Abstract:
The invention is related to analog to digital conversion of a multi-level analog signal at a very low sampling rate. The analog signal is sampled by a recovered clock to produce a succession of samples of the analog signal. The low sampling rate may be within an order of magnitude of the symbol rate of the analog signal. Each sample is converted to a digital word. A phase detector reference circuit determines from peak values of the analog signal at least two allowable levels of the analog signal including a reference-crossing level. The phase detector defines a zero band of amplitude ranges of the analog signal including the reference-crossing level. It further defines an error band of amplitude ranges of the analog signal extending from said zero band to a fraction of the amplitude of the next allowable level. The phase detector then infers either a positive or negative phase error for each pair of successive samples of the analog signal. If the current and prior samples fall within the error and zero bands respectively, a negative phase error is inferred. If the current and prior samples fall within the zero and error bands respectively, a positive phase error is inferred. The phase error is used in a control feedback loop to adjust the recovered clock.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention is related to signal processing of a received multi-state signal such as a pulse amplitude modulated signal, and in particular to phase detection as part of the clock recovery of such a signal using a very low sampling rate such as the baud rate of the signal. 
     2. Background Art 
     In a serial data communication channel, the clock is embedded in the data or information (rather than being transmitted as a separate signal in parallel with the data). Therefore, a receiver must recover the transmitter&#39;s clock from the data-containing signal in order to process the data. Typically, this means that the receiver must be able to discern each edge in a pulse amplitude modulated signal (for example), compare it to the edge of a locally generated clock, and use this comparison, or “phase detection”, to adjust the local clock, until the local clock is frequency locked to the transmitter&#39;s clock and the phase of the local clock is optimally positioned so as to minimize the errors in recovering the data. When a DSP receiver is used, the most straight-forward way to ensure that this can happen is for the receiver to sample the received signal at a sampling rate much higher than the frequency of the transmitter&#39;s clock. In this way, the receiver can closely observe the zero-crossing time of each edge in the received signal, accurately perform phase detection with that zero-crossing and the phase of the local clock using conventional techniques, adjust the local clock, and thereby recover the exact frequency and phase of the transmitter&#39;s clock. Of course, such a high sampling rate (e.g., 5 or 10 times the baud rate of the received signal) is a luxury that a competitive system cannot easily afford. This is because, at least in part, the power consumption of the receiver increases with the sampling rate. Therefore, it would be desirable to reduce the sampling rate, but such a reduction would seem to entail a loss of accuracy in the determination of the zero-crossing of each edge. Such a loss of accuracy leads to clock recovery failure and therefore general failure-of the receiver. 
     Assuming such difficulties can be overcome, it would be desirable to sample a pulse amplitude modulated signal at a rate the same as or not significantly exceeding the symbol (baud) rate of the received signal. This may be thought of as sampling once per edge. It would be further desirable to be able to do this with a multi-state signal (e.g., a pulse amplitude modulated signal) which is allowed to change from any one of its allowed levels to any other allowed level within one clock period. 
     SUMMARY OF THE INVENTION 
     The invention is related to analog to digital conversion of a multi-level analog signal at a very low sampling rate. The analog signal is sampled by a local clock (the “recovered” clock) to produce a succession of samples of the analog signal. These samples themselves are used to adjust the phase and frequency of the local clock in a feedback loop known as “clock recovery.” Part of any clock recovery feedback loop is a phase detector that detects the difference in phase between the local clock and the clock that transmitted the signal that is being sampled. The low sampling rate may be within an order of magnitude of the symbol rate of the analog signal. Each sample is converted to a digital word. A phase detector reference circuit determines from peak values of the analog signal at least two allowable levels of the analog signal including a reference-crossing level. The phase detector defines a zero band of amplitude ranges of the analog signal including the reference-crossing level. It further defines an error band of amplitude ranges of the analog signal extending from said zero band to a fraction of the amplitude of the next allowable level. The phase detector then infers either a positive or negative phase error for each pair of successive samples of the analog signal. If the current and prior samples fall within the error and zero bands respectively, a negative phase error is inferred. If the current and prior samples fall within the zero and error bands respectively, a positive phase error is inferred. For any other values of the current and prior samples, the phase error is unknown. 
     A negative or positive phase error causes a phase correction feedback control loop to adjust the recovered clock. If the recovered clock is being generated by means of selecting a phase of a local clock, known to be very close in frequency to the far end transmitter clock, then the phase correction feedback control loop is used to increment or decrement the phase of the local clock. If the recovered clock is being generated by means of adjusting a local VCO, then the phase correction feedback control loop is used to adjust the voltage on the VCO. 
     Typically, there are three allowable levels determined by the determining step symmetrically disposed about zero amplitude. In-one embodiment, the phase detector takes the absolute value of each of the samples, whereby the pair of successive samples are absolute values of the digitized analog signal. The reference crossing level is zero amplitude and is a zero-crossing level. 
     The invention relies upon the randomness of the received signal in that no determination or correction of phase error is made if neither or both of the pair of samples are in the error band, since this condition will obtain sporadically and not continuously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a state diagram of a multi-level signal of the prior art having rules restricting transitions. 
     FIG. 2 is a time domain graph illustrating a method of the prior art for detecting phase error using baud rate sampling of the signal of FIG.  1 . 
     FIG. 3 is a time domain graph illustrating a multi-level signal without any rules restricting transitions. 
     FIG. 4 is a time domain graph illustrating how phase error is detected with baud rate sampling of the signal of FIG.  3 . 
     FIG. 5 is a diagram illustrating of the operation of the phase detector embodying the invention. 
     FIG. 6 is a block diagram illustrating a receiver system embodying the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Prior Phase Detection Schemes 
     As employed herein, the term baud rate sampling refers to analog-to-digital conversion carried out at a sampling rate equal to the symbol or “baud” rate of the signal. Essentially, this usually entails sampling at most once for every possible edge in the signal. For certain types of multi-state signals, phase detection using baud rate sampling of the received signal has been demonstrated. Specifically, it has been demonstrated for three-level pulse amplitude modulated signals in which only one-way transitions between the two extreme levels are permitted, such as the MLT 3  signal in Fast Ethernet. 
     A state diagram of such a signal is illustrated in FIG.  1 . In FIG. 1, there are three valid levels (also referred to herein as “states” or “outputs”) of the pulse amplitude modulated signal, namely +1, 0 and −1. In FIG. 1, the +1 level may be repeated (branch  110 ) or may transition to the 0 level (branch  120 ). From there, the signal may either repeat the 0 level (branch  130 ) or may transition to the −1 level (branch  140 ). The −1 level may be repeated (branch  150 ) or the signal may transition to the 0 level (branch  160 ). From there, the signal may repeat the 0 level (branch  170 ) or may transition to the +1 level (branch  180 ). 
     FIG. 2 illustrates a time domain waveform of the signal of FIG.  1 . The analog waveform of FIG. 2 includes about 3 bauds or symbols, namely a +1 centered around time  365 , a 0 centered around time  400  and a −1 centered around time  430 . The time axis of FIG. 2 is arbitrarily divided into 32 units per baud. Since the goal here is to employ baud rate sampling, there is only one sample of the signal taken for the 0 centered around time  400 . The phase error is deduced from this single sample as follows. Initially, the vertical axis is divided into three regions, depending upon the three valid signal levels. If for example the valid levels are +1, 0 and −1 amplitudes, then a zero phase error region is established in the neighborhood of 0 amplitude. This corresponds to a sample taken at the zero crossing of the signal, which would be a recovered clock with zero error. For example, the zero phase error region  210  may be defined between +0.125 and −0.125 amplitudes. A positive phase error region  220  is defined from −0.125 to −0.5 amplitude and a negative phase error region  230  is defined from +0.125 to +0.5 amplitude. If the sample falls within the positive phase error region, then a positive phase error is inferred and the phase of the “recovered” clock is decremented or decreased. If the sample falls within the negative phase error region, then a negative phase error is inferred and the phase of the “recovered” clock is incremented or increased. If the sample falls within the zero phase error region, no error is inferred and no correction is made to the recovered clock phase. 
     The difficulty arises when baud rate sampling is attempted with a multi-state or multi-level signal that is not subject to transition rules or restrictions like those of FIG.  1 . For example, a more general multi-level signal may be permitted to make transitions between any two levels during any clock period without regard to prior transitions. In such a case, it is impossible to apply the scheme of FIG. 2 without encountering anomalies in interpreting the phase error. Such an attempt is illustrated in FIG.  3 . Since the signal may have positive-going or negative-going edges after any zero-crossing, an attempt to simplify the situation is made by taking the absolute value of the received signal in FIG.  3 . The problem is that because of the unpredictability of the signal&#39;s transition sequence, it is impossible to assign a particular amplitude range (such as the ranges  310 ,  320  and  330  corresponding to the regions  210 ,  220 ,  230  of FIG. 2) to a particular polarity of phase error (positive, negative or zero) as could be done in the case of FIG.  2 . 
     The problem is overcome in the invention by looking not at the mere state amplitude of the current sample but instead by looking at the transition history of the two most recent samples. Specifically, as illustrated in FIG. 4, the signal amplitude range for the absolute value of the signal is divided into a zero band  403  and an error band  405 . In the-present example in which there are three allowed levels (+1, −1 and 0), the zero band may be defined as being between about 0 and +0.1, while the error band may be defined as being between about +0.1 and +0.5. Two transitions of the pair of most recent samples are recognized. One recognized transition is one in which the current sample lies in the error band and the previous sample lies in the zero band. For this transition, a positive phase error is inferred. The other recognized transition is one in which the current sample lies in the zero band and the previous sample lies in the error band. For this latter transition, a negative phase error is inferred. Of course, if a negative phase error is inferred the recovered clock phase is incremented while if a positive phase error is inferred the recovered clock phase is decremented. A significant feature of the invention is that not all transitions are recognized. If for example if both samples are in the zero band, no determination of phase error is attempted. Moreover, if neither sample is in the zero band, then no determination of phase error is attempted. The invention relies upon the randomness inherent in the data signal to render highly unlikely a long repeated continuous string of unrecognized transitions from preventing frequent correction of the recovered clock phase. The randomness of the data virtually guarantees the frequent occurrence of the two recognized transitions to ensure continuous correction and control of the recovered clock phase. 
     FIG. 4 illustrates a time domain waveform of the absolute value of a 3-level pulse amplitude modulated signal with no transition restrictions. In one case, the first sample may occur at about time  380  and the next sample at time about  410 , so that the two samples occur within the error band and within the zero band, respectively. In this case, a negative phase error is inferred. In a different case, the first sample may occur at about time  400  and the next sample at about time  430 , so that the two samples occur within the zero band and within the error band, respectively. In this case, a positive phase error is inferred. 
     FIG. 5 illustrates the phase detection process of the invention. First, the allowed signal levels are detected (block  510  of FIG.  5 ). Next, a zero error band is defined near the signal zero crossing amplitude (block  520 ). Then, an error band is defined above the zero band (block  530 ). The current and previous sample amplitudes are stored for the current clock period (block  540 ). A determination is then made whether the pair of samples occurred in the error and zero bands in that order (block  550 ). If so, a negative phase error is returned (block  560 ) and the recovered clock phase is incremented or increased(block  570 ). Otherwise, a determination is made whether the pair of samples occurred in the zero and error bands in that order (block  580 ) and if so a positive phase error is returned (block  590 ) and the recovered clock phase is decremented or decreased(block  595 ). Otherwise, no determination is made (block  597 ). 
     2. System Overview 
     FIG. 6 illustrates a receiver that includes a phase detector embodying the present invention. The receiver communicates in a pulse amplitude modulation gigabit-per-second computer network. In such a system, the same cable (the cable  400  of FIG. 6) carries the transmitted and received signals simultaneously. Therefore, in order to isolate the received signal, an analog subtractor  402  subtracts the analog transmitted signal (the input labeled “analog tx”) from the signal on the cable, producing the received signal (“rx”) at the output of the subtractor  402 . An analog-to-digital converter  404  samples the analog received signal rx in synchronism with a recovered clock signal produced by a clock recovery circuit  406 . The analog-to-digital converter  404  converts each analog sample to a digital word (e.g., an eight-bit digital word) in accordance with an analog reference level from a conventional reference generator  408 . The digital output of the analog-to-digital converter  404  is processed by a feed-forward equalizer  410  having a transfer function specified in accordance with industry standards to remove filtering imposed on the signal by the transmitter on the other side of the cable (the far end). 
     In order to compensate for distortions imposed on the received signal during its transit over the cable  400 , such as those attributable to filtering effects of the cable discussed above in this specification, a cable feedforward equalizer  412  imposes a selected transfer function on the signal output by the equalizer  410 . The equalizer  412  is of the conventional type whose transfer function may be represented in the complex plane with plural poles and zeroes corresponding to a desired frequency response. Preferably, the equalizer stores a number of such transfer functions, one of which may be selected at any one time. A cable feed forward equalizer adapter  414  chooses the best one of the transfer functions or settings of the cable feedforward equalizer  412 . 
     The equalized digital signal produced by the cable feedforward equalizer  412  is combined in an adder  416  with a crosstalk correction signal produced by a crosstalk correction circuit  418 . The crosstalk correction circuit  418  produces the crosstalk correction signal so as to compensate or cancel crosstalk from the transmitted signal when combined with the equalized digital signal in the adder  416 . The crosstalk correction circuit has two inputs, namely the corrected signal from the output of the adder  416  and the transmitted signal tx, as indicated in FIG.  6 . The crosstalk correction circuit- 418  consists of a near end crosstalk (“NEXT”)/echo canceller  420  and a NEXT/echo adapter  422  that controls the canceller  420 . The crosstalk correction circuit  418  including the canceller  420  and the adapter  422  are described in U.S. patent application Ser. No. 09/636,047 entitled “ADAPTER FOR NEAR-END CROSSTALK AND ECHO CANCELLER FOR BI-DIRECTIONAL DIGITAL COMMUNICATIONS” filed Aug. 10, 2000, by Duy PHAM et al and U.S. patent application Ser. No. 09/636,042 filed Aug. 10, 2000, by Duy PHAM et al, both applications being assigned to the assignee of the present application, the disclosures of which are incorporated herein by reference. 
     The output of the adder  416  is fed back as an input to the crosstalk correction circuit  418 , as referred to above, and to the cable feedforward adapter  414  at feedback input  414   a . The receipt of the succession of samples of step  310  refers to the successive digitized samples furnished to the input  414   a  of the adapter  414 . The adapter  414  chooses the best equalizer setting so as to maximize the number of digitized samples received at the input  414   a  falling within the three allowed levels discussed above. 
     The digital signal output by the adder  416  is also applied as a feedback signal to the clock recovery circuit  406 , and specifically to a phase detector  432  which is the subject of the present application. The output of the phase detector  432  is applied to the input of a conventional loop filter  434  whose output controls a voltage controlled oscillator  436 . The voltage controlled oscillator  436  generates the recovered clock signal applied to the analog-to-digital converter  404 . The phase of the voltage controlled oscillator  436  is incremented or decremented depending upon the polarity of the phase error detected by the phase detector  432  in the manner described above with respect to FIGS. 4 and 5. 
     A conventional slicer  450  makes a decision for each digital sample as to which one of the allowed levels the sample represents (i.e., is closest to). It does this in accordance with a conventional threshold generator  452 . It should be noted that during the training period of the equalizer adapter  414 , 3-level pulse amplitude modulation is employed, but the actual data may be transmitted using a different number of levels, such as 5-level pulse amplitude modulation. 
     A peak detector  454  determines the prevailing or current peak amplitude (positive and negative) of the digital samples output by the adder  416 . The positive and negative peak amplitudes define the upper and lower valid levels of the  3- level signal used during training of the adapter  414 . In the example described above, the positive peak was 100, the negative peak was −100, defining the upper and lower valid levels, while the middle level between them was 0. The adapter  414  deduces the three valid levels of the 3-level pulse amplitude modulation signal by assigning the positive peak value sensed by the peak detector  454  to the upper valid level, the negative peak value sensed by the peak detector  454  to the lower valid level and the amplitude midway between the two peaks as the middle valid level. Conventional circuitry is employed to carry out this task. 
     The output of the peak detector  454  is also utilized in conventional well-known fashion by the conventional analog-to-digital reference generator  408 . The reference generator  408  deduces from the peak magnitudes sensed by the detector  454  the current analog range of the incoming signal, and in conventional manner cause the maximum digital range of the analog-to-digital converter  404  to match the sensed analog range of the incoming signal. 
     The output of the peak detector  454  is also applied to a phase detector reference circuit  430  of the clock recovery circuit  406 . The phase detector reference circuit  430  uses the peak magnitudes sensed by the peak detector  454  to deduce the allowable levels of the digitized signal at the output of the adder  416 . The allowable levels thus deduced are then provided to the phase detector  432 . The phase detector  432  compares each digital sample received from the adder  416  to the allowable levels provided by the phase detector reference circuit  430  in order to deduce the current phase error. This operation has been described above with reference to FIG.  6  and will now be elaborated upon with reference to the system of FIG.  6 . 
     The peak detector  454  provides the phase detector reference circuit  430  the positive and negative peaks of the received signal. From these, the phase detector reference circuit deduces the three allowed levels of the signal and provides this information to the phase detector  432 . The phase detector reference circuit is preferably dedicated hardwired logic circuitry implemented using conventional design techniques to carry out the foregoing function. This function corresponds to the step of block  510  of FIG.  5 . 
     The phase detector  432  establishes from the allowed signal levels the zero band  403  and the error band  405 . In the examples discussed above, the zero band extended from 0 to about 10% of the maximum amplitude and the error band extended from 10% to 50% of the maximum amplitude. However, other selections for the width of these bands could be made. The zero and error bands are computed from the peak of the absolute value of the signal, but bands from other processed versions of the signal could be used, such as bands of the square of the signal, or the square root of the absolute value, or other non-linear processes of the signal that would increase the sensitivity to phase error. 
     The boundary between the error band and the zero band could be located at any amplitude between about 5% and 20% of the peak amplitude. These latter steps correspond to blocks  520  and  530  of FIG.  5 . The phase detector  432  during each clock cycle stores the current sample from the adder  416  as well as the previous sample. It takes the absolute value (or square, or square root, or similarly suitable function of the absolute value, etc.) of those samples and then performs the steps of blocks  550  through  597 . The phase detector may be a custom hardwired logic circuit designed to carry out those steps. The output of the phase detector  432  determines whether the phase of the voltage controlled oscillator  436  is decremented (block  595 ) or incremented (block  570 ) in accordance with the process of FIG.  5 . 
     While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.