signal. The variable gain amplifier applies a variable gain to the input signal in order to generate an equalizer core input signal having a pre-determined signal swing. The equalizer core is coupled to the equalizer core input signal, and applies a frequency dependant gain to the amplifier output in order to compensate for attenuation of the input signal that is caused by losses incurred in the transmission medium and generates a core output signal. The fixed-swing slicer is coupled to the core output signal, and converts the core output signal into a digital output signal having the pre-determined signal swing. The variable gain amplifier control loop is coupled to the core output signal and the digital output signal, and compares the core output signal with the digital output signal in order to generate the variable gain control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the use of an equalizer in a typical serial digital data communication system in which an input signal is transmitted through a transmission medium; 
         FIG. 2  is a graph showing the loss (in dB) incurred in the transmission medium, plotted as a function of both the length (l) of the medium and the frequency (f) of the signal; 
         FIG. 3  is a graph showing the inverse loss function G(f) plotted on the same axes as the loss function L(f); 
         FIG. 4  is a block diagram of an equalizer core that implements the inverse loss function G(f); 
         FIG. 5  is a block diagram of an alternative equalizer core that implements a bandwidth-limited inverse loss function; 
         FIG. 6  is a graphical representation of the bandwidth-limited inverse loss function, plotted on the same axes as the loss function L(f); 
         FIG. 7  is a block diagram showing a known multiple-stage equalizer core in which each stage implements the inverse loss function G(f); 
         FIG. 8  is a graph showing how the gain control signals in the multiple-stage equalizer core are adjusted according to the length of the transmission medium; 
         FIG. 9  is a block diagram showing an exemplary equalizer system such as described in the referenced &#39;515 Application; 
         FIG. 10  is a block diagram of an exemplary transmit amplitude independent adaptive equalizer having a controllable-swing slicer according to one embodiment of the claimed invention; 
         FIG. 11  is a block diagram of an exemplary transmit amplitude independent adaptive equalizer having a fixed-swing slicer according to another embodiment of the claimed invention; 
         FIG. 12  is a block diagram of a transmit amplitude leveling circuit for an equalizer; 
         FIG. 13  is a block diagram illustrating an exemplary implementation of the received data swing detection circuit and the core input swing detection circuit shown in  FIG. 12 ; and 
         FIG. 14  is a block diagram of an exemplary transmit amplitude independent adaptive equalizer utilizing a variable gain amplifier. 
     
    
    
     DETAILED DESCRIPTION 
     Referring again to the drawing figures,  FIG. 10  is a block diagram of an exemplary transmit amplitude independent adaptive equalizer  200  having a controllable-swing slicer  204 . The equalizer  200  includes an equalizer core  202 , a controllable-swing slicer  204 , a gain control loop (G-Loop)  206 , and an amplitude control loop (A-Loop)  208 . The G-Loop  206  includes two high band-pass filters  220  and  222 , two envelope detectors  224  and  226 , an adder  228 , and a sequencer  230 . The amplitude control loop (A-Loop)  208  includes two low band-pass filters  240  and  242 , two envelope detectors  244  and  246 , and an adder  248 . 
     Operationally, the amplitude independent adaptive equalizer  200  tracks the amplitude at which its input signal  210  was transmitted by varying the swing of its digital output signal  214  to approximate the swing of the transmitted data. Because the loss incurred in a transmission medium is frequency dependant, the low frequency portion of the equalizer&#39;s input signal  210  shows substantially less attenuation than the higher frequency portions. The equalizer  200  thus detects the amplitude of its input signal  210  at a low frequency, and uses this low-frequency amplitude to approximate the swing of the transmitted data. 
     The equalizer core  202  receives the input signal  210  from a transmission medium and generates a core output signal  212 . The input signal  210  is preferably a digital signal that has been attenuated during transmission over a transmission medium. The equalizer core  202  compensates for attenuation and distortion in the input signal  210  by applying an inverse loss function G(f) as described above with reference to  FIGS. 2-8 . The equalizer core  202  also receives a gain control signal (Ki)  234  from the gain control loop (G-Loop)  206 , which controls the gain applied by the inverse loss function G(f). The core output signal  212  is then coupled to the controllable-swing slicer  204 , which converts the core output signal  212  into a digital output signal  214  having a swing that is controlled by the amplitude control loop (A-Loop)  208 . The A-Loop  208  isolates the low frequency portions of the core output signal  212  and the digital output signal  214 , and compares the signal energies of the two low band-limited signals to set the controllable-swing slicer  204  to approximate the swing of the equalizer input  210  prior to transmission. Similarly, the high frequency portions of the core output signal  212  and the digital output signal  214  are isolated by the gain control loop (G-Loop)  206 , which compares the signal energies of the high band-limited signals to generate the gain control signal (Ki)  226 . Preferably, the low band-limited signals are centered towards the low end of the transmit spectrum at which the transmission losses are minimal, and the high band-limited signals are centered towards the higher end of the transmit spectrum at which the transmission losses are more significant. However, the low and high band-limited signals may be centered at alternative points within the transmit spectrum as long as the A-Loop  208  isolates a lower spectral range than the G-Loop  206 . 
     Within the A-Loop  208 , the low frequency portions of the core output signal  212  and the digital output signal  214  are isolated by the low band-pass filters  240  and  242 . The low band-limited signals are each coupled to one of the envelope detectors  244  and  246 , which detect the signal energies. The envelope detectors  244  and  246  may preferably be comprised of rectifiers, but could alternatively be any device or combination of devices capable of generating an output signal proportional to the signal energy of its input. The difference between the energy-level outputs from the envelope detectors  244  and  246  is then determined by the adder  248 . Preferably, the energy difference is calculated by coupling the energy-level output from one envelope detector  244  as a positive input to the adder  248 , and the energy-level output from the other envelope detector  246  as a negative input to the adder  248 . In this manner, the adder  248  generates a swing control signal  249  that is proportional to the energy difference between the low band-limited signals. The swing control signal  249  is coupled to the controllable-swing slicer  204  to control the energy level of the digital output signal  214 . Operationally, the A-Loop swing control signal  250  forces the swing of the digital output  214  to match the swing of the low band-limited core output. Because the low band-limited core output is typically not significantly attenuated by the transmission medium, the swing of the digital output  214  is thus made to approximate the swing of the equalizer input  210  prior to transmission. 
     Within the G-Loop  206 , the higher frequency portions of the core output signal  212  and the digital output signal  214  are isolated by the high band-pass filters  220  and  222 . The bandwidths of the high band-pass filters  220  and  222  are preferably set to isolate the frequency band in which the equalizer input signal  210  is most significantly effected by transmission losses. Once the core and digital outputs  212  and  214  have been band-limited by the high band-pass filters  220  and  222 , the signals are respectively coupled to the input of the envelope detectors  224  and  226 , each of which generates an energy-level output proportional to the signal energy of its input signal. The difference between the energy-level outputs of the envelope detectors  224  and  226  is preferably determined by coupling one energy-level output as a negative input to the adder  228  and coupling the other energy-level output as a positive input to the adder  228 . The adder  216  then generates a single-stage gain control signal (K)  232  that is proportional to the energy difference between the band-limited core and digital output signals. Because this energy difference approximates the energy lost during transmission over the transmission medium, the single-stage gain control signal (K)  232  settles to a value proportional to the transmission loss which is a function of the length of the transmission medium. The single-stage gain control signal (K)  232  is coupled to the sequencer  230 , which generates the gain control signal Ki  234  that is fed back to control the gain of the equalizer core  202  as described above with reference to  FIGS. 7 and 8 . Alternatively, if the equalizer core  202  is a single-stage equalizer core as described above with reference to  FIG. 4  or  FIG. 5 , then the single-stage gain control signal (K)  232  may be directly fed back to the equalizer core  202 . 
     It should be understood that many types of controllers could be used to implement the A-Loop  208  and the G-Loop  206  shown in FIG.  10 . For instance, the A-Loop  208  and the G-Loop  206  may be implemented as either a proportional type controller (P-Type), an integral type controller (I-Type) or a combination PI-type controller. To implement the G-Loop  206  as an I-Type controller, for example, an integrator (K I /s) could be coupled between the adder  228  and the sequencer  230 . In addition, to maintain stability in the system, the A-Loop  208  and the G-Loop  206  are preferably implemented as different controller types such that one control loop  206  or  208  has a dominant time constant/pole. For example, the G-Loop  206  may preferably be implemented as a slower I-type loop, while the A-Loop  208  is implemented as a faster P-Type loop. 
       FIG. 11  is a block diagram of an exemplary transmit amplitude independent adaptive equalizer  300  having a fixed-swing slicer  304 . In this exemplary embodiment, the amplitude independent adaptive equalizer  300  includes an equalizer core  302 , a slicer  304 , and a control loop  305 . The control loop includes a core-side low band-pass filter  306 , a core-side high band-pass filter  308 , a slicer-side low band-pass filter  310 , a slicer-side high band-pass filter  312 , four envelope detectors  314 ,  316 ,  318  and  320 , a core-side multiplier  340 , a slicer-side multiplier  342 , an adder  322 , and a sequencer  324 . Operationally, the equalizer  300  utilizes a fixed-swing slicer  304 , which is independent from the amplitude of the equalizer&#39;s input signal  332 . To accomplish amplitude independence and maintain optimal equalization, the equalizer  300  calculates the necessary gain by first normalizing the energy level at the outputs of the equalizer core  302  and the fixed-swing slicer  304  with respect to their low-frequency amplitudes, and then comparing the normalized signals. 
     The equalizer core  302  receives an input signal  332  from a transmission medium and a gain control signal (Ki)  303  from the control loop  305 . As described above, the equalizer core  302  applies a frequency dependant gain to the equalizer input signal  332  as a function of the gain control signal (Ki), and generates a core output signal  326 . The core output signal  326  is then coupled to the fixed-swing slicer  304 , which converts the core output signal  326  into a digital output signal  328  having a fixed swing. Preferably, the swing (A) of the digital output signal is fixed at a nominal value at which data is most often transmitted to the equalizer  300 . Because the equalizer  300  is independent of the amplitude of the input signal  332 , however, the swing (A) of the digital slicer output  328  may not approximate the swing of the equalizer input signal  332  prior to transmission. Therefore, to accurately determine the losses incurred in the transmission medium and achieve optimal equalization, the control loop  305  sets the gain control signal (Ki) by normalizing the energy level of the core and digital output signals  328  and  326  with respect to their respective low-frequency amplitudes before comparing the signals to determine the necessary gain to be implemented by the core  302 . The control loop  305  may be implemented, for example, as either a proportional type controller (P-Type), an integral type controller (I-Type) or a combination PI-type controller. 
     Within the control loop  305 , the low frequency portions of the core output signal  326  and the digital output signal  328  are isolated by the core-side low band-pass filter  306  and the slicer-side low band-pass filter  310 , respectively. The center frequency of the low band-pass filters  306  and  310  is preferably chosen to match the frequency band at which the equalizer input signal  332  exhibits minimal attenuation. Similarly, the higher frequency portions of the core output signal  326  and the digital output signal  328  are isolated by the core-side high band-pass filter  308  and the slicer-side high band-pass filter  312 , respectively. The bandwidths of the high band-pass filters  308  and  312  are preferably chosen to isolate the frequency band at which the equalizer input signal  332  is attenuated and distorted by the transmission medium. The energy level of the output of each band-pass filter  306 ,  308 ,  310  and  312  is then determined by one of the envelope detectors  314 ,  316 ,  320  and  318 , each of which generates an energy-level signal (a, x, b, and y) proportional to the energy of its input. 
     In amplitude dependant equalizer systems, such as described above with reference to  FIG. 9 , the energy level signals (x and y) for the high frequency portion of the core and slicer outputs  326  and  328  are compared to force the energy level of the core output equal to the known energy level of the slicer output (x=y). This relationship between the energy-level signals (x and y) loses its significance, however, when the swing (A) generated by the slicer  304  is not substantially equal to the swing of the data prior to transmission. Thus, to compensate for an unknown input signal amplitude, the energy-level signal (y) of the high frequency portion of the digital output signal  328  is weighted with the energy-level signal (a) of the low frequency portion of the core output signal  326 . Similarly, the energy-level signal (x) of the high frequency portion of the core output signal  326  is weighted with the energy-level signal (b) of the low frequency portion of the digital output signal  328 . With reference to  FIG. 11 , the core-side multiplier  340  multiplies the signals a and y to generate the output a*y, and the slicer-side multiplier  342  multiplies the signals b and x to generate the output b*x. This multiplication function is equivalent to normalizing the energy levels of the core and slicer outputs  326  and  328  with respect to their amplitudes (x/a is the normalized core signal and y/b is the normalized slicer signal). 
     To determine the single-stage gain control signal (K)  330 , the energy-level difference between the weighted signals (a*y and b*x) is measured by the adder  322 . If the equalizer core  302  is multiple-stage, then the single-stage gain control signal (K)  330  is coupled to the sequencer  324 , which generates the gain control signal (Ki)  303  that is fed back to the equalizer core  302  as described above. Alternatively, if the equalizer core  302  is single-stage, then the single-stage gain control signal is preferably fed back directly to the equalizer core  302 . In either case, because the energy-level signals (x and y) corresponding to the lossy portions of the core and digital output signals  326  and  328  are normalized with respect to their low-frequency amplitudes (a and b), the values of K  330  and Ki  303  are made proportional to the transmission losses even though the amplitude of the equalizer input signal  332  is an unknown. In this manner, the gain control signal (Ki)  303  (or K  330 ) forces the normalized energy level of the core output (x/a) to approximate the normalized energy level of the slicer output (y/b), and the input signal  332  is correctly equalized. 
       FIG. 12  is a block diagram of a transmit amplitude leveling circuit  400  for an equalizer. This circuit  400  includes a variable gain amplifier  402 , an adder  404 , a received data swing detection circuit  406 , and a core input swing detection circuit  408 . The variable gain amplifier  402  receives an input signal  410  from a transmission medium with an unknown transmit amplitude (B), and generates an equalizer core input signal  412  that settles to a fixed amplitude (A). The variable gain amplifier  402  also receives a variable gain control signal  414  that sets the gain of the amplifier  402  to either amplify or attenuate the input signal  410  to the fixed swing (A) expected by the equalizer core. The variable gain control signal  414  is preferably generated by the adder  404 , which compares the transmit swing (B) of the input signal  410  from the transmission medium with the swing (A) of the equalizer core input signal  412 . The transmit swing (B) of the input signal  410  is calculated with the received data swing detect circuit  406 , which generates an energy-level output that is preferably coupled as a negative input to the adder  404 . The swing (A) of the equalizer core input signal  412  is calculated with the core input swing detect circuit  408 , which generates an energy-level output that is preferably coupled as a positive input to the adder  404 . It should be understood, however, that many types of control circuits could be used to generate the variable gain control signal  414 , such as a proportional type controller (P-Type), an integral type controller (I-Type) or a combination PI-type controller. 
     The transmit amplitude leveling circuit  400  may be implemented, for example, in the equalizer system  100  described above with reference to  FIG. 9  by coupling the output of the variable gain amplifier  412  as the input to the equalizer core  102 . For example, if the equalizer system  100  is configured to equalize data transmitted at 800 mV and the data is instead transmitted at 1200 mV, then the variable gain control signal  414  would preferably adjust the gain of the variable gain amplifier  402  to 0.666 in order to reduce the signal swing of the received data signal. If, however, the data is transmitted at 800 mV as expected in the equalizer core, then the gain of the variable gain control signal  414  would preferably be set to unity (1) by the variable gain control signal  414 , and thus the amplifier  402  would be operating as a buffer. 
       FIG. 13  is a block diagram illustrating an exemplary implementation of the received data swing detection circuit  406  and the core input swing detection circuit  408  shown in FIG.  12 . The exemplary received data swing detection circuit  406  includes a low band-pass filter  502  and an envelope detector  504 . The low band-pass filter  502  preferably filters the input signal  410  from the transmission medium to a frequency range centered towards the low end of the transmit spectrum at which the transmission losses are minimal. In this manner, the output from the low band-pass filter  502  approximates the transmit swing (B) of the input signal  410 . The output from the low band-pass filter  502  is then coupled to the envelope detector  504 , which detects the energy level of the signal and generates the output of the received data swing detection circuit. 
     The exemplary core input swing detection circuit  408  includes a fixed-swing slicer  506 , a low band-pass filter  508 , and an envelope detector  510 . The fixed-swing slicer  506  is coupled to the equalizer core input signal  412 , which is converted by the slicer  506  into a digital output signal having the swing (A) expected in the equalizer core. This digital signal is then filtered by the low band-pass filter  508 , which preferably has a bandwidth substantially the same as that of the low band-pass filter  502  in the received data swing detection circuit  406 . The filtered output from the low band-pass filter  508  is coupled to the envelope detector  510 , which detects the energy level of the signal and generates the output of the swing detection circuit  408 . The output from the core input swing detection circuit  408  is preferably coupled as the positive input to the adder  404 , and the output from the received data swing detection circuit  406  is preferably coupled as the negative input to the adder  404 . The variable control signal  414  generated by the adder  404  is thus proportional to the difference between the transmit swing (B) of the input signal  410  and the swing (A) expected in the equalizer core. 
       FIG. 14  is a block diagram of an exemplary transmit amplitude independent adaptive equalizer  600  utilizing a variable gain amplifier  402 . The equalizer  600  includes a variable gain amplifier  402 , an equalizer core  202 , a fixed-swing slicer  602 , a gain control loop  206 , and a variable gain amplifier control loop (VG-Loop)  604 . The VG-Loop  604  includes two low band-pass filters  606  and  608 , two envelope detectors  610  and  612 , and an adder  614 . 
     The variable gain amplifier  402  preferably receives an input signal  410  that has been attenuated from its transmit swing (B) as a result of losses incurred in a transmission medium. The variable gain amplifier  402  also receives a variable gain control signal  616  from the VG-Loop  604  that controls the amount of gain applied by the amplifier  402  in order to generate an equalizer core input  412  that settles to a fixed swing (A). The equalizer core  202  operates as described above to compensate for transmission losses incurred in the transmission medium, and generates a core output signal  618 . The core output signal is then coupled to the fixed-swing slicer  602 , which converts the core output signal  618  into a digital output signal  620  having a fixed swing (A). The gain control loop  206  operates as described above with reference to  FIG. 10  to control the gain applied by the equalizer core  202  in order to compensate for frequency dependent losses incurred in the transmission medium. The VG-Loop  604  preferably isolates the low frequency portions of the core output signal  618  and the digital output signal  620 , and compares the signal energies of the two low band-limited singles to generate the variable gain control signal  616 . Preferably, the low band-limited signals are centered towards the low end of the transmit spectrum at which the losses from the transmission medium are minimal. However, the low band-limited signals may be centered at alternative points within the transmit spectrum as long as the VG-Loop  604  isolates a lower spectral range than the gain control loop  206 . 
     Within the VG-Loop  604 , the low frequency portions of the core output signal  618  and the digital output signal  620  are isolated by the low band-pass filters  606  and  608 . The low band-limited signals are each coupled to one of the envelope detectors  610  and  612 , which generate energy-level outputs that are proportional to the signal energies. The energy-level output from one envelop detector  612  is preferably coupled as a positive input to the adder  614 , and the energy-level output from the other envelop detector  610  is preferably coupled as a negative input to the adder  614 . The adder  614  generates the variable gain control signal  616 , which is proportional to the difference between the energy levels of the low band-limited signals. In this manner, the variable gain control signal  616  forces the output of the variable gain amplifier to settle at a swing level (A) substantially equal to the fixed swing (A) of the slicer. 
     It should be understood that many types of controllers could be used to implement the VG-Loop  604  and the gain control loop  206  shown in FIG.  14 . For instance, the VG-Loop  604  or the gain control loop  206  may be implemented as a proportional type controller (P-Type), an integral type controller (I-Type) or a combination proportional-integral type controller (PI-Type). In addition, the VG-Loop  604  and the gain control loop  206  are preferably implemented as different controller types in order to maintain stability in the system. 
     The embodiments described herein are examples of structures, systems or methods having elements corresponding to the elements of the invention recited in the claims. This written description may enable those skilled in the art to make and use embodiments having alternative elements that likewise correspond to the elements of the invention recited in the claims. The intended scope of the invention thus includes other structures, systems or methods that do not differ from the literal language of the claims, and further includes other structures, systems or methods with insubstantial differences from the literal language of the claims.