Abstract:
An apparatus of compensating for offset in a received signal generated from a forward path stage, includes a first peak detector for receiving a first signal from the forward path stage and capable of detecting a peak of the first signal; a second peak detector for receiving a second signal from the forward path stage and capable of detecting a peak of the second signal; a differential amplifier coupled to the first peak detector and the second peak detector and capable of generating an offset control signal; and a compensation stage coupled to the differential amplifier and capable of compensating for offset in the received signal in response to the offset control signal. A method of compensating for offset in a received signal includes detecting a level of a first signal from a forward path stage; detecting a level of a second signal from the forward path stage; and generating an offset control signal to reduce an offset in the received signal in response to the level difference of the first signal and the second signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The subject matter of this application is related to the subject matter of the following co-pending U.S. Applications: (1) U.S. application Ser. No. 09/322,668, filed May 28, 1999 by Jung-Chen Lin, entitled “A DELAY LOCKED LOOP FOR SUB-MICRON SINGLE-POLY DIGITAL CMOS PROCESSES” which is fully incorporated herein by reference; (2) U.S. application Ser. No. 09/321,983, filed May 28, 1999 by Menping Chang and Vuong Kim Le, entitled “UNIVERSAL OUTPUT DRIVER AND FILTER” which is fully incorporated herein by reference; (3) U.S. application Ser. No. 09/321,403, filed May 28, 1999 by Menping Chang and Hai T. Nguyen, entitled “ADAPTIVE EQUALIZER AND METHOD” which is fully incorporated herein by reference; (4) U.S. application Ser. No. 09/321,935, filed May 28, 1999 by Menping Chang and Hai T. Nguyen, entitled “SELECTIVE SAMPLED PEAK DETECTOR AND METHOD” which is fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of data communication and more particularly to a circuit and method for compensating for baseline wander occurrences in data signals. 
     BACKGROUND OF THE INVENTION 
     In 100 Base TX applications of a Copper Distributed Data Interface (CDDI), a data signal is transmitted across a twisted-pair copper wire that is commonly known as CAT-5 cable. The data signal is then AC coupled through a primary side of a transformer and received by a receiver connected to the secondary side of the transformer. This method of AC coupling the data signals through the transformer may cause the average DC value of the signal to drift (i.e., wander) significantly over time, as shown in FIG. 1 a . The data signal  100  is a waveform of an ideal MLT-3 signal, while the data signal  105  is a waveform of an MLT-3 signal that has been affected by Baseline Wander. 
     Baseline Wander occurs due to the high pass characteristics of the transformer. The transformer will suppress the DC level if data signal remains in the same level for a long time. This can cause the data signals to “droop” over time, as illustrated in FIG. 1 a . This drooping behavior may accumulate over time and may cause a maximum offset of 750 mV, differentially. 
     In the process of receiving and recovering data, uncompensated Baseline Wander offset can cause serious signal distortions due to the limited linear range of the amplifiers along the data path and can cause high jitter. This makes data slicing difficult and results in received errors. Therefore, Baseline Wander correction is important in the recovery of transmitted data signals. 
     An overview of a typical offset cancellation circuit is now discussed to provide better understanding of conventional approaches. FIG. 1 b  is a schematic block diagram of an offset cancellation circuit  120 , which includes a forward path block  121  with a gain of A 1 , feedback path block  122  with a feedback gain of A 2 , and a summing point  125 . Equation (1) expresses the differential output voltage Vo of circuit  120  and explains the offset cancellation function. 
     
       
         [(V IN +Δv)−(A 2 )(Vo)]A 1 =Vo  (1) 
       
     
     The term V IN  is the input signal in circuit  120  while Δv is the offset voltage added to the input signal V IN . The equations (2) and (3) can then be subsequently derived from equation (1). 
     
       
         A 1 (V IN +Δv)=[1+(A 1 )(A 2 )]Vo  (2) 
       
     
     
       
         Vo=(V IN +Δv)A 1 /[1+(A 1 )(A 2 )]  (3) 
       
     
     For stability of the feedback system in FIG. 1 b , the feedback gain A 2  has a low pass characteristic such that it has a high gain (i.e., A 2 &gt;&gt;1) at low frequency operation and has a very small gain (i.e., A 2 &lt;&lt;1) at high frequency operation. For a low frequency gain, A 2 &gt;&gt;1, the term [1+(A 1 ×A 2 )] represents the denominator in the equation (3) and, therefore, the denominator also has a large value. As a result, the effect of offset is minimized. However at high frequency, the denominator of equation (3) approaches a unity value and equation (3) can be approximated as equation (4). 
     
       
         Vo=(V IN +Δv)×A 1   (4) 
       
     
     It is noted that in the case where equation (4) is satisfied, the offset voltage Δv can not be canceled. Since the baseline wander effect is not a constant DC effect but an AC phenomenon, it is not cancelable by use of the offset cancellation circuit  120 . 
     A conventional Baseline Wander compensation version of an offset cancellation circuit  120  is shown in the circuit  150  of FIG. 1 c . The circuit  150  includes a feed forward path block  121  with a gain of A 1 , the feedback path block  155  with a gain of A 2  and a summing point  125 . The feedback path block  155  compares the output signal Vo with a replicated ideal signal  130 , which has the same DC bias, DC gain and AC gain as an ideal output signal Vo (ideal)  with no Baseline Wander offset. As the Baseline Wander event occurs at the input signal V IN  and consequentially at the output signal Vo, the feedback stage  155  detects this offset by comparing the output voltage Vo with the replica  130  and outputs a feedback signal  140  to compensate for Baseline Wander. As a result of this compensation, the offset or baseline wander is minimized at the input of the feed forward path block  121 . 
     However in practice, there is a phase difference between the replica signal  130  and the output signal Vo, as shown in FIG. 1 d . Although the effects of a phase difference can be “smoothed” out by using the low pass filter in the feedback path  155 , the phase difference still causes some unwanted ripple in the output signal  140 . As a result, the data output Vo has excessive jitter (noise) due to the offset ripple into the summing point  125  and then amplified by  121 . 
     Another problem, which is more severe, is the variations and mismatches in either DC gain and AC gain of the output signal Vo and its ideal replica  130  as shown in FIG. 1 e . Even though there is no phase shift in this case, the gain mismatch causes a residue signal that translates into offset ripple (Vo-Replica signal  130 ). Notice that Vo is the output of an analog signal and replica signal  130  is synthesized from a digital data. The two waveforms cannot be match perfectly in terms of overshoot and rise/fall time and it is very difficult for the forward block  121  to have a fix gain over process, temperature and supply voltage. This results in unwanted ripples and potential errors. These ripples are the results of phase delay and gain mismatch and are always present regardless of the effect of baseline wander. Consequently, the performance of this replica approach is very limited. 
     Therefore, there is a need for a baseline wander compensation approach that overcomes the unwanted ripples and errors in conventional baseline wander compensation systems. 
     SUMMARY OF THE INVENTION 
     The apparatus and method of the present invention operates to track and dynamically compensate for the Baseline Wander event. The apparatus includes a first peak detector for receiving a first signal from a forward path stage and capable of detecting a peak of the first signal, and a second peak detector for receiving a second signal from the forward path stage and capable of detecting a peak of the second signal. The apparatus also includes a differential amplifier coupled to the first peak detector and the second peak detector and capable of generating an offset control signal, and a compensation stage coupled to the differential amplifier and capable of compensating for offset in the received signal in response to the offset control signal. 
     In another aspect of the present invention, the apparatus may also include a pair of selective sample peak detectors that are capable of sampling a portion of the input signal. The sampled portion of the input signal is defined by a “smart window” (timing window) which is received by a sample and block circuit of the peak detector. By making uses of these peak detectors and the smart or timing windows, the present invention is able to detect correctly the peak levels of the positive and negative signals independently and compares these levels for any potential offset caused by a Baseline Wander event. The present invention then outputs a Baseline Wander control signal to compensate for Baseline Wander. 
     The method of Baseline Wander compensation in accordance with the present invention is able to compensate the offset in data signals independently of its AC and DC gain. Furthermore, the present invention provides a Baseline Wander compensation approach that is fully differential, thereby permitting a circuit in accordance with the present invention to have improved immunity to noise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a waveform diagram comparing an ideal MLT-3 signal with an MLT-3 signal affected by baseline wander; 
     FIG. 1 b  is a schematic block diagram of a typical DC offset cancellation circuit; 
     FIG. 1 c  is a schematic block diagram of the conventional offset cancellation circuit by using a replicated signal; 
     FIGS. 1 d - 1   e  are waveform diagrams in the conventional circuit of FIG. 1 c ; they illustrate the phase and gain mismatches causing a ripple effect; 
     FIG. 2 a  is a schematic block diagram of an offset or baseline wander compensation circuit in accordance with an embodiment of the present invention; 
     FIG. 2 b  is a schematic block diagram of the offset or baseline wander compensation circuit of FIG. 2 a  shown in additional details; 
     FIG. 2 c  is a waveform diagram that illustrates the conversion of data signals from MLT-3 into binary as triggers for the window generator; 
     FIG. 2 d  is a schematic block diagram of one embodiment of a smart window generator; 
     FIG. 2 e  illustrates the waveforms of the window generator in FIG. 2 d;    
     FIG. 3 is a waveform diagram that shows the use of sampling windows to generate the average peak level of a signal; 
     FIG. 4 a  is a waveform diagram of data signals and their peaks before Baseline Wander compensation; 
     FIG. 4 b  is a waveform diagram of data signals and its peaks after Baseline Wander compensation; 
     FIG. 5 is flowchart illustrating the operation of a baseline wander compensation circuit in accordance with an embodiment of the present invention; and 
     FIG. 6 is an embodiment of the sample and block circuit coupled to the charge pump. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 a  is a schematic block diagram of a circuit  200  for performing baseline wander and offset compensation, in accordance with an embodiment of the present invention. The circuit  200  includes a forward path stage  202  for generating differential output signals P and M, peak detectors  204   a  and  204   b  for receiving the P signal and M signal, respectively, a differential amplifier  206  for comparing the outputs of peak detectors  204   a  and  204   b , and a summing stage  207  to subtract the differential amplifier outputs from input signals  209 . In one embodiment, the circuit  200  applies a sampled peak detection approach to extract the peak levels of the signals P and M, as described in additional details below and in the above-referenced U.S. Application, filed May 28, 1999 Menping Chang and Hai T. Nguyen, and entitled “SELECTIVE SAMPLED PEAK DETECTOR AND METHOD”. The peak detector  204   a  or  204   b  has a low-pass filter function, but with enough bandwidth to track the Baseline Wander event. By comparing the peak levels of signals P and M, the present invention is able to detect and compensate the Baseline Wander offset dynamically. Therefore, the present invention advantageously does not rely on fixed amplitude signals or replica signal to compensate for Baseline Wander offset as required by the conventional approach. 
     FIG. 2 b  is a schematic block diagram of the present invention shown in additional details that expand the peak detectors  204   a  and  204   b  into several components and introduces the additional circuit blocks, as described below, to correctly extract the peak information of the data signals P and M. 
     The comparators  215   a  and  215   b  compare the forward path  202  output signals P and M with an internal generated reference voltage Vref to slice out the output signals out_P and out_M, respectively. As shown in FIG. 2C, the MLT-3 data signal P is sliced against a voltage level Vref to generate output signal out_P, while the MLT-3 data signal M is sliced against a voltage level Vref to generate output signal out_M. 
     The circuit  200  also includes a pair of window generators  220   a  and  220   b  and a pair of peak detectors  204   a  and  204   b . Similar window generators and peak detectors are also shown and described in the above-referenced U.S. Patent Application entitled “Selective Sampled Peak Detector” by Hai T. Nguyen and Menping Chang. 
     The pair of window generators  220   a  and  220   b  receives the output signals out_P and out_M, respectively, as triggers to generate timing control signals,  230   a  and  230   b . An example of such window generator is shown in the schematic block diagram of FIG. 2 d  and the waveform for such a window generator is shown in FIG. 2 e . The operation of the window generator is described as follows. A signal out_P is delayed by a delay stage  260  by time ΔT and inverted by inverter  265  into an inverted/delayed signal out_P′. The input signal out_P and the inverted/delay signal out_P′ are ANDed by AND gate  270  to generate the control signal  230   a.    
     Similarly for input signal out_M, the window generator  220   b  outputs the control signal  230   b . As a result of using a window generator as shown in FIGS. 2 d - 2   e , the timing control signals  230   a  and  230   b  are simply pulses with time interval ΔT. 
     The timing control signals  230   a  and  230   b  are then used as a timing windows to selectively “sample” the top peaks of the forward path output signals P and M through the peak detectors  204   a  and  204   b , respectively. The peak detector  204   a  uses the timing control signal  230   a  to “sample” the top peak of forward path output signal P to output the average top peak voltage P_top, while the peak detector  204   b  uses the sample/block control signal  230   b  to “sample” the top peak of forward path output signal M to output the average top peak voltage M_top. 
     Reference is now made to FIGS. 2 b  and  3  to describe the operation of a peak detector in FIG. 2 b . In this example, only the operation of the peak detector  204   a  is described. The peak detector  204   a  includes a comparator  240  for comparing the positive signal P with the average peak detector output P_top that is generated at the output of peak detector  204   a . The comparator  240  generates a comparing value signal  242  based upon the comparison of the detected peak level of P signal (P_top) and the positive P signal. A sample and block circuit  245  receives pulses  335   a - 335   c  (FIG. 3) of time interval ΔT from the control signal  230   a , as sampling windows, to timely “ΔT” charge the charge pump  250  (FIG. 2 b ), which may include a pull-up current source  255 , a pull-down current source  260 , and a output capacitor  265  having a voltage value corresponding to P_top. The charge pump  250  either charges up or charges down the capacitor  265  during ΔT time interval depending on whether the comparing value  242  is high or low, respectively. The charge pump  250  is inactive outside the timing interval ΔT and the output voltage P_top remains the same during that time. As a result of selectively sampling the signal P by the peak detector, the output signal P_top averages out the top peaks  305   a - 305   c  of signal P within the pulses  335   a - 335   c.    
     A differential amplifier  206  (FIG. 2 b ) then compares the values of the top peaks P_top and M_top for any potential offset caused by a Baseline Wander event and generates the compensation control signal  208  to the baseline wander compensation block  207  to compensate for BLW. The top peaks P_top and M_top have the same voltage level if there is no offset or baseline wander event. 
     As baseline wander occurs, the offset starts to build up in the input signals  209 , in the forward path input signals  210  and consequently in the forward path output signals P and M. The peak detectors  204   a  and  204   b  then track the peaks of output signals P and M with this offset. The differential amplifier  206  sees this offset through the peak levels (P_top and M_top) and outputs the compensation control signals  208  to permit the baseline wander (BLW) compensation stage  207  to eliminate this offset by introducing a same amount of offset but in an opposite direction to the input signals  209 . As a result, both output signals  210  of the baseline wander compensation (summing) stage  207  are balanced. Consequentially, the output signals P and M and its top peaks P_top and M_top are also balanced. The waveforms in FIG. 4 a  illustrate the peak detector levels P_top and M_top of the MLT-3 signals P and M before Baseline Wander compensation is provided to the signals P and M. FIG. 4 b  illustrates the peak detector levels P_top and M_top of the MLT-3 signals P and M after Baseline Wander compensation is provided to the signals P and M. 
     By using the peak detector approach to detect and compensate for baseline wander offset, the present invention eliminates the needs for a replica, stringent fixed amplitude and gain requirement of the data signals for the conventional approaches. Furthermore, the present invention can properly perform offset and baseline wander cancellation, even if the received signals are distorted due to overshoot and undershoot distortion. These distortions are either filtered out by the sampling window or averaged out by the peak detectors described above in present invention. 
     Another advantage achieved by the present invention is that it substantially reduces the circuit elements and complex algorithm implemented by generating the replica in the conventional approach, thereby reducing the operating current and die size. The peak detectors and window generators discussed above are very small and simple building block. In fact, the comparators, window generators and peak detectors in the present invention are existing circuit elements in the receiver block, which performs the equalized function and is described in the above-referenced U.S. Patent Application entitled “Adaptive Equalizer and Method” by Hai T. Nguyen and Menping Chang. 
     Additionally, the present invention provides a baseline wander compensation approach that is fully differential, thereby permitting a circuit in accordance with the present invention to have improved immunity to noise. 
     The present invention also does not have the problem of the conventional circuit  160  of FIG. 1 d  wherein a phase difference created between the data signal Vo and the replica signal  130 . Thus, the present invention avoids the phase difference problem that can lead to improper baseline wander compensation and data recovery errors. 
     It is noted that other embodiments and configurations may be used to implement the circuits  220   a  and  220   b  for generating and controlling a smart window (FIG. 2 b ), depending on the application of the invention. The implementation of circuits  220   a  and  220   b  may be varied, for example, to select a specific portion and/or pattern of pulses that is more meaningful for the applications. 
     Although the present invention has been described in considerable detail with the comparison of the top peaks of the data signals, other versions are possible. For example, the present invention may be set to track either the bottom peaks of the data signals or any voltage level extracted from the signals using the sampling window. 
     FIG. 5 is a flowchart  500  describing an operation of a baseline wander compensation circuit in accordance with the present invention. A data signal is first received  505  by the baseline wander compensation circuit. The pulse peaks of a positive signal P of the data signal and the pulse peaks of a negative signal M are then tracked  510 . A comparison  515  is then made for the pulse peak values of the positive signal P and the negative signal M. Based on the comparison  515 , a proper amount of compensation can be determined  520  so that baseline wander or offset occurrence are eliminated from the data signal. 
     FIG. 6 is a schematic block diagram of an embodiment of a sample and block circuit  245  that is integrated with charge pump  250 . It is noted that other embodiments and configurations of the sample and block circuit  245  may be implemented depending on the application of the invention. In the embodiment shown in FIG. 6, the smart windows  335   a - 335   c  (FIG. 3) of signal  230   a  control a pair of transmission gates  605  and  610 . The gate  605  includes n-channel transistor  615  and p-channel transistor  620 , while the gate  610  includes n-channel transistor  625  and p-channel transistor  630 . When the control signal  230   a  is high (i.e., a smart window is high or asserted), the comparing value signal  242  (from comparator  240  in FIG. 2 b ) is passed to pump control transistors  635  and  640 . Thus, if the comparing value signal  242  is high, then the pump control transistor  635  is on and the pump control transistor  640  is off, thereby permitting the charge pump  250  to pump up. If the comparing value signal  242  is low, then the pump control transistor  635  is off and the pump control transistor  640  is on, thereby permitting the charge pump  250  to pump down. The pump-up current value I 1  is set by the fixed current source  255  including a current mirror formed by transistors  645  and  650 . The pump-down current value I 2  is set by the fixed current source  260  including a current mirror formed by transistors  655  and  660 . 
     When the control signal  230   a  is low (i.e., a smart window is not present), both transmission gates  605  and  610  are off. The p-channel transistor  665  is on and will pull the gate of pump control transistor  635  high, thereby turning off transistor  635 . The n-channel transistor  670  is on and pulls the gate of pump control transistor  640  low, thereby turning off the transistor  640 . Since the pump control transistors  635  and  640  are off, the value of I out  is zero and the value of P_top remains the same.