Abstract:
A phase detector is adapted to receive first and second signals and generate third and fourth signals representative of the difference between the phases of the first and second signals. The phase detector assert the third signal in response to the assertion of the first signal and unasserts the third signal in response to the assertion of the second signal. The phase detector asserts the fourth signal in response to the assertion of the third signal and unasserts the fourth signal in response to unassertion of the first signal. The phase detector may include combinatorial logic gates, thereby to generate the third and fourth signals in response to logic levels of the first and second signals. The phase detector may include sequential logic gates, thereby to generate the third and fourth signals in response to transitions of the first and second signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present application is a continuation in-part of and claims priority under 35 U.S.C. 120 from application Ser. No. 10/896,372 filed Jul. 20, 2004, entitled “Phase Detector”, the content of which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to electronic circuits, and more particularly to phase detectors adapted to operate with return-to-zero (RZ) or pulse position modulation data in clock and data recovery (CDR) system. 
   BACKGROUND OF THE INVENTION 
   The increasing speed with which multiple types of data, such as text, audio and video, are transported over existing communication networks has brought to the fore the reliability with which such data transportation is carried out. In accordance with one conventional method, to ensure reliable data transfer, the data is first encoded with a reference clock signal at the transmitting end of the network to generate a composite signal. Thereafter, the composite signal is transmitted over the network to the receiving end. At the receiving end, the data and clock signals are recovered from the composite signal to ensure that the data and clock signals remain synchronous with respect to each other. 
   The clock and data recovery (CDR) is typically carried out, for example, by a phase locked loop (PLL). In operation, a phase locked loop maintains a fixed relationship between the phase and frequency of the signal it receives and those of the signal it generates. A phase-locked loop often includes a phase detector (PD) that receives a pair of signals, and in response, generates a pair of output signals representative of the difference between the phases of the two received signals. 
   One widely known phase detector, referred to as Hogge phase detector, and which can only rely on the non-return to zero (NRZ) or pulse width modulation (PWM) property of data to re-time the input data at the optimal sampling point is shown in  FIG. 1 . Phase detector (PD)  50  requires that the duty cycle distortion of the recovered clock be kept at minimum. The operation of PD  50  is described further below. 
   A timing diagram with the clock aligned nearly perfectly aligned to the input data transitions is shown in  FIG. 2  for a NRZ or PWM data stream. The input NRZ data stream signal is provided to PD  50  on data signal line labeled Rdata, which is applied to the input terminal D of flip-flop  12 . The output QN 1  of flip-flop  12  is supplied to the input terminal D of flip-flop  14 . 
   Clock signal Rclk is applied to the input clock terminal CP of flip-flop  12 , and the inverse of clock signal Rclk is applied to the input clock terminal of flip-flop  14 . The input and output of the flip-flop  12  are provided to an exclusive-OR gate  24  to provide signal P_UP signal. The input and output of the flip-flop  14  are provided to a second exclusive-OR gate  26 , to provide signal P_DN. Signals P_UP and P_DN are provided to a charge pump (not shown). 
   As can be seen from the block diagram of  FIG. 1  and the timing diagram of  FIG. 2 , the P_UP pulses are generated during the time interval between data transitions and the next rising edge of the clock. Every P_UP pulse generates a P_DN pulse with a fixed width of half of the clock period. Ideally the width of the P_UP and P_DN pulses should be equal to half of the clock period. When the clock leads or lags from this ideal position, the P_UP pulse becomes smaller or larger than the P_DN pulse, respectively. The P_UP and P_DN pulses are fed to the charge pump and loop-filter which are part of a phase-locked loop (PLL) (not shown). The difference between the pulse width of the signals P_UP and P_DN is processed and delivered as a feedback signal to control the frequency of the oscillator in the PLL 
   When the clock is aligned nearly perfectly to the input data transitions, the difference between the pulse widths of P_UP and P_DN is equal to nearly zero, and the PLL is in a phase-locked condition. It is seen that the sampling point of the data is optimal since the sampling (rising) edge of the clock is located near the center of the data windows, thus providing the maximum noise margin. Referring to  FIG. 2 , it is seen that each NRZ of data RDATA pulse provides two P_UP signals and two P_DN signals, resulting in two ramp-up and ramp-down transitions of the loop filter voltage, as shown at the bottom of  FIG. 2 . 
     FIG. 3  illustrates what happens if RZ or PPM type data (the pulse width of the signal could be much smaller or much bigger than one period of the clock) is used for the input RDATA signal shown at the top of  FIG. 3 . As can be seen, since the clock rising edge  32  is aligned with the falling edge  34  of the RZ data, a misalignment may result in the one pulse not being sampled. Accordingly, PD  50  is very susceptible to noise for RZ data and is also not suitable for RZ or Pulse Position Modulation (PPM) data format used in the magnetic recording technology or the communication technology. 
     FIG. 4  is a simplified block diagram of a tri-wave phase detector  100 , as known in the prior art. Tri-wave phase detector  100  is shown as including three flip-flops and three XOR gates. Tri-wave detector  100  provides a reduced sensitivity to data transition density. However, tri-wave detector  100  is more sensitive to duty cycle distortion in the clock signal than is Hogge&#39;s phase detector and it is also more complex than Hogge&#39;s PD. 
     FIG. 5  is a simplified block diagram of a modified tri-wave phase detector  150 , as known in the prior art. Modified tri-wave phase detector  150  uses two distinct down-integration intervals clocked on opposite edges of the clock, rather than a single down-integration of twice the strength clocked on a single edge. This enables tri-wave phase detector  150  to have a relatively improved duty cycle performance compared to tri-wave phase detector  100 . However, phase detectors that are based on Hogge (both tri-wave phase detector  100  as well as modified tri-wave phase detector  150 ) do not function properly with RZ data stream. Moreover, the center offsets of both these detectors are dependent on the duty cycle of the clock signal CLK. 
   U.S. Pat. No. 6,324,236 also describe examples of different circuits adapted to detect the phase of a RZ data signal. However, the circuits disclosed in this patent utilize the pulse width of the clock and hence their performance suffers from the clock duty cycle distortion. Moreover, they cannot function properly if the pulse width of the RDATA is greater than one period of the clock. 
   BRIEF SUMMARY OF THE INVENTION 
   A phase detector in accordance with the present invention receives first and second signals and, in response, generates third and fourth signals representative of the difference between the phases of the first and second signals. The phase detector asserts the third signal in response to the assertion of the first signal and unasserts the third signal in response to the assertion of the second signal. The phase detector asserts the fourth signal in response to the assertion of the second signal and unasserts the fourth signal in response to unassertion of the first signal. The first and second signals represent data and clock signals. The phase detector in accordance with the present invention is adapted to operate even if the pulse width is greater than twice the clock period. 
   In some embodiments, the phase detector includes combinatorial logic gates, such as AND gates. In these embodiments, the phase detector generates the third and fourth signals in response to logic levels of the first and second signals. In some embodiments, the phase detector includes a first combinatorial logic gate adapted to receive the first and second signals and generate the third signal, and a second combinatorial logic gate adapted to receive the first signal and an inverse of the second signal and generate the fourth signal. Each of the first and second logic gates may be an AND gate further adapted to receive an enabling signal. 
   In some embodiments, the phase detector includes, in part, sequential logic gates, such as flip-flops gates. In these embodiments, the phase detector generates the third and fourth signals in response to transitions of the first and second signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of a Hogge phase detector, as known in the prior art. 
       FIG. 2  is a timing diagram of various signals as the phase detector of  FIG. 1  receives NRZ data or PWM data stream. 
       FIG. 3  is a timing diagram of various signals as the phase detector of  FIG. 1  receives RZ data stream. 
       FIGS. 4 and 5  show tri-wave phase detectors, as known in the prior art. 
       FIG. 6  shows a phase detector, in accordance with one exemplary embodiment of the present invention. 
       FIG. 7  shows an exemplary timing diagram of signals associated with the phase detector of  FIG. 6 . 
       FIG. 8  shows a phase detector, in accordance with another exemplary embodiment of the present invention. 
       FIG. 9  shows an exemplary timing diagram of signals associated with the phase detector of  FIG. 8 . 
       FIG. 10  shows an exemplary timing diagram of signals associated with the phase detector of  FIG. 8 . 
       FIG. 11A-11C  are exemplary timing diagrams of a number of signals associated with the phase detector of  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6  is a schematic diagram of a phase detector  200  adapted to detect the phase of data signal Rdata relative to the phase of signal Rclk where the pulse width of signal Rdata is smaller than one half period of the clock, in accordance with one exemplary embodiment of the present invention. Phase detector  200  is shown as including combinatorial logic gates, namely AND gates  202 ,  204  and inverter  206 . Signal Rdata is applied to input terminals of both AND gates  202 , and  204 . Signal Rclk is applied to an input terminal of AND gate  204 . Inverse of signal Rclk is applied to an input terminal of AND gate  202 . Signal Enable is also applied to an input terminal of each AND gates  202 , and  204 . AND gate  202  generates output signal P_UP, and AND gate  204  generates output signal P_DN. Signals P_UP an P_DN are generated in response to the logic levels of signals Rdata and Rclk. 
   Signal Enable is used to enable or to disable phase detector  200 . Accordingly, when signal Enable is, e.g., in a logic high state, phase detector  200  is enabled, and when signal Enable is, e.g., in a logic low state, phase detector  200  is disabled. 
     FIG. 7  shows a timing diagram of some of the signals received by or generated by phase detector  200 , when signals Rdata and Rclk are in-lock, in accordance with one exemplary embodiment. As seen from this timing diagram, assuming clock signal Rclk is in a logic low state (e.g., due to high-to-low transition  210 ) after signal Rdata transitions to a high logic state (becomes high)  215 , signal PULP also goes high  220 . Thereafter, when signal Rclk goes high  225 , signal P_UP goes low  230 , and signal P_DN goes high  235 . Signal P_DN remains high until signal Rdata goes low  240 , thereby causing signal P_DN to go low  245 . In conventional Hogge type phase detectors, such as that shown in  FIG. 1 , the high-to-low transition  255  of a signal corresponding to signal P_DN (shown in  FIG. 7  as signal P_DN_PriorArt) occurs in response to high-to-low transition  250  of signal Rclk. However, in accordance with the present invention, the high-to-low transition  245  of signal P_DN occurs in response to high-to-low transition  240  of signal Rdata. 
     FIG. 8  is a schematic diagram of a phase detector  260  adapted to detect the phase of data signal Rdata relative to the phase of signal Rclk, in accordance with another exemplary embodiment of the present invention. Phase detector  260  is shown as including sequential logic gates, namely flip-flops  262 ,  264 , as well as combinatorial logic gates, namely NOR gates  266 ,  267 ,  268 ,  269 ,  270 , and inverters  280 ,  281 ,  282 ,  283 ,  284 , and three delay buffers  285 ,  286 ,  287 . 
   There are three pulse generators  210 ,  220  and  230 . Pulse generator  210  includes inverter  284 , delay buffer  285  and NOR gate  267  whose output drives one of the input terminals of NOR gates  268  and  269 . Pulse generator  220  includes inverter  281 , delay buffer  286  and NOR gate  266  whose output drives the CK clock input terminal of flip-flop  262 . Pulse generator  230  includes inverter  283 , delay buffer  287  and NOR gate  270  whose output drives the CK clock input terminal of flip-flop  264  and input terminal of NOR gate  268 . Pulse generator  230  is adapted to output pulses if signal RDATA signal is at high logic level. 
   There are three conditions in which NOR gate  268  resets flip-flop  262 : The first condition occurs whenever RDATA signal makes a logic high to low. The second condition occurs when signal RCLK signal makes a logic low to high transition. The negative transition of signal RDATA also resets the flip-flop  264  via NOR gate  269 . The third condition occurs when signal RST resets both flip-flops  262  and  264  via NOR gates  268  and  269  respectively. 
   The first flip-flop  262  is set whenever signal RDATA makes a low to high transition via the inverter gate  280  and pulse generator  220 . The second flip-flop  264  is set only when signal RCLK makes a low to high transition and signal RDATA is asserted via inverter  282  and pulse generator  230 . 
     FIG. 9  shows an exemplary timing diagram of some of the signals received by or generated by phase detector  260 , when signal Rdata lead signals and Rclk. As seen from this timing diagram, when signal Rdata goes high  305 , signal P_UP also goes high  310 . Signal P_UP remains high until signal Rclk goes high  315  to reset flip-flop  262 , thereby causing signal P_UP to go low  320 . Also, in response to the low-to-high transition  315  of signal Rclk, signal P_DN makes a low-to-high transition  325 . Signal P_DN remains in a high states until signal RDATA goes from high to low  330  to reset flip-flop  264 , thereby causing signal P_DN to go low  335 . In conventional Hogge type phase detectors, such as that shown in  FIG. 1 , the high-to-low transition  345  of a signal corresponding to signal P_DN (shown in  FIG. 8  as signal P_DN_PriorArt) occurs in response to a high-to-low transition  340  of signal Rclk. However, in accordance with the present invention, the high-to-low transition  335  of signal P_DN occurs in response to high-to-low transition  330  of signal Rdata. 
     FIG. 10  shows another exemplary timing diagram of some of the signals received by or generated by phase detector  260 , when signals Rdata lags signals and Rclk. As seen from this timing diagram, when signal Rdata goes high  405 , signal P_UP also goes high  410 . Signal P_UP remains high until signal Rclk makes a low-to-high transition  415  to reset flip-flop  262 , thereby causing signal P_UP to go low  420 . Also, in response to the low-to-high transition  415  of signal Rclk, signal P_DN makes a low-to-high transition  425 . Signal P_DN remains in a high states until signal Rdata goes from high to low  445 , at which point signal P_DN goes low  440 . In conventional Hogge type phase detectors, such as that shown in  FIG. 1 , the high-to-low transition  450  of a signal corresponding to signal P_DN (shown in  FIG. 10  as signal P_DN_PriorArt) occurs in response to high-to-low transition  435  of signal Rclk. However, in accordance with the present invention, the high-to-low transition  440  of signal P_DN occurs in response to high-to-low transition  445  of signal Rdata. 
   Unlike the prior art phase detectors which cause signal P_DN to become inactive when signal Rclk becomes inactive (e.g., when signal Rclk transitions from high to low), a phase detector in accordance with the present invention, causes signal P_DN to become inactive (e.g., from active high to inactive low) in response to transitions (e.g. from active high to inactive low level) of data signal Rdata. Accordingly, a phase detector in accordance with the present invention, causes transitions from the inactive levels (e.g., low) to the active levels (e.g., high) of signal Rclk to be positioned nearly at the center of the transitions of signal Rdata for RZ or pulse-position modulation data. Accordingly, a phase detector in accordance with the present invention, is immune to dependency of the duty cycle of the received clock signal and thus is adapted to restore the 50% duty cycle of the clock. 
   The present invention is adapted to be operative even in the absence of setup or hold times between the RDATA and RCLK signals. The present invention is further operative to detect phase differences if the pulse width of input signal RDATA signal is less than two periods of the clock. As is known, RZ or PPM data stream do not have pulse widths that are equal or longer than two periods of the clock. 
     FIG. 11A  shows that transition  525  on signal Rclk of phase detector  500  occurs near the center  530  of signal A. Accordingly, signals P_UP and P_DN generated by phase detector  500  have the same pulse width.  FIG. 11B  shows that transition  535  on signal Rclk of phase detector  500  occurs prior to the center  530  of signal A. Accordingly, signal P_UP generated by phase detector  500  has a smaller pulse width than signal P_DN.  FIG. 11C  shows that transition  545  on signal Rclk of phase detector  500  occurs after to the center  550  of signal A. Accordingly, signal P_UP generated by phase detector  500  has a greater pulse width than signal P_DN. 
   The above embodiments of the present invention are illustrative and not limitative. The invention is not limited by any particular arrangement of logic gates used to generate the phase signals. The invention is not limited by the logic level with which defines whether a signal is active or inactive. Thus, in some embodiments, a high logic level may be an active level while in other embodiments, a low logic level may be an active level. The invention is not limited by any particular combinatorial or sequential logic. Other additions, subtractions or modification are obvious in view of the present invention and are intended to fall within the scope of the appended claims.