Abstract:
The present invention relates generally to the field phase detectors and particularly to a phase detector being formed by a flip-flop.  
     Phase detectors being formed by a flip-flop are known. However, for the realization of the known phase detectors several parameters have to be dimensioned individually in order to achieve an optimized phase detector.  
     With the present invention it is no longer necessary to explicitly dimension these parameters.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims priority of European Patent Application No. 98306189.6, which was filed on Aug. 4, 1998.  
         TECHNICAL FIELD  
         [0002]    The present invention relates generally to the field phase detectors and particularly to a phase detector being formed by a flip-flop.  
         BACKGROUND OF THE INVENTION  
         [0003]    Phase detectors realized with a flip-flop are known. The patent DE 40 16 429 C2, which is incorporated by reference herein, shows a phase detector being realized with a D-flip-flop. The feedback clock is coupled to a clock input of the flip-flop. The reference clock is coupled via a mono-flop, which generates a pulse, to an asynchronous reset input of the flip-flop. At the output of the flip-flop a control signal is available, for controlling e.g. a voltage controlled oscillator. The data input of the flip-flop is coupled to the inverted output of the flip-flop. Pig.  1  shows a phase locked loop (PLL) having the known phase detector. The PLL consists of a voltage controlled oscillator  1  including a PLL filter, a D-flip-flop  2  forming the known phase detector, a low pass filter  3  and a mono-flop  4 . A clock signal FB produced by the voltage controlled oscillator  1 , being the feedback clock, is coupled to a clock input C of the flip-flop  2  and a pulse RES, derived e.g. from the rising edge of a reference clock REF by mono-flop  4 , is coupled to an asynchronous reset input R of the flip flop  2 . A data input D of the flip-flop  2  is coupled to an inverse output Q′ of the flipflop  2 . Therefore, an output signal OUT at output Q of the flip-flop  2 , forming a control signal, goes to a high level, e.g. a system voltage, with the rising edge of the feedback clock FB and goes to a low level with the rising edge of the reference clock REF.  
           [0004]    The output signal OUT is filtered by the low pass filter  3  consisting of a resistor R 1  and a capacitor C 1 , and forms a control signal OUT 3  for the voltage controlled oscillator  1 , which corresponds to the mean value of the output signal OUT, formed by the low pass filter  3 . A useful nominal operating point is given by a duty cycle of 1:1. Other operating points are also possible. Phase delay between the reference clock REF and the feedback clock FB is then given with a phase of π in a locked high gain PLL circuit.  
           [0005]    In case of a reference clock REF failure the signal RES goes inactive, the flip-flop  2  works as a divider by two. Therefore the output signal OUT has a duty cycle of exactly 1:1, i.e. the phase detector  2  works at the nominal operating point. For that reason the phase detector formed by flip-flop  2  is self biasing in case of loss of the reference clock REF.  
           [0006]    [0006]FIG. 2 shows the transfer function of the phase detector, i.e. the phase deviation φ of reference clock REF to feedback clock FB versus the filtered phase detector output signal OUT 3 . The phase detector uses almost the whole phase deviation from O to 2π for detection. There is only a small and well defined dead-zone Z caused by the pulse width of the reset pulse RES derived from the reference clock REF by mono-flop  4 . As long as the reset pulse RES is active, the feedback clock FB on the clock input C of the flip-flop  2  cannot set the output Q of the flip-flop  2 . Therefore, the state of output Q is well defined under all conditions, including a phase difference 0 of reference and feedback clock.  
           [0007]    However, the proposed generation of the reset pulse RES for the phase detector flip-flop  2  has the disadvantage, that the mono-flop  4  has to be dimensioned in a way that the reset pulse RES generated is neither to short nor to long. If it is to short the phase detector flip-flop  2  will not be reset. If it is too long the dead-zone Z will be unnecessarily long. In addition it has to be secured, so that no matter how the reference clock signal REF fails—static high or static low, the signal at the reset input R of the phase detector flip-flop  2  goes inactive (low).  
           [0008]    Another drawback of the known phase detector flip-flop  2  is that a skew (phase error) is present. The skew depends on the different delay times of the clock to data valid transition, i.e. the delay caused by the phase detector flip-flop  2  after a rising edge of the feedback clock FB, and of the reset to data valid transition, i.e. the delay caused by the mono-flop  4  and the phase detector flip-flop  2  after a rising edge of the reference clock REF.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, it is an object of the present invention to provide a phase detector being formed by a flip-flop. It is the aim of the inventive phase detector under consideration to avoid the drawbacks known from the state of the art.  
           [0010]    The object is achieved by providing a phase detector having a D-flip-flop with a first output for a control signal, a first input for a feedback clock, a second input for a pulse, generated from a reference clock and a third input to which a second output is coupled, by a gate for generating the pulse from the reference clock and that an output of said gate is coupled back to an input of said gate.  
           [0011]    An advantage of the present invention is that it provides a reset pulse for a phase detector being formed by a D-flip-flop that has an optimum width, without the necessity of explicitly dimensioning it. Another advantage of the present invention is that it allows the design of a skewless phase detector.  
           [0012]    The present invention will become more fully understood from the detailed description given hereinafter and further scope of applicability of the present invention will become apparent. However, it should be understood that the detailed description is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The following detailed description is accompanied by drawings of which  
         [0014]    [0014]FIG. 1 is a schematic diagram of a phase detector known form the state of the art,  
         [0015]    [0015]FIG. 2 is a graph of the transfer function of the phase detector of FIG. 1,  
         [0016]    [0016]FIG. 3 is a schematic diagram of a first embodiment of a gate for generating a reset pulse for the phase detector according to this invention,  
         [0017]    [0017]FIG. 4 is a schematic diagram of a second embodiment of a gate for generating a reset pulse for the phase detector according to this invention, and  
         [0018]    [0018]FIG. 5 is a schematic diagram of a skewless phase detector according to this invention. 
     
    
       [0019]    Identical denotations in different Figures represent identical elements.  
       DETAILED DESCRIPTION  
       [0020]    Depicted in FIG. 1 is a phase locked loop as described in the opening portion having a phase detector formed by a D-flip-flop  2 , a voltage controlled oscillator  1 , a PLL-filter  3  and a gate  4  for generating a reset pulse RES from a reference clock REF. The reset pulse RES is coupled to a reset input R of the flip-flop  2 . The function of gate  4  for generating the reset pulse RES according to this invention will now be explained for two embodiments with reference to FIG. 3 and  4 .  
         [0021]    [0021]FIG. 3 shows a schematic diagram of a first embodiment of a gate  4  for generating a reset pulse RES from the reference clock REF. Gate  4  for generating the reset pulse RES is formed by a D-flip-flop. The reference clock REF is coupled to a clock input C of the flip-flop  4 . A high level, e.g. a system voltage VCC, is coupled to a data input D of the flip-flop  4 . At an output Q of the flip-flop  4  the reset pulse RES is available. The output Q is also coupled back to a reset input R of the flip-flop  4 . The back coupled output Q therefore immediately resets the flip-flop  4  each time a high signal is present at the output Q, i.e. after a rising edge of the reference clock. In that way a reset pulse RES is produced having a width equivalent to the delay time of the flip-flop  4 .  
         [0022]    [0022]FIG. 4 shows a schematic diagram of a second embodiment of a gate  4  for generating a reset pulse RES from the reference clock REF. Gate  4  for generating the reset pulse RES is formed by an AND-gate. The reference clock REF is coupled to a first input of AND-gate  4 . The output signal OUT of the phase detector flip-flop  2  of FIG. 1 is coupled to a second input of the AND-gate  4 . At an output of the AND-gate  4  the reset pulse RES is available. The back coupled output signal OUT of the phase detector flip-flop  2  thus is “anded” with the reference signal REF. The output of the AND-gate  4  goes high with REF going high and forms the reset pulse RES that resets the phase detector flip-flop  2 . After reset of the phase detector flip-flop  2  the output signal OUT of the phase detector flip-flop  2  goes low and thus the output of the AND-gate  4 . In that way a reset pulse RES is produced having a width equivalent to the delay time of the phase detector flip-flop  2  and the delay time of the AND-gate  4 .  
         [0023]    As can be seen from the two different embodiments explained above, it is possible by coupling back the output signal RES of gate  4  or a signal derived from the output signal RES to an input of gate  4 , to produce a reset signal RES having an optimum reset pulse width. Because the components of gate  4  are realized in the same technology as used as for the components of the phase detector flip-flop  2 , the pulse width has the minimum width necessary for the reset of the phase detector flip-flop  2 .  
         [0024]    Now reference is made to FIG. 5 which shows a schematic diagram of a skewless phase detector, having a phase detector D-flip-flop  2 , a D-flip-flop  4  for generating the reset pulse RES form the reference clock REF, a third D-flip-flop  5 , an AND-gate A 1  having an inverted input and two AND-gates A 2  to A 3 . Flip-flop  2  is the phase detector flip-flop as known from the above explanations. Flip-flop  4  generates the reset pulse RES for the phase detector flip-flop  2  from the rising edge of the reference clock REF. With the rising edge of the reference clock REF, the signal RES goes high. As the output Q of the flip-flop  4  is directly fed back to the reset input R, the output will go low as soon as the flip-flop  4  goes high.  
         [0025]    The pulse width of the reset signal RES only depends on the reset input R to output Q data valid delay time. Flip-flop  5 , which data input D and clock input C are coupled to a low level, is used to minimize the phase detector output skew. Looking at the phase detector flip-flop  2 , the output skew depends on the different delay times of ‘clock to data valid’ (C 2 D) and ‘reset to data valid’ (R 2 D). The delay time for the rising edge of the reference clock REF to the falling edge of the output signal OUT (REF 2 OUT) is the sum of the delay time of AND-gate A 3  and the delay times of  15  ‘clock to data valid’ and ‘reset to data valid’ (A 3 +R 2 D+C 2 D). In order that the delay time of the rising edge of the feedback clock FB to the rising edge of the output signal OUT (FB 2 OUT) is equal to the delay time REF 2 OUT, a delay time R 2 D+A 3  has to be added to the path of the feedback clock FB (FB 2 OUT). This is achieved with the help of flip-flop  5 . Assuming that the ‘set to data valid’ (S 2 D) delay equals the ‘reset to data valid’ delay (R 2 D), flip-flop  5  adds exactly the same delay time. The output of flip-flop  5  is set by the rising edge of the feedback clock FB and reset by coupling back outputs Q and Q′ of flip-flop  5  via AND-gates A 2  and A 1  to the set and reset inputs S and R respectively. The additional AND-gate A 3  is used only to delay the reference clock REF to compensate for the delay time of AND-gate A 2  in the path of the feedback clock FB.  
         [0026]    The phase detectors as explained above can be implemented as integrated circuits (ICs) or with discrete components. The logic structure of the phase detector guarantees, that the delays of the signal paths of the input signals FB and REF, caused by the phase detector, will be equalized. If it is assured that like logic elements have like signal delays, the delays for the input signals FB and REF are equalized. For achieving maximum advantage of the phase detector as explained above, it should be implemented in a monolithic structure, e.g. ASIC, PLD, FPGA etc. In that case no additional efforts, like defining timing constraints will be necessary, as the skew of a phase detector realized in a monolithic structure is already minimized.  
         [0027]    For reason of testability it sometimes is desirable to test the characteristics of the phase detector in the absence of the reference clock REF. This can easily be achieved for a phase detector having a gate  4  as shown in FIGS.  3  to  5  by coupling a low level signal to input VCC. For a phase detector having a gate  4  as shown in FIG. 4, this can be easily achieved by using an AND-gate having an additional input. In operational mode a high level signal is coupled to the additional input, whereas in test mode a low level signal is coupled to the additional input.  
         [0028]    As explained in the incorporated document DE 40 16 429 C2, instead of the asynchronous reset input R used with the phase detector flip-flop  2  also an asynchronous set input can be used.  
         [0029]    In addition it should be understood that a phase detector as explained above also could be realized by inverting all logical levels and inputs of the components used to achieve the same operability.