Abstract:
A phase detection system allows the capture range, lock range and jitter tolerance to be extended beyond ±360°. The capture range for the phase detection system may be extended in programmable amounts up to several thousand clock cycles or can be set to any desired maximum capture range in steps of approximately 360°. The phase detection system circuit utilizes a coarse phase detector and a fine phase detector. The phase detection system uses the digital cycle slip counter phase detector to provide a wide phase capture and lock range for a large jitter tolerance. The phase detection system combines this detector with a fine phase measurement from a PFD (phase and frequency detector) for very accurate phase control and low output jitter. The PFD operates in the approximately ±540° range and provides overlap in response with a coarse phase detector using a digital cycle counter approach. The PFD allows the digital counter, used for coarse cycle slip tracking, to precondition the PFD so that the coarse and fine detectors work together with no dead-band and no conflict in responses.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/367,792, filed Mar. 26, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic circuits, and, more specifically, to phase detection circuits. 
     2. Description of Related Art 
     A PLL (phase locked loop ) refers to a feedback loop in which the input and the feedback parameters of interest are the relative phases of the waveforms. The function of a PLL is to track small differences in phase between the input and feedback signal. A conventional PLL typically includes a phase detector, low-pass filter and a VCO (voltage-controlled oscillator). The phase detector measures the phase difference between its two inputs. The phase detector output is then filtered by the low-pass filter and applied to the VCO. The VCO input voltage changes the VCO frequency in a direction that reduces the phase difference between the input signal and the local oscillator. The loop is in phase lock or locked when the phase difference between the input signal and the VCO frequency is reduced to zero. 
     A phase detector only accepts phase information in comparing two signals. A PFD (phase/frequency detector) is also able to accept frequency information in comparing two signals. A digital PLL is a PLL system in which the VCO and loop filter are built from digital components such as gates or flip-flops. A PFD is typically made from an exclusive OR gate, or an AND gate and D-type flip-flops or a tri-state phase/frequency comparator. PLL circuits have two ranges for acquisition, a pull-in range and a capture range (also known as lock-in range). The acquisition time is the total time the PLL takes to acquire both frequency and phase lock. 
     A PLL circuit will produce the lowest output jitter level if it can perform phase comparisons and can phase lock using the highest input clock available. As phase measurements become more regular, the loop is updated more regularly and control is thereby maintained to reduce internal noise. Unfortunately, some systems, such as telecommunications networks, require the PLL system to be tolerant to a large amount of jitter on the clock input and still be able to maintain a lock. To maintain a lock, the system must remember the location of the 0° position and continuously pull the PLL in the direction of that location. Often the required jitter tolerance values will extend to tens of clock cycles or unit intervals (1 unit interval (UI)=1 clock cycle=360°). A conventional PLL would not be able to maintain a lock with over ±1 UI of jitter on the input. Thus, with ±10 UI of input jitter, the PLL would not know the location of the original 0° position and, as a result, cycle slippage would occur. Cycle slippage causes timing problems for the system, such as, for example, buffer overflows. Accordingly, conventional PLL circuits may not meet the requirements of systems that require a high jitter tolerance, such as telecommunications systems. Jitter tolerance is particularly difficult to provide in high frequency applications. Conventional techniques for providing jitter tolerance for high frequency applications include dividing the clock down to a fraction of the original frequency. Although the lower frequency results in proportionately lower jitter, the system suffers a loss in performance. 
     FIG. 10 shows a conventional type-4 PFD, indicated generally at  500 , with a pair of output sampling DFF (D-type flip flop) units  505  and  510  (“Down_s” and “Up_s”, respectively). The output sampling DFF units  505  and  510  are optional and would not be needed in an APLL (analog PLL) system. Note that AND gate  525  (“I1”) and inverter  530  (“I2”), shown in FIG. 10, are idealized representations and in reality would incorporate some delay so that the reset pulse lasts for a sufficient duration, typically a few nanoseconds, to effectively reset DFF units  535  and  540 . Generally, the output of DFF units  535  and  540  each go high on the leading edge of their respective clock inputs and remain high until they are reset. The reset signal occurs when inputs A  515  and B  520  have both gone from a low to a high state, which makes signals ‘up’ and ‘down’ both high. When both input signals A  515  and B  520  are in phase and of the same frequency, both outputs will be low for most of the time, with signals ‘up’ and ‘down’ both pulsing high only for a few nanoseconds, and no signal will be applied to the VCO (not shown in FIG.  10 ). If the two signal frequencies are not the same, then the output pulse widths will depend on both the relative frequency difference and the phase difference. The type-4 PFD  500  is common because of its simplicity, accuracy and ability to perform both frequency and phase locking. But, the phase capture range of the type-4 PFD is generally limited to ±360°. 
     FIGS. 11 and 12 show timing diagrams that illustrate the behavior of conventional PFD  500  shown in FIG.  10 . If the rising edge of input A  515 , shown in FIG. 11, occurs before the rising edge of input B  520 , then the “up” pulse is wider than the “down” pulse, as shown in FIG.  11 . The width of the up pulse is proportional to the phase difference between input A  515  and input B  520 . Conversely, if the rising edge of input B  520  occurs before the rising edge of input A  515 , then the down pulse is wider and has a width proportional to the phase difference. An inspection of FIG. 10 in conjunction with the timing diagrams shown in FIGS. 11-12 reveals that the inherent range of the conventional PFD  500  is limited to one cycle or UI as discussed above. The diagram shown in FIG. 12 shows the waveforms at the extreme ±360° limit. Beyond this limit, the signal begins to resemble that of FIG.  11 . At this point, the 0° reference point has been lost and a cycle slip has occurred. Thus, PFD  500  is unable to operate past ±360°. Therefore it would be desirable to provide a phase detection circuit that provides an operating range that extends beyond ±360° and provides a large amount of jitter tolerance. 
     SUMMARY OF THE INVENTION 
     The present invention provides a phase detection circuit that allows the capture range, lock range and jitter tolerance to be extended beyond the ±360° limit associated with conventional PLL circuits. In an embodiment of the invention, the phase detection circuit includes a PFD (phase and frequency detector) that operates in the ±540° range. 
     In another exemplary embodiment of the invention, the phase detection system combines two types of phase detectors, including a coarse phase detector and a fine phase detector, e.g., the PFD, in an advantageous manner. The phase detection system uses the coarse phase detector, e.g., a digital cycle slip counter phase detector, to provide a wide phase capture and lock range for a large jitter tolerance. The phase detection system combines this detector with a fine phase measurement from the PFD for very accurate phase control and low output jitter. 
     The PFD allows the coarse phase detector to precondition the PFD so that the coarse and fine detectors work together with no conflict in responses and no dead-band, e.g., phase ranges not captured by either detector. The capture range for the presently disclosed phase detection circuit may be extended in programmable amounts up to several thousand clock cycles or can be set to any desired maximum capture range in steps of 360°. In an exemplary embodiment of the invention, the system and method of the present invention may be implemented in PLL systems that have some digital component such that the logical merging and arithmetic combining of phase detector results may be more easily accomplished with the digital components than with analog components. 
     The presently disclosed phase detection system provides a number of advantages over conventional phase detection circuits. One advantage of the present phase detection system is a wide phase capture and lock range with an unlimited maximum phase capture range. Another advantage is that the system provides accurate phase measurement in addition to the wide range. An additional advantage of the present system is a frequency lock capability. A further advantage of the present invention is the easy programmability of a maximum capture range. Another advantage of the present invention is the relative ease of programming additional options for a ±180°, ±360° or ±540° phase range. In addition, users may program additional options for ranges in multiples of 360°. Yet another advantage of the present invention is that the system may be implemented in both digital and analog systems even though some of the techniques used may be digital. 
     A more complete understanding of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present disclosure and its numerous objects, features, and advantages may be better understood by reference to the following description of an illustrative embodiment, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of an exemplary embodiment of the phase frequency detector (PFD); 
     FIGS. 2A and 2B show an exemplary embodiment of the PFD; 
     FIG. 3 is a timing diagram illustrating the performance of an exemplary embodiment of the PFD at a +90° input phase difference; 
     FIG. 4 is a timing diagram illustrating the performance of an exemplary embodiment of the PFD at a +270° input phase difference; 
     FIG. 5 is a timing diagram illustrating the performance of an exemplary embodiment of the PFD at a +450° input phase difference; 
     FIG. 6 is a timing diagram illustrating the performance of an exemplary embodiment of the PFD at approximately a +540° input phase difference; 
     FIG. 7 is a timing diagram illustrating the performance of an exemplary embodiment of the PFD from a +90° to a +540° input phase difference and back to a 0° position; 
     FIG. 8 is an exemplary embodiment of a system incorporating a digital phase detection scheme with the PFD; 
     FIG. 9 is a timing diagram illustrating the performance of the system shown in FIG. 7 during a phase sweep from 0 to 250 UI (900°) and back to a 0° position; 
     FIG. 10 shows a prior art PFD; 
     FIG. 11 is a timing diagram illustrating the performance of the prior art PFD with a minor phase difference; and 
     FIG. 12 is a timing diagram illustrating the performance of the prior art PFD at the 360° phase difference limit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention satisfies the need to provide a PFD that allows the capture range, lock range and jitter tolerance to be extended beyond the ±360° limit associated with conventional phase detection circuits. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more of the drawings. 
     FIG. 1 shows a block diagram of an exemplary embodiment of the PFD of the present invention, indicated generally at  10 . PFD  10  detects the phase and frequency differences between a first input signal  15  (input signal B) and a second input signal  20  (input signal A). PFD  10  contains a first stage down phase capture unit  315  to capture negative phase differences down to −360°. The output of first stage down phase capture unit  315  goes active when it detects an input B  15  rising edge. First stage down phase capture unit  315  is connected to a second stage down phase capture unit  325 . The output of second stage down phase capture unit  325  may go active when a second input B  15  rising edge is detected at a point when the output of first stage down phase capture unit  315  is already active. As a result, second stage down phase capture unit  325  may capture negative phase differences down to −540°. Resetting control blocks  335  control the reset sequence of down phase capture units  315  and  325  to ensure that all phase values in the negative phase range are captured. Down sum unit  340  combines the outputs from down phase capture unit  315  and  325  to produce down signal  215 , which represents the input phase difference in a negative direction. 
     A similar interaction occurs between the first stage up phase capture unit  320 , second stage up phase capture unit  330  and resetting control blocks  335 . The output of first stage up phase capture unit  320  goes active when it detects an input A  20  rising edge. This allows first stage up phase capture unit  320  to capture positive phase differences up to +360°. First stage up phase capture unit  320  is connected to a second stage up phase capture unit  330 . The output of second stage up phase capture unit  330  may go active when a second input A  20  rising edge is detected at a point where the output of the first stage unit  320  is already active from the detection of the first rising edge. This allows PFD  10  to capture phase differences up to +540°. Up sum  345  combines the outputs of units  320  and  330  to produce up signal  220 , which represents the input phase difference in a positive direction. 
     FIGS. 2A and 2B show another exemplary embodiment of the PFD of the present disclosure, indicated generally at  10 . The operation of PFD  10  may be illustrated by reference to the timing diagrams depicted in FIGS. 3-7, depicting various gradually increasing phase offsets in one direction (e.g., the input A  20  rising edge occurs first). The design of PFD  10  is symmetrical in that the response to the phase offsets in the opposite direction is substantially identical, but uses flip flops  30 ,  35 ,  80  and  65  (D 1 , D 2 , DR 2  and DOWN_s, respectively), shown in FIG. 2A, instead of flip flops  150 ,  155 ,  130 , and  180  (U 1 , U 2 , UR 2  and UP_s, respectively), shown in FIG.  2 B. It should be noted that the exemplary embodiment of PFD  10  shown in FIGS. 2A and 2B is one example of implementing the required PFD functionality. One of ordinary skill in the pertinent arts will recognize that other types of gates and gate arrangements may be used to provide an equivalent functionality. 
     DFF units  30  (D 1 ) and  150  (U 1 ) allow PFD  10  to phase capture input phase difference of up to ±360°. DFF units  35  (D 2 ) and  155  (U 2 ) extend the phase capture range up to ±540°. PFD  10  also includes resetting control blocks to control the reset of these DFF units. Units  90 ,  95  and  100  serve as resetting control blocks to control the resetting of DFF  30  (D 1 ) with signal resetd 1 . The resetting of DFF  150  (U 1 ) is controlled by units  110 ,  100  and  95  with signal resetu 1 . Units  105 ,  80  and  75  control the resetting of DFF  35  (D 2 ) with signal resetd 2 . Units  115 ,  130  and  140  control the resetting of DFF  155  (U 2 ) with signal resetu 2 . 
     PFD  10  includes components to sum outputs from selected components to produce a final output signal. Block  60  combines the outputs from DFF blocks  30  and  35  (D 1  and D 2 ) to produce the final output signal Down  215 . In the exemplary embodiment shown in FIG. 2A, block  60  may be an OR logical component. The output signal Down  215  represents the input phase difference, in a negative direction. Output signal Down  215  may be a pulse width varying signal. The negative direction indicates that the input B  15  rising edge occurs before the input A  20  rising edge. 
     Block  145  combines the outputs from DFF blocks  150  and  155  (U 1  and U 2 ) to produce the final output signal Up  220 . In the exemplary embodiment shown in FIG. 2B, block  145  may be an OR logical component. The output signal Up  220  represents the input phase difference, in a positive direction. Output signal Up  220  may be a pulse width varying signal. Generally, the positive direction indicates that the input A  20  rising edge occurs before the input B  15  rising edge. 
     In order to provide the extended range of PFD  10 , the output and resetting of the DFF components must be properly timed or sequenced. The sequencing of outputs from DFF units  30 ,  35  and  80  (D 1 , D 2  and DR 2 , respectively), shown in FIG.  2 A,provide for the extended range of PFD  10  for negative phase differences. The extended range is made possible by the output of DFF  35  (D 2 ) going high, i.e., qpfdD 2 =1, in response to a second B input  15  rising edge, when the output of DFF  30  (D 1 ), signal qpfdD 1 , is already high from the first B input  15  rising edge. The output signal for DFF  35  (D 2 ), qpfdD 2 , goes high because the phase difference is less than −360°. As a result, PFD  10  may keep track of the 0° position past −360°. 
     In order to prevent the output signal from DFF  30  (D 1 ) from being overlooked, components  40 ,  45 ,  50  and  55 , shown in FIG. 2A, are used to provide a delay that is longer than the reset time for DFF  30  (D 1 ). For example, the function of the delay components may be observed in the situation where an input A  20  rising edge closely follows an input B  15  rising edge. First, the output signal qpfdD 1  from DFF  30  (D 1 ) goes high in response to the input B  15  rising edge. Next, the output signal qpfdU 1  from DFF  150  (U 1 ) goes high in response to the input A  20  rising edge. This sequence triggers reset signal resetd 1  to go low to reset DFF  30  (D 1 ). If DFF  30  (D 1 ) is reset too soon, then the output signal qpfdD 1  from DFF  30  (D 1 ) will not be captured by DFF  35  (D 2 ) and, as a result, the fact that there was an input B  15  rising edge would be lost. Accordingly, the delay introduced by components  40 ,  45 ,  50  and  55  provides that qpfdD 1  is high for long enough that a high signal (signal d_d 2 ) may be read into the D input of DFF  35  (D 2 ) before signal qpfdD 1  goes low in response to reset signal resetd 1 . 
     In order to ensure that all phase values are being recorded, PFD  10  provides for a specific reset sequence of both DFF  30  (D 1 ) and DFF  35  (D 2 ). For example, DFF  80  (DR 2 ) resets DFF  35  (D 2 ) on the falling edge of the input B  15  signal when both qpfdD 1  and qpfdD 2  are both high. This interaction results in a phase capture range that extends to −540°. Gate  90  (I 8 ) ensures that DFF  30  (D 1 ) is not reset when the output signal qpfdD 2  of DFF  35  (D 2 ) is high. In order to move back from −370° to −350°, DFF  35  (D 2 ) must be reset and not set again before DFF  30  (D 1 ) is reset. Both of these mechanisms ensure that the range from −540° to −350° is not overlooked. 
     A similar sequence of events occurs for DFF units  150 ,  155 , and  145  (U 1 , U 2  and UR 2 ), shown in FIG.  2 B. The extended range for positive phase differences is made possible by the output of DFF  155  (U 2 ) going high, i.e., qpfdU 2 =1, in response to a second A input  20  rising edge, when the output of DFF  150  (U 1 ), signal qpdfU 1 , is already high from the first A input  20  rising edge signal. The high status of qpfdU 2  in this case indicates that the phase difference has exceeded +360°. Accordingly, PFD  10  may keep track of the 0° position at phase differences of over +360°. 
     For the situation in which an input B  15  rising edge closely follows an input A  20  rising edge (the reverse of the scenario described above), components  160 ,  165 ,  170  and  175  provide a sufficient delay to ensure that signal qpfdU 1 , the output signal from DFF  150  (U 1 ), is high for long enough so that a high signal (signal d_u 2 ) may be read into the D input of DFF  155  (U 2 ) before signal qpfdU 1  goes low. As with DFF units  30  and  35  (D 1  and D 2 ), PFD 10  provides a specific set of sequences for resetting DFF units  150  and  155  (U 1  and U 2 ) to ensure that phase values are not missed. For example, DFF  130  (UR 2 ) resets DFF  155  (U 2 ) on the falling edge of the A input when the outputs qpfdU 1  and qpfdU 2 , from DFF  150  (U 1 ) and  155  (U 2 ), respectively, are both high. This provides for the +540° phase range limit. Gate  110  (I 11 ) ensures that DFF  150  (U 1 ) is not reset when the output of DFF  155  (U 2 ) signal qpfdU 2  is high. In moving from 350° to 370°, DFF  155  (U 2 ) is reset and not set again until DFF  150  (U 1 ) is reset. Accordingly, the phase value from 350° to 540° is not overlooked. 
     FIGS. 3-7 show timing diagrams of an exemplary embodiment of the present invention. In FIG. 7, the phase offset between input A and input B starts at 90° and increases to 540°. When the phase position returns to 0°, the up and down pulses are balanced. Accordingly, the system  10  is able to remember the 0° phase position even for a phase offset of 540°. As a result, the system  10  has a capture range of ±540°. FIGS. 3-6 show the operation of PFD  10  at a +90°, +270°, +450° and +540° input phase difference, respectively. As shown in FIGS. 4-6, at phase offsets of over 360° the final “up” and “down” signals show that the “up” signal is continuously on during this time. This particular embodiment of PFD system  10  provides a proportional pulse width modulated phase measurement up to 360°. 
     In another exemplary embodiment of PFD  10 , the circuit may include output sampling DFF units  180  (UP_s) and  65  (DOWN_s). DFF units  10  (Up_s) and  65  (DOWN_s) sample Up signal  220  and Down signal  215 , respectively, in accordance with clock signal  205 , to produce Up_sync signal  185  and Down_sync signal  70 , respectively. These DFF units  180  and  65  are used for sampling into a digital system and are not necessarily required in an APLL (analog PLL) system. In another exemplary embodiment, PFD  10  may accept preconditioning signals  210  (preconU_b) and  25  (preconD_b) from a digital coarse phase detector (not shown in FIGS.  2 A and  2 B). The operation of the preconditioning signals is discussed below in connection with FIG.  8 . PFD  10  may also have optional enabling signals engaging a ±180°, ±360° or ±540° locking range. For example, in an exemplary embodiment of PFD  10 , the circuit includes signal  120  (enable_ 180 ) to enable the phase range of ±180°. Similarly, PFD  10  may include signal  125  (disable_ 540 ) to disable the extended ±540° locking range. 
     PFD  10  may be implemented in a phase detection system with other types of phase detectors. FIG. 8 shows an exemplary embodiment of a phase detection system, shown generally at  280 . Phase detection system  280  includes PFD  10  and a coarse digital phase detector. In this particular embodiment, the coarse digital phase detector is an up/down digital counter that may be used to measure complete cycle slips. The digital counter may be based on any suitable numbering scheme or concept. Generally, a digital counter can count cycles but does not track the location of a 0° position as well as a fine phase detector such as, for example, PFD  10 . Accordingly, phase detection system  10  combines the digital counter&#39;s ability to track cycle slips with the ability of PFD  10  to track and lock in to a 0° position. Because PFD  10  and the coarse phase detector provide an overlap in response, there is substantially no dead-band in the transition between the two phase detectors. 
     Phase detection system includes edge detect components  240  and  245 . The rising edge of input clock signal B  15  and input clock signal A  20  is detected by input B edge detect  240  and input A edge detect  245 , respectively. The outputs of edge detect  240  and  245  are connected to count control  250 . Count control  250  is connected to counter  255 . Count control  250  handles the decision to increment, decrement or make no change to counter  255 . Counter  255  increments when the rising edge of input A  20  is detected. Counter  255  decrements when the rising edge of input B  15  is detected. When both are detected at the same time, no counter change is made. Counter  255  is associated with a programmable limit set  270 . Limit set  270  defines the maximum and minimum counter value. This maximum and minimum counter value corresponds to the required phase capture range. For example, with a maximum counter value of 8191, the phase capture range of the whole system would be 8191×360°=2,948,760°. 
     Generally, counter  255  will produce +1, −1 or 0 count values when the A and B input edges are close to each other. If the distance between the edges exceeds a defined phase difference, then counter  255  may increment or decrement by larger count values. For example, at +360° phase offset, the counter may produce +2, 0 or +1 count values. The coarse phase detector value may be used when the count value is greater than +1 or less than −1. 
     Counter  255  is connected to output control  260 . Output control  260  handles the decision of whether to use the coarse phase detector value. If output control  260  decides to use the coarse phase detector value from counter  255 , then this value will be sent to digital filter  265 . Digital filter  265  averages the count value along with the up and down output signals from PFD  10 . Accordingly, the output of digital filter  265  is a representation of the input phase. Digital filter  265  averages the PWM (pulse width modulated) signals of Up_sync  185  and Down_sync  70  from PFD  10  in addition to the different count values from counter  255 . The count values are also effectively PWM signals because, for example, the +2 and +3 values would provide greater precision after averaging and resolving down to fractions of a period. For instance, in a stream of 100 counter values, with 35 values of +2 and 65 values of +3, the measured phase value from the averaging digital filter  265  would be +2.35. Note that in this exemplary embodiment, the sampled signals  185  and  70  are used because PFD  10  is connected with a digital component, i.e., digital filter  265 . 
     Output control  260  may also send preconditioning signals  25  and  210  to PFD  10 . As discussed above, the preconditioning signals are used to ensure that PFD  10  and the coarse phase detector operate in a harmonious fashion. Without this link, the two phase detectors may pull in opposite directions. The manner in which PFD  10  is preconditioned by signals  210  (preconU_b) and  25  (preconD_b) is based on the counter value currently tallied by digital counter  255 . If the counter value is a positive, non-zero number, then there is a phase difference in the positive phase direction between the two input signals  15  and  20 . Accordingly, preconditioning signal  210  (preconU_b) should be asserted to indicate that to PFD  10  that there is a phase change in the positive phase direction. On the other hand, if the counter value is a negative number, then there is a phase difference in the negative direction. As a result, preconditioning signal  25  (preconD_b) should be asserted to indicate to PFD  10  that there is a phase change in the negative phase direction. In one exemplary embodiment, PFD  10  is preconditioned according to the logical description shown in Table I based on the exemplary embodiment of PFD  10  shown in FIGS. 2A and 2B (e.g., the preconditioning signals are active low). 
     
       
         
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Counter 
                 PreconD_b 
                 preconU_b 
               
               
                   
               
             
             
               
                 &lt;−1 
                 goes to 0 
                 goes to 1 
               
               
                  &gt;1 
                 goes to 1 
                 goes to 0 
               
               
                 0, 1 or −1 
                 goes to 1 
                 goes to 1 
               
               
                   
               
             
          
         
       
     
     The application of the preconditioning signals ensures a smooth continuation of phase measurement across the 360° boundary and maintains the memory of the 0° phase position. 
     In the exemplary embodiment shown in FIG. 8, PFD  10 , Edge detects  240  and  245 , counter  255 , output control  260  and digital filter  265  are all clocked components. The resolution of the system is determined by the system clock rate that drives all of the digital blocks and whether the system clock is synchronized with the main clock. 
     FIG. 9 depicts a phase sweep showing a range from 0 UI to +2.5 UI (+900°) and back to 0 U 1 . The simulation waveform illustrates the interaction of the components of the system shown in FIG. 8 to track a varying input phase difference. With preconditioning from digital phase counter  255 , the system can keep track of many cycles. As shown in FIG. 9, the system goes to +2.5 UI and then completely recovers to the same position back at 0 UI (0°). The counter value from counter  255  steps from 0 to −3 and back to 0 again. The digital count value is represented by an analog waveform. When the count value is less than −1, then signal  25  (preconD_b) activates, e.g., active low, which preconditions DFF  35  (D 2 ) and output qpfdD 2  to go high. This high signal notifies PFD  10  that digital counter  255  is indicating that the phase is at least less than −360°. Accordingly, FIG. 9 shows that phase detection system  280  remembers the 0° position and is able to return to this position rather than lock one or more cycles away. 
     Having thus described a preferred embodiment of the phase detection system, it should be apparent to those skilled in the art that certain advantages of the described method and system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. For example, particular gates and gate arrangements have been illustrated, but it should be apparent that the inventive concepts described above would be equally applicable to alternate gates and gate arrangements that provide equivalent functionality. The invention is further defined by the following claims.