Abstract:
A method and apparatus for eliminating dead zone in a phase locked loop with binary quantized detectors are described. Dead zone can be eliminated by changing the threshold used to quantize the cross point sample. A quantized cross point sample is integrated in order to set a new threshold. The integration may be performed during data transitions to eliminate threshold drift during long sequences where no transitions occur.

Description:
BACKGROUND INFORMATION 
   1. Field of Invention 
   The invention relates generally to phase locked loops, particularly to the elimination of dead zone for phase locked loops using binary quantized phase detectors. 
   2. Description of Related Art 
   Clock recovery circuits (CRC) are often used in communication systems and other electronic systems to synchronize a local clock to an external system clock. Currently, a phase locked loop (PLL) is the standard approach to constructing a CRC. 
     FIG. 1  shows a conventional phase locked loop, including the basic components: a phase and frequency detector denoted  1 , a low pass filter (LPF) denoted  3 , and a voltage-controlled oscillator (VCO) denoted  5 . 
   The phase and frequency detector  1  compares a reference signal denoted  7  to a feedback signal denoted  11  in order to produce a phase error signal denoted  9 . The phase error signal  9  is then filtered through the low pass filter  3  and subsequently input to the VCO  5 . The VCO  5  generates a signal  11  with a frequency controlled by the filtered phase error signal. The output  11  is fed back into the phase and frequency detector  1 . If the two frequencies for the signals  7  and  11  do not equal, the filtered phase error signal would cause the VCO  5  to shift to the frequency of the reference signal  7 . When the shift is completed, the output of the VCO  5  is used as the synchronized signal. 
   Several types of phase locked loops have been developed using phase detectors such as Alexander phase detector and Hogge phase detector. Particularly, the Alexander phase detector has been widely adopted due to its ease of implementation. 
   A CRC measures the clock phase and aligns it to the reference clock to minimize bit-error-rate, and the optimum sampling instant is at the center of the data-eye.  FIG. 2  illustrates a data-eye diagram with two data-eyes having centers denoted  17  and  19  and a cross point denoted  13 . An Alexander phase detector uses the cross point  13  sample as a reference to locate the data-eye centers. 
   However, due to duty cycle distortion, the cross point  13  in  FIG. 2  is offset from the threshold  0  by a dead zone denoted  15 . The duty-cycle distortion causes an Alexander phase detector to wander in the interval denoted  14 , searching for the cross point, thereby offsetting the locations of the data-eye centers  17  and  19 . 
   SUMMARY OF THE INVENTION 
   The present invention provides the method and apparatus for eliminating dead zone in a phase lock loop using a binary quantized phase detector. The present invention allows a binary quantized phase detector to locate a cross point in a data-eye diagram by gradually shifting a threshold value towards the threshold at which a cross point occurs. 
   In a first embodiment of the present invention, a binary quantized phase detector first receives a signal sampled in a wandering interval caused by duty-cycle distortion, and compares the value of the signal to a threshold value. The binary result of the comparison is then level shifted and subsequently integrated. The output of the integration is set as the new threshold and the process to locate the cross point restarts with the new threshold. 
   In a second embodiment of the present invention, the integration step takes place only when the data sequence transitions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings that are incorporated in and form a part of this specification illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention: 
       FIG. 1  illustrates a prior art phase locked loop with basic components. 
       FIG. 2  illustrates a data-eye diagram with duty-cycle distortion. 
       FIG. 3  illustrates a first embodiment of the present invention that integrates quantized values sampled in the wandering interval. 
       FIG. 4  illustrates a second embodiment of the present invention including transition detection. 
       FIG. 5  illustrates the second embodiment of the present invention as implemented in a clock data recovery circuit. 
       FIG. 6  illustrates a flow diagram for one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  illustrates a set-threshold circuit  100  used to locate a cross point in a data-eye diagram, comprising: a comparator  37  with two inputs  16  and  39  and an output  22 , an integrator block  4 , and a unit delay block  6 . 
   The first embodiment shown in  FIG. 3  sets 0 as the default starting value for the threshold value  39 . Referring now to  FIG. 3  in view of  FIG. 2 , the phase detector first takes a sample  16  in the wandering interval denoted  14  in  FIG. 2  and compares the value of the sample with the threshold value  39 ; the comparison returns an output  22  of 0 if the comparison result is FALSE and an output  22  of 1 if the comparison result is TRUE. The binary output  22  is then subtracted by 0.5 to obtain an average output value of 0; the resulting value  24  is integrated in the integrator block denoted  4 . 
   Moreover, the scale factors such as 0.0001 in  FIG. 3  are selected in the integration equation as the amount to shift the threshold value of the phase detector up or down, and the value may vary depending on the desired precision with which the detector uses to locate a cross point in the data-eye diagram. 
   As an example, the cross point shown in  FIG. 2  is offset to approximately an amplitude of 0.09, therefore the average value of transition samples in the wandering interval  14  is a positive value and greater than the default starting threshold  0 . 
   Moreover, the output of the integrator block  4  is delayed one cycle in a delay block  6  and then used as the new threshold input  39  for the next comparison in the comparator  37 , and the process to locate the cross point restarts with the new threshold  39 . For the example shown in  FIG. 2 , the phase detector searches for the cross point at incrementally higher amplitudes with each new threshold value  39 . The increment or decrement amount depends on the scale factor in the integration equation, which is set as 0.0001 for the embodiment shown in  FIG. 3 . 
     FIG. 4  illustrates a set-threshold circuit  200  as a second embodiment of the present invention, comprising: a comparator  37  with two inputs  16  and  39  and an output  22 , a XOR function block  8  with two inputs  18  and  20 , a multiplication block  10 , an integrator block  4 , and a unit delay block  6 . 
   In addition to the integration and shifting to find a cross point in a data-eye diagram, the second embodiment performs the integration step only when data transitions are present in order to eliminate threshold drift during long periods of data sequences without transitions. 
   The values  18  and  20  are binary results of comparing 0 to sample values taken at data-eye centers, with respect to the location of the wandering interval sample  16 . If the values  18  and  20  differ, indicating a transition in the data-eye diagram, the multiplier  10  multiplies 1 with the quantized value  24 , allowing the integration to proceed. Conversely, if the values  18  and  20  are equal, indicating a data sequence without transitions, the multiplier  10  multiplies 0 with the quantized value  24  and effectively eliminates the integration step. 
   Furthermore, since the samples  18  and  20  are located with reference to the location at which sample  16  is taken, the accuracy of the locations for the samples  18  and  20  is improved as the phase detector gradually shifts the amplitude at which it samples  16  towards the desired cross point. 
     FIG. 5  shows the second embodiment of the present invention implemented as part of an Alexander phase detector based clock data recovery circuit. The clock data recovery circuit in  FIG. 5  comprises the basic elements of a PLL, including: an Alexander phase detector denoted  64 , a filter denoted  62 , and a VCO denoted  61 . 
   As shown in  FIG. 5 , data inputs are received at block  21 . The sample taken at the center of a first eye is first received in the circuit, and subsequently compared to 0 as illustrated by blocks  49  and  51 . The resulting signal  20 , whose value is either 0 or 1, is delayed one cycle and then released at the output of block  53  as signal  18 . 
   Referring to  FIG. 5  in view of  FIG. 2 , a sample  16  is taken in the wandering interval  14 , and the sample value is compared to a threshold input  39  whose initial value is set as 0. The resulting signal  22  of the comparison, whose value is either 0 or 1, is the output of the comparator block  37 . 
   As the sample signal  16  is processed, a third sample taken at the center of a second eye is received in the circuit, and subsequently compared to 0 as illustrated by blocks  49  and  51 . The resulting signal  20 , whose value is either 0 or 1, is the output of the block  51 . Subsequently, the signals  22 ,  18 , and  20  are inputs to the circuit  41 , whose components are explicitly illustrated in  FIG. 4 . 
   As shown by blocks  41  and  37 , the output  39  of the circuit  200  is the new threshold input for comparison with the wandering interval sample  16 . Moreover, the comparison process continues with all three sample-points as the phase detector adjusts the sampling threshold through the integration performed in the circuit  41 . Since the wandering interval sample  16  is used as a reference to locate the data-eye centers  18  and  20 , as the phase detector adjusts the sampling amplitude to locate the cross point, it consequently adjusts the locations at which the samples  18  and  20  are taken and effectively allows the phase detector to self-center. 
   As illustrated in  FIG. 5 , the circuit also includes a constant block  27  that feeds into an integrator block  29 , the output of which is combined with the output of VCO  61  to produce a result. This result, along with data input  21 , is input to a transport delay block  23  and a unit delay block  25 . From unit delay block  25 , the signal branches to unit delay block  47 , which outputs to comparator block  51 , and to transport delay block  33 , which outputs to unit delay block  35  that in turn outputs to comparator block  37 . 
   The circuit of  FIG. 5  also includes an XOR block  45  that receives signals  20  and  22  as inputs, and an XOR block  55  that receives signals  18  and  22  as inputs. 
     FIG. 6  is a flow diagram illustrating a method for eliminating dead zone using a binary quantized phase detector according to one embodiment of the present invention. Step  63  shows that a signal B in the wandering interval is sampled and compared to a threshold value (step  65 ). If the value of B is less than the threshold value, B is set to −0.5 (step  67 ); if the value of B is greater than or equal to the threshold value, B is set to 0.5 (step  69 ) as shown in  FIG. 4 . 
   A delay is then inserted to retime the Alexander phase detector in step  71 . While the signal B is processed, two other signals A and C are sampled in steps  75  and  77  at the center of the eyes before and after B, using B as a reference mid-point location. 
   Both samples A and C are subsequently compared to 0 in steps  79  and  81 . If the value of either signal is less than 0 (zero), the signal is set to 0 (steps  83 ,  87 ); otherwise, the signal is set to 1 (steps  85 ,  89 ). Step  91  then applies an XOR function to the resulting signals A and C to determine if a data transition has occurred in the interval between the two sample points. 
   Step  91  shows that a temporary variable T holds the result of the XOR function; if a transition exists in the data sequence, T equals 1; else T equals 0. T is then multiplied with the value of B in step  73  to determine whether integrating the value of B is necessary. 
   If the output O of step  73  is 0 (zero), the integrating step is overlooked, and the phase detector restarts the sampling process for all three signals. If the output O of step  73  is 1, the value of B is integrated in step  93 , and the result is set as the new threshold for the phase detector (step  95 ). 
   Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications that would be apparent to a person skilled in the art. 
   For example, the Alexander phase detector and the XOR function shown in the figures are intended as illustrations, other binary quantized phase detectors and means to determine if two values are equal may be used in conjunction with the present invention without departing from the spirits of the present invention. Moreover, the multiplication block  10  in  FIG. 4  may be replaced with other means such as a table look up or a gain stage, to filter out the wandering interval sample. 
   It is understood that common practice such as representing binary values with “0” and “1” or TRUE and FALSE may be implemented by various means without departing from the desired representation. 
   Temporary variables shown in  FIG. 6  such as T and O are used to clarify the details of the embodiment and may not be necessary in actual implementation. 
   Furthermore, constants such as 0.5 used in  FIG. 2  and  FIG. 3  to level shift the input signal so that its mean value is zero, may be replaced by other constants, the comparisons may substitute its operators such as &gt;= to &gt;, the default threshold value 0 for block  39  may be altered, and the scale factor for the integrator may be adjusted to desired precision requirements without altering the essence of the present invention. 
   The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the arts to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.