Abstract:
A novel phase detector for use in a timing recovery circuit of pulse amplitude modulation communication system. A filter internal to the phase detector as preliminary stage for operating on a signal stream of pulse-shaped symbols to reduce pattern-dependent jitter of the output of the phase detector. The filter may have plural taps, delays, multipliers, and summers.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to phase detectors and, more specifically, relates to phase detectors having a filter for reducing systematic and pattern dependent jitter. 
     In pulse amplitude modulation (PAM) communication systems, precise symbol timing is essential for reliable performance. Generally, error in circuits for recovering symbol timing is due to temperature drift, aging, and misadjustment. To combat these problems and to correct timing error automatically, a timing recovery circuit such as a phase locked loop may include a sampling unit and a digital timing phase detector. 
     For digital timing phase detectors, a main source of error in symbol timing is due to systematic or pattern-dependent jitter. Generally, systematic or pattern-dependent jitter occurs as a result of a signal stream of symbols having a sequence or pattern with few signal transitions (e.g., the signal is not periodic in nature). A typical prior art phase detector receiving a completely periodic signal stream (e.g., a clock signal) provides highly accurate timing information. However, for a non-periodic or partially periodic signal stream, the typical prior art detector provides much less meaningful timing information. In multilevel PAM communication systems, systematic or pattern-dependent jitter is specially problematic due to the inherently fewer zero-transitions of a PAM signal stream. 
     With reference to FIG. 1, in one prior art phase detector for detecting symbol timing by a wave differential method, also known as the Gardner phase detector, the phase detector  20  receives a signal stream of pulse-shaped symbols. Sampler  24  samples the signal stream at twice the symbol rate where f s  is the symbol rate. The three most recent samples are processed and re-sampled at the symbol rate to provide an output signal indicating symbol timing error. 
     In operation, the first sample in a set of three most recent samples is subtracted from the last sample in the set. The resulting subtracted signal is multiplied by the middle sample to provide an output signal. If the received signal stream of pulse-shaped symbols has zero timing error, the average output of the phase detector  20  is zero. However, even under such ideal conditions the variance of the phase detector output signal is not zero. The non-zero variance is due to the intermediate samples being dependent on adjacent symbols and hence contributing to systematic or pattern-dependent jitter. 
     With reference to FIG. 2, in another prior art technique, phase detector  30  may include a signal estimator  32  and a subtractor  34 . The estimator  32  estimates the influence of adjacent symbols on intermediate samples. The subtractor  34  subtracts the output of the estimator  32  from the intermediate samples to reduce the effect of systematic and pattern dependent jitter on the output signal of the phase detector  30 . 
     Accordingly, it is an object of the present invention to provide a novel phase detector including a filter for filtering a received signal stream of symbol before symbol timing is extracted. 
     It is another object of the present invention to provide a novel filter for operating on a received signal stream of pulse-shaped symbols to thereby provide a filtered received signal to a phase detector for reducing pattern dependent jitter of the phase detector. 
     It is yet another object of the present invention to provide a novel PAM communication system having a receiver including a phase detector for providing accurate symbol timing information with less pattern-dependent jitter. 
     It is still another object of the present invention to provide a novel receiver including a phase detector having a filter for increasing the periodicity of a received signal stream of pulse-shaped symbols before symbol timing is extracted from the signal stream. 
     It is a further object of the present invention to provide a novel method for reducing pattern-dependent jitter in the output of a phase detector. 
     It is yet a further object of the present invention to provide a novel phase locked loop having a filter for filtering a received signal stream of pulse-shaped symbols as an initial stage before phase detection. 
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram illustrating a first prior art phase detector. 
     FIG. 2 is a functional block diagram illustrating a second prior art phase detector. 
     FIG. 3 is a function block diagram illustrating an embodiment of a phase detector of the present invention. 
     FIG. 4 is a functional block diagram illustrating an embodiment of a communication system of the present invention. 
     FIG. 5 is a functional block diagram illustrating an embodiment of a filter inside a phase detector of the present invention. 
     FIG. 6 is a functional block diagram illustrating an embodiment of a PAM communication system of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 3, a phase detector  40  may include filter  42 , input sampler  44 , output sampler  46 , and a Gardner phase detector  48 . The input sampler  44  or output sampler  46  may be external to the phase detector  40 . 
     In operation, a signal stream of pulse-shaped symbols are sampled at twice the symbol rate by input sampler  44  to provide a received signal stream of samples x(n). The filter h(n)  42  operates on the received signal stream of samples x(n) to provide an output signal stream of samples y(n) that is generally zero every 1/f s  seconds (y(n)=x(n) * h(n), where * is the convolution operator). The output y(n) of filter  42  is then provided to the Gardner detector  48  to detect timing information. The Gardner phase detector  48  may provide an output signal at the symbol rate. 
     In typical PAM communication systems, to reduce intersymbol interference, a signal stream of symbols is pulse-shaped with a pulse-shaping filter such as a “raised cosine” filter or a Nyquist filter. Typically, pulse-shaping to reduce intersymbol interference is partially performed at a transmitter with the remainder of the pulse shaping being performed at a receiver. The impulse response of the pulse-shaping filter may be represented by g(t) and g(n) is the sampled version of g(t) at twice the symbol rate. 
     The unit sample response of the filter  42  may be: 
     
       
         h(n)=g(n)cos(πn)=g(n)(e jπn +e −jπn )/2  (1) 
       
     
     where n is integer and g(n) is a sampled version at twice the symbol rate of the impulse response of the combined transmit and receive pulse shapers. 
     With reference to FIG. 4, a PAM communication having symbol timing detection may include a Gardner phase detector  68 , filter  60 , and pulse shaper  62 . The pulse shaper  62  may include a transmit pulse shaper  64  located in a transmitter and a receive pulse shaper  66  located in a receiver. 
     In operation, a signal stream of symbols may be operated on by the pulse shaper  62  and filter  60 . Together, the unit sample response of the pulse shaper  62  and filter  60  is s(n), where the equation for s(n) may also be expressed as:                      s        (   n   )       =                    g        (   n   )       *     h        (   n   )         =       g        (   n   )       *       (         g        (   n   )                 jπ                 n         +       g        (   n   )                   -   jπ                   n           )     /   2                     =                    g        (   n   )       *       (       g        (   n   )                 jπ                 n         )     /   2       +       g        (   n   )       *       (       g        (   n   )                   -   jπ                   n         )     /   2                     =                         jπ                   n   /   2              (       (       g        (   n   )                   -   jπ                     n   /   2           )     /   2     )       +                                       -   jπ                     n   /   2              (       (       g        (   n   )                 jπ                   n   /   2           )     *       (       g        (   n   )                   -   jπ                     n   /   2           )     /   2       )                   =                  (       (       g        (   n   )                   -   jπ                     n   /   2           )     *     (       g        (   n   )                 jπ                   n   /   2           )       )            (            jπ                   n   /   2         +            -   jπ                     n   /   2           )     /   2                   =                  (       (       g        (   n   )                   -   jπ                     n   /   2           )     *     (       g        (   n   )                 jπ                   n   /   2           )       )            cos        (     π                   n   /   2       )       .                     (   2   )                                
     In equation (2), “cos(πn/2)” causes s(n) to be zero every 1/f s  seconds. The output of filter  60  will be zero every 1/f s  seconds because symbol input rate is also 1/f s  seconds. Therefore, under ideal channel conditions and zero phase error, the output signal of the phase detector will have an average value of zero with zero variance. 
     In a PAM communication system having pulse-shaping means for pulse-shaping a signal stream of symbols having a “raised-cosine” filter to prevent intersymbol interference among the symbols, the impulse response of the “raised-cosine” filter may generally be expressed as: 
      g(t)=sinc(t/T)(cos(απt/T)/(1−(2αt/T) 2 )) 
     where t is time, T is a constant representing the symbol period, and α is a real constant between 0 and 1. The selection of α is a design choice. The sampled version of g(t) at twice the symbol rate is: 
     
       
         g(n)=sinc(n/2)(cos(απn/2)/(1−(αn) 2 )) 
       
     
     The impulse response of filter h(n) may then be: 
     
       
         h(n)=sinc(n/2)(cos(απn/2)/(1−(αn) 2 ))cos(πn)  (3) 
       
     
     In general, h(n) has infinite duration. For practical implementation, the duration may be truncated to be finite. Then h(n) may be: 
     
       
         h(n)=g(n)cos(πn)w(n)  (4) 
       
     
     where w(n) is a rectangular window function. The window function w(n) can be expressed as:          w        (   n   )       =       ∑     k   =     -   L       L          δ        (     n   -   k     )                                
     where δ(n) is a unit sample function, L is a positive integer constant, and 2L+1 is the duration of w(n). For the case of “raised cosine” pulse-shaping, h(n) may be: 
     
       
         h(n)=sinc(n/2)(cos(απn/2)/(1−(αn) 2 ))cos(πn)w(n)  (5) 
       
     
     With reference to FIG. 5, phase detector  80  may include input sampler  90 , filter  91 , Gardner detector  88 , and output sampler  92 . The filter  91  may include plural taps  82 , plural multipliers  84 , a summer  86 , and plural delays  94 . Input sampler  90  and output sampler  92  may be internal or external to the phase detector  80 . Filter  91  is an implementation of equation (4) with L=1. 
     In operation, input sampler  90  receives and samples at twice the symbol rate a signal stream of symbols which have been shaped with a “raised cosine” filter g(t). A series of samples may be stored in the delays  94 . Each tap  82  may receive one of the stored samples and provides the stored sample to one of the multipliers  84 . Each multiplier  84  may receive one stored sample and multiply the sample with a value of h(n). The multiplied signal from each multiplier  84  is then provided to summer  86 . The sum of the multiplied signal is then provided to the Gardner phase detector  88 . FIG. 5 illustrates a three tap embodiment (i.e., L=1) as an example. Preferably, the phase detector may be implemented with eleven or more taps. 
     With reference to FIG. 6, a PAM communication system may include a transmitter  100  and a receiver  102 . The receiver  102  may include a timing recovery circuit  104 , a sampler  108 , and a receive pulse shaper  110 . The timing recovery circuit  104  may include a phase detector  112 , a loop filter  114 , a voltage controlled oscillator  116 , and a frequency divider  118 . The transmitter  100  may include a transmit pulse shaper  120  and means for providing a signal stream of symbols (not shown). 
     In operation, a signal stream of symbols are received by the transmit pulse shaper  120 . The transmit pulse shaper  120  shapes the signal stream of symbols. The transmitter  100  transmits the signal stream of pulse-shaped symbol through the channel  122 . The receiver  102  receives the signal stream of pulse-shaped symbols. 
     In the receiver, the receive pulse shaper  110  pulse-shapes the received signal stream. Together the operation of the transmit pulse shaper  120  and the receive pulse shaper  110  result in pulse-shaping the signal stream of symbols with a pulse-shaping filter such as the “raised-cosine” filter to provide a received signal stream of pulse-shaped symbols having no intersymbol interference under ideal conditions. Sampler  108  samples the received signal stream of pulse-shaped symbols at the symbol rate to recover the transmitted symbols. The received signal stream of pulse-shaped symbols may be provided to the phase detector  112 . The phase detector  112  may be one of the phase detectors illustrated in FIGS. 3 or  5 . The phase detector  112  produces an output signal indicating the phase offset of sampled symbols. The output of the phase detector  112  may be received by loop filter  114  to average the phase detector output. The output of the loop filter  114  may be supplied to the voltage controlled oscillator  116 . The voltage controlled oscillator  116  generates a timing signal at twice the symbol rate. The timing signal generated by the voltage controlled oscillator  116  is provided to the phase detector  112  and the timing signal is also provided to the frequency divider  118  to generate a timing signal at the symbol rate. The timing signal at the symbol rate generated by the frequency divider  118  is provided to the phase detector  112 . This timing signal is also provided to the sampler  108  to trigger the sampling of the received signal stream. The output of the sampler  108  is expected to be the symbol which had been transmitted by the transmitter. 
     For quadrature amplitude modulation applications, outputs of the I and Q phase detectors are combined through addition into one signal. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.