Abstract:
A high-speed, half rate phase detector provides an effective solution to the problem of XOR gate response to the minimum width signal precursors (Q 1  and Q 2 ) of a phase signal that indicates a phase difference between a data signal and a clock signal by combining the precursor signals in a multiplexer and combining the multiplexed signal with the data signal in an XOR gate. This affords the transition information in the transitions of the precursor signals, which is significant of phase difference, without requiring the XOR gate to respond to the minimum widths of those pulses.

Description:
BACKGROUND 
   The invention concerns the operation and architecture of a linear half-rate phase detector, and more particularly concerns a half-rate phase detector for a phase-locked loop which produces a linear phase difference output signal by combining signals indicative of the phase relationship between a recovered clock signal and a received non-return to zero (NRZ) data signal, with a delayed version of the NRZ data signal. 
   Phase synchronization and clock recovery, an important component of digital communications, is a manifold process by which a receiver is synchronized to an incoming signal in order that the receiver be enabled to reliably extract clock information from the input signal. One communication component used for such synchronization is the phase-locked loop (PLL). In NRZ signaling schemes, a PLL is a servo loop which compares a version of a clock signal embedded in the NRZ signal with a version of the clock signal synthesized from the incoming signal by the PLL. The PLL operates to measure and correct phase difference between the two clock signals. A typical PLL architecture with a frequency aided acquisition loop is illustrated in  FIG. 1 . 
   In  FIG. 1 , a PLL  100  includes a frequency detector  102  and phase detector  103  as would be found, for example, in a digital receiver. The PLL  100  further includes a loop filter  104 , voltage controlled oscillator (VCO)  106  and frequency divider  109  with division value N. The PLL has an input  110  to receive a divided-down clock signal (VCODIVCLK) and an input  111  to receive a reference clock (RCLK), which aid in a frequency acquisition process. For phase acquisition, the PLL has two inputs,  112  and  113 , for respectively receiving an incoming data signal (DATA) and a synthesized clock signal (CLK) recovered by the PLL  100  from the incoming data signal. The frequency acquisition process is conducted as follows. The frequency detector  102  receives RCLK and VCODIVCLK (the clock output of the VCO  106 , divided by N) as inputs. The frequency detector  102  has UP and DOWN outputs. If the frequency of VCODIVCLK is higher than the frequency of RCLK, the average of the UP-DOWN outputs is negative. If the frequency of VCODIVCLK is lower than the frequency of RCLK, the average of the UP-DOWN outputs is positive. The UP-DOWN outputs are averaged by the loop filter  104 . In response to the UP-DOWN outputs, the loop filter  104  generates a voltage signal V control  that controls the response of the PLL  100  to indicated errors in frequency difference measured by the frequency detector  102 . The VCO  106  produces the CLK signal at a frequency determined by the voltage level of the V control  signal. The condition where the average of the UP-DOWN outputs is zero is referred to as “frequency lock”. At frequency lock the frequency of the CLK signal is exactly “N” times the frequency of RCLK. When frequency lock occurs, control of the frequency/phase lock process is passed to the phase detector  103 , which operates to acquire phase lock between the DATA and CLK signals, and to extract clock signal information from the DATA signal. In operation, the phase detector  103  produces a phase synchronization signal (PHASE) and a data reference signal (REF). The PHASE signal indicates the degree and direction of any phase difference between the DATA and CLK signals. The REF signal indicates the degree of synchronization between a data pattern, or symbol interval in the DATA signal and a corresponding integration interval in the PLL. The REF signal provides a reference comparison for the PHASE signal for different data patterns in the DATA signal. In a phase acquisition lock condition, the phase detector  103  produces a net average zero signal, wherein PHASE−REF=0. The loop filter  104  receives the PHASE and REF signals and generates the voltage signal V control  in response thereto. The V control  signal controls the response of the PLL  100  by indicating errors in phase between the DATA and CLK signals as measured by the phase detector  103 . The VCO  106  produces a synthesized recovered clock signal (CLK) on an output  108  that is aligned with the phase of the DATA signal (“phase aligned”) at a frequency determined by the voltage level of the V control  signal. The CLK signal is provided to the input of the frequency divider  109 , which divides it by N to produce VCODICCLK. The output is also connected to the input  113  of the phase detector  103  in order to provide the CLK signal as an input to the phase detector  103 . The phase detector  103  also responds to the DATA and CLK signals by extracting data from the DATA signals and providing the extracted data as a RECOVERED DATA signal on output. 
   The phase detector  103  of the PLL  100  may be practiced as a prior art phase detector  200 , illustrated in  FIG. 2 . Although illustrated as a discrete, or stand-alone apparatus comprising a combination of elements, the prior art phase detector  200  (and the novel phase detector later described) would preferably be found as a component of an integrated circuit (IC), often, although not necessarily, in combination with a frequency detector, manufactured using semiconductor technology, and intended for use in an integrated electronics appliance such as a receiver. 
   The phase detector  200  is a high-speed linear half rate phase detector. In this regard, it is high-speed in that it must be able to respond to pulse signals having pulse widths that may be as narrow as a few tens of picoseconds. The phase detector is said to be “half rate” in that the CLK signal is equal in frequency to the fundamental rate of the DATA signal. In this regard, there are “full rate” phase detectors that operate at twice the rate of an incoming data signal. These full rate phase detectors are manufactured using exotic semiconductor process technologies that produce very high speed devices capable of responding to pulse signals having pulse widths that are less than ten picoseconds wide; however, such devices use non-standard process technologies and are very expensive to integrate with standard process technologies. Half rate phase detectors, on the other hand, are manufactured using standard process technology. A linear half rate phase detector may have an architecture that is easy to integrate with larger chips while still delivering optimum speed and power performance comparable to full rate phase detectors. 
   Referring to  FIG. 2 , the phase detector  200  detects a phase difference between the DATA signal and the CLK signal recovered from the DATA signal. The phase detector  200  includes a first latch  202  and a second latch  204 . A latch is a data storage device that samples an input signal in response to a clock signal. In this regard, each of the latches  202  and  204  has an input (D) for a data signal, a input (CLK) for a clock signal. The output (Q) of each latch is enabled by a first transition of a CLK input such that the signal on the output follows (“samples”) the signal at the latch&#39;s input until the CLK input transitions at the transition (the second transition) immediately following the first transition. Following the second transition, the output stays at the level the input signal had at the second transition. The output of the first latch  202  is connected to a first input of a first exclusive-OR (XOR) gate  205 , and to the input (D) of a third latch  206 . The output of the second latch  204  is connected to a second input of the XOR gate  205 , and to the input (D) of a fourth latch  208 . The first XOR gate  205  produces the PHASE signal at its output. The output (Q) of the third latch  206  is connected to a first input of a second XOR gate  209 , and to the input of a buffer  210 . The output (Q) of the fourth latch  208  is connected to a second input of the second XOR gate  209 , and to the input of a buffer  211 . The second XOR gate  209  produces the REF signal at its output. The data in the incoming DATA signal is produced, in differential form, at the outputs of the buffers  210  and  211 . Although the circuit architecture of the phase detector  200  depicted in  FIG. 2  is single-ended, those skilled in the art will appreciate that corresponding circuit architecture could be deployed in differential form. 
   The operation of the prior art phase detector  200  is represented by the waveforms of  FIG. 3 . In  FIG. 3 , the labels on the several waveforms correspond with identical labels at various locations in the phase detector  200  of  FIG. 2  and represent the waveforms of signals at those locations. The CLK 1  signal that is provided to the CLK inputs of the first and fourth latches  202  and  208  may be derived, for example, from the CLK of the PLL  100  of  FIG. 1 . The inverse of the CLK 1  signal is provided to the CLK inputs of the second and third latches  204  and  206 . When phase lock occurs, one of the transitions of the CLK 1  signal is centered in a bit of the DATA signal. The half-rate architecture of the phase detector  200  uses the opposite transitions of the CLK 1  signal in the first and second latches to sample the input DATA signal in the latches  202  and  204 , causing the production of a signal Q 1  at the output of the first latch  202  and a signal Q 2  at the output of the second latch  204 . The Q 1  and Q 2  signals are precursors to the REF signal. The PHASE signal is generated by combining Q 1  and Q 2  signals in the first XOR gate  205 . Similarly, the opposite transitions of the CLK 1  signal are used to sample the outputs of the first and second latches, with their respective outputs Q 3  and Q 4  combined by the second XOR gate  209  to produce the REF signal. As shown in  FIG. 3 , the precursor signals Q 1  and Q 2  used to generate the REF signal exhibit minimum pulse widths when the DATA signal transitions at its highest possible rate. The minimum width pulse  212  of Q 1  is generated by transition  210  of CLK 1  signal and transition  208  of DATA signal. The minimum pulse width  213  of Q 2  is generated by transition  211  of CLK 1  signal and transition  209  of DATA signal. The first XOR gate  205  must respond to these minimum widths in order to faithfully track the difference in phase between the CLK signal and the incoming data signal. However, when these minimum widths begin to approach tens of picoseconds in width, the linear response of the first XOR gate  205  is severely degraded, limiting the linearity of the PHASE outputs in reaching full DC level for different DATA patterns. This contributes to phase offset errors and data pattern dependent phase offsets. This limitation on the performance of the XOR gate leads to an advancing reduction in the accuracy with which the PHASE signal represents the actual phase difference being measured. This limits the linear range of operation and the jitter tolerance of the half-rate phase detector  200 . 
   SUMMARY 
   A high-speed, half rate phase detector provides an effective solution to the problem of XOR gate response to the minimum width of the PHASE signal precursors (Q 1  and Q 2 ) by combining those signals in a multiplexer and combining the multiplexed signal with the incoming delayed DATA signal in an XOR gate. This yields the transition information in the transitions of the precursor signals, which is significant of phase difference, without requiring the XOR gate to respond to the minimum pulse widths of those signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating the architecture of a phase-locked loop. 
       FIG. 2  is a logic diagram illustrating the architecture of a prior art half-rate phase detector. 
       FIG. 3  is a set of waveform diagrams illustrating the operation of the prior art half-rate phase detector of  FIG. 2 . 
       FIG. 4  is a logic diagram of an embodiment of a half-rate phase detector according to the invention. 
       FIG. 5  is a set of waveform diagrams illustrating the operation of the half-rate phase detector of  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   A novel phase detector is illustrated in one or more of the above-described drawings, and is disclosed in detail in the following description. Although these illustrations and the description may show and describe elements that constitute a preferred embodiment and a best mode of practicing the invention, they are not intended to foreclose other equivalent implementations of the invention. 
   The invention is a high speed linear half-rate phase detector in which a PHASE signal indicative of a difference in phase between a data signal and a clock signal is generated by combining two precursors of the PHASE signal and subjecting the result to exclusive disjunction with a delayed version of the data signal in an exclusive-OR gate. 
   Although the half-rate phase detector to be described is illustrated as a circuit or device composed of discrete elements, the expected mode of deployment or utilization would be in combination with other circuits. Thus, the invention is most likely to be found as a component of an integrated circuit (IC), often, although not necessarily, in combination with a frequency detector, manufactured using semiconductor technology, and intended for use in an integrated electronics appliance such as a receiver. 
   The invention is illustrated in the logic circuit of  FIG. 4 , in which a half-rate phase detector according to the invention receives a data signal (DATA) and responds to various versions of a clock signal (CLK) in measuring a phase difference between the data signal and the clock signal. In this example, and in many implementations of the invention, the clock signal is recovered from the data signal, as would occur in the case where the phase detector is a component of a phase-locked loop. 
   The data signal is received on an input  402 . The input  402  feeds the data signal to a buffer  403  which provides a first delayed version of the data signal as DATAR on signal line  404 . The output of the buffer  403  is connected via signal line  404  to the input of a buffer  405  which provides a second delayed version of the data signal as DATAD on signal line  406 . The clock signal is received on an input  410  which is connected to the inputs of buffers  411   a  and  411   c . The buffer  411   a  provides a first delayed version of the clock signal as CLK 1 . The output of the buffer  411   a  is connected to the input of a buffer  411   b  which provides a second delayed version of the clock signal as CLK 2 . The buffer  411   c  provides a third delayed version of the clock signal as CLK 3 , and its output is connected to the input of the buffer  411   d , which provides a fourth delayed version of the clock signal as CLK 4 . Although not shown explicitly in  FIG. 4 , it is asserted that the buffers  411   a ,  411   b ,  411   c , and  41  id also provide the inverse or complementary forms of the respective delayed versions of the clock signal. 
   The output of the buffer  403  is connected to the data (D) inputs of first and second latches  415  and  419 , which together comprise a first latch circuit that combines DATAR with alternate transitions of CLK 1  to produce the precursor signals Q 1  and Q 2 . The output of the latch  415  is connected through a buffer  416  to a signal line  417 . The latch  415  has a clock input (CLK) which receives CLK 1 . The output of the latch  419  is connected through a buffer  420  to a signal line  421 . The latch  419  has a clock input (CLK) which receives the inverse of CLK 1 . A multiplexer  423  has a first input (0) connected to the signal line  417 , a second input (1) connected to the signal line  421 , and an output (Q). The multiplexer  423  has a control input for receiving CLK 2 . As seen in  FIG. 5 , when CLK 2  transitions positively, at  502 , for example, the multiplexer  423  connects its input (1) to its output (Q); when CLK 2  transitions negatively, as at  503 , the multiplexer  423  connects its input (0) to its output (Q). A first exclusive-OR (XOR) gate  425  has a first input (I 1 ), a second input (I 2 ) and an output (Q). The first input of the XOR gate  425  is connected via signal line  424  to the output of the multiplexer  423 ; the second input of the XOR gate  425  is connected to the signal line  421 , and thereby to the output of the buffer  420 . 
   Refer now to  FIGS. 4 and 5  for an understanding of how the half-rate phase detector of the invention generates, or produces a PHASE signal indicative of a linear phase difference between the data and clock signals. The latches  415  and  419  sample DATAR at their respective inputs in response to successive opposite transitions of CLK 1 . These latches produce the PHASE precursor signals Q 1  and Q 2  through the buffers  416  and  420 , respectively. These may be denoted as first precursor signals. The multiplexer  423 , controlled by CLK 2 , multiplexes the first precursor signals, producing a multiplexed signal Q 12 . The multiplexed signal Q 12  is combined with DATAD by an exclusive disjunction (exclusive-OR operation) performed by the XOR gate  425 , which yields the PHASE signal. As can be appreciated with reference to  FIG. 5 , the control of the multiplexer  423  by CLK 2  produces a signal Q 12 . The rising edge of Q 12  is generated by the rising edge of CLK 2  that samples the region  513   a  in  FIG. 5 . The falling edge of Q 12  is generated by the falling edge of CLK 2  that samples the region  512   a  in  FIG. 5 . As seen in  FIG. 5 , the sampled regions  512   a  and  513   a  are of wider pulse width than the minimum possible pulse widths of the precursor signals. Thus, although the minimum pulse widths  512  and  513  of the precursor signals Q 1  and Q 2  are produced in the phase detector of  FIG. 4 , the critical transitions of those signals are produced in the transitions  515  and  516  of the multiplexed signal Q 12  on a pulse of greater width. This provides the transition information to the first XOR gate  425  without the burden imposed by the minimum pulse widths of Q 1  and Q 2 . 
   The half-rate phase detector of  FIG. 4  generates the REF signal by means of third and fourth latches  430  and  432 , which together comprise a second latch circuit that combines the first precursor signals Q 1  and Q 2  with alternate transitions of CLK 2  to produce second precursor signals Q 3  and Q 4  from which the reference signal REF is derived. The latch  430  has a data input (D) connected to the output of the latch  415  via signal line  417  and inverter  416 , a clock input for receiving the inverse form of CLK 2 , and an output (Q) connected via signal line  431  to the first input (I 1 ) of a second XOR gate  434 . The latch  432  has a data input (D) connected to the output of the latch  419  via signal line  421  and inverter  420 , a clock input for receiving the positive form of CLK 2 , and an output (Q) connected via signal line  433  to the second input (I 2 ) of the second XOR gate  434 . The XOR gate  434  has an output (Q) connected to a signal line  435 . As may be understood with reference to  FIGS. 4 and 5 , the latch  430  samples Q 1  in response to the inverse form of CLK 2  to produce Q 3 , and the latch  432  samples Q 2  in response to the positive form of CLK 2  to produce Q 4 . The precursor signal Q 3  is combined with the precursor signal Q 4  by an exclusive disjunction (exclusive-OR operation) performed by the XOR gate  434 , which yields the reference signal. 
   Data is decoded from the data signal by a third latch circuit including fifth, sixth, and seventh latches  440 ,  442 , and  443 , which provide a decoded data signal DATA_OUT 1  having a first (preferably, odd) bit polarity, and eighth and ninth latches  450  and  452 , which provide a decoded data signal DATA_OUT 0  having a second (preferably, even) bit polarity. In this regard, the latch  440  has a data input (D) on which it receives DATAR on signal line  404 , a clock input for receiving CLK 3 , and an output (Q) connected to the input of a buffer  441 . The latch  442  has an input (D) connected via the buffer  441  to the output of the latch  440 , a clock input for receiving the inverse form of CLK 4 , and an output. The latch  443  has an input (D) connected to the output of the latch  442 , a clock input for receiving the positive form of CLK 4 , and an output (Q) connected to the signal line  444 . The latch  450  has a data input (D) on which it receives DATAR on signal line  404 , a clock input for receiving the inverse form of CLK 3 , and an output (Q) connected to the input of a buffer  451 . The latch  452  has an input (D) connected via the buffer  451  to the output of the latch  450 , a clock input for receiving the positive form of CLK 4 , and an output (Q) connected to the signal line  453 . 
   Additional desirable features include two offset features incorporated into the buffer  411   c  that produces CLK 3 , and the second XOR gate  434  that produces REF. Either feature, or both features, may be operated by a user to fine tune the operation of the half-rate phase detector. The PHASE_OFFSET_CONTROL input to the buffer  411   c  on signal line  412  enables a user to independently offset the phase of the clock signal CLK 3  from the center of a data pulse DATAR in order to optimize jitter tolerance of the data decoding operation of the third latch circuit without affecting the control operation of a phase locked-loop incorporating the phase detector. The DC_OFFSET_CONTROL input to the XOR gate  434  on signal line  436  enables a user to change or adjust the DC voltage reference with which the second XOR gate  434  operates. This symmetrically adjusts the current source ratios of the XOR gate  434  without affecting the linear phase transfer curve, enabling control of DC offset of the XOR gate  434  to near zero. This enhances the ability of the half-rate phase detector to tolerate large numbers of successive identical patterns in the data signal as smaller offsets are integrated by a PLL filter. Both of the PHASE_OFFSET_CONTROL and DC_OFFSET_CONTROL signals are preferably provided as dc voltages and user programmable control signals.