Abstract:
In general, in one aspect, an apparatus includes a phase frequency detector, a charge pump, a voltage controlled oscillator, an integral capacitor to maintain an integral charge and provide an integral voltage, and a mutual-charge canceling sample reset (MCSR) capacitor to maintain a proportional charge and provide a proportional voltage each reference clock cycle. The MCSR includes a first proportional capacitor, a second proportional capacitor in parallel to, and having substantially identical capacitance value as, the first proportional capacitor, a first set of switches to provide direct coupling of the first and second proportional capacitors, and a second set of switches to provide cross coupling of the first and second proportional capacitors. The first and second set of switches alternatively turn on and off every reference clock cycle so that set of switches coupling the first and second proportional capacitors alternates every reference clock cycle.

Description:
BACKGROUND 
   The phase-locked loop (PLL) is a versatile electronic circuit used in a wide variety of applications, including frequency synthesis, clock recovery, clock multiplication, and clock regeneration. In large, high-speed integrated circuits (including application-specific integrated circuits, field-programmable gate arrays, network processors, and general purpose microprocessors), PLLs have become commonplace. On-chip phase-locked loop clock multipliers are used on these chips to generate a high-frequency clock signal that is a multiple of, and in phase with, a system clock or I/O clock. PLLs may also be used on these chips to resynchronize and realign clocks in deep clock distribution trees to reduce clock skew. 
   PLLs utilize a phase frequency detector (PFD) to compare a reference clock to a clock generated by a voltage controlled oscillator (VCO) and feed back to the PFD. Reference clock feedthrough degrades high frequency phase noise performance of PLLs so that they achieve worse short term clock jitter. PLLs may implement sample-reset techniques to improve the reference clock feedthrough performance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the various embodiments will become apparent from the following detailed description in which: 
       FIG. 1  illustrates an example mutual-charge canceling sample-reset (MCSR) loop filter phase locked loop (PLL), according to one embodiment; 
       FIG. 2  illustrates an example timing diagram of an MCSR capacitor, according to one embodiment; 
       FIG. 3  illustrates an example operational implementation of the MCSR capacitor, according to one embodiment; 
       FIG. 4  illustrates an example MCSR loop filter PLL, according to one embodiment; and 
       FIG. 5  illustrates an example MCSR loop filter PLL, according to one embodiment. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an example mutual-charge canceling sample-reset (MCSR) loop filter phase locked loop (PLL)  100 . The MCSR loop filter PLL  100  includes a phase-frequency detector (PFD)  110 , an integral charge pump (CPi)  120 , a proportional charge pump (CPp)  130 , an integral capacitor (Ci)  140 , an MCSR capacitor  150 , a voltage combiner  160 , a voltage controlled oscillator (VCO)  170 , and a divider  180 . 
   The PFD  110  compares the phase-frequency of a feedback clock to a reference clock. The feedback clock is the signal from the VCO  170  divided by the divider  180 . The PFD  110  generates UP or DOWN pulses based on a comparison of the feedback clock and the reference clock. The generation of the UP or DOWN pulses is dependent on the VCO  170  design (e.g., what control voltage is referenced to). The pulse width of the UP or DN pulses are based on the amount of lead or lag between the clocks. 
   The CPi  120  provides or withdraws integral charge (Qi) into or from the Ci  140  according to the UP or DN signals received from the PFD  110 . The amount of the Qi driven by the CPi  120  is based on the pulse width of the UP or DN signal. The Ci  140  keeps integrating the Qi from the CPi  120 , and generates an integral voltage (Vi) corresponding to the entire charge accumulated in the Ci  140 . 
   The CPp  130  provides or withdraws proportional charge (Qp) into or from the MCSR capacitor  150  according to the UP or DN signals received from the PFD  110 . The amount of the Qp driven by the CPp  130  is based on the pulse width of the UP or DN signal. The MCSR capacitor  150  includes a primary proportional capacitor (Cp 1 )  152 , a secondary proportional capacitor (Cp 2 )  154 , and first and second switch pairs S 1 , S 2 . The Cp 1  and Cp 2   152 ,  154  are in parallel and should be of equal (or substantially equal) capacitance value. The first switch pair S 1  provides a direct parallel coupling of the Cp 1   152  and the Cp 2   154  while the second switch pair S 2  provides a reversed parallel coupling of the Cp 1   152  and the Cp 2   154 . 
   The MCSR capacitor  150  receives and captures the Qp and generates a proportional voltage (Vp) based thereon. The Qp is captured in the MCSR capacitor  150  by exclusively turning on one of the first and second switch pairs S 1 , S 2 . The Qp is maintained in the MCSR capacitor  150  and the Vp is valid for one reference clock cycle. Both the first and second switch pairs S 1 , S 2  remain turned off after capturing the Qp. After every Qp maintenance cycle (e.g., one reference clock cycle), the first and second switch pairs S 1 , S 2  provide a discharging path to the Cp 1   152  and the Cp 2   154  by commutating the polarity of the Cp 2   154 . The discharging process occurs simultaneously (or substantially simultaneously) with the next cycle of capturing the Qp. The MCSR capacitor  150  provides a sample-reset scheme by mutually cancelling charges with the two equal (or substantially equal) proportional capacitors (Cp 1   152 , Cp 2   154 ) on alternating cycles of the reference clock. 
   The operation of the MCSR capacitor  150  provides a proportional path gain in the form of the Vp by means of the Cp 1   152  and the Cp 2   154  responding to the Qp injected from the CPp  130 . The MCSR capacitor  150  determines the proportional path gain as the amount of time the Qp is held in the MCSR capacitor  150 , such that the proportional path gain can be linearly scaled by Tref/Cp, where Tref is the time period of the reference clock (input signal to the MCSR loop filter PLL  100 ) and Cp is the total capacitance in the parallel connection of the Cp 1   152  and the Cp 2   154 . 
   The voltage combiner  160  sums the Vi and the Vp and provides the sum to the VCO  170 . The VCO  170  adjusts the clock generated based thereon. 
   The nature of discrete time processing of the MCSR capacitor  150  enables the MCSR loop filter PLL  100  to achieve a high degree of reference clock feedthrough rejection. The MCSR loop filter PLL  100  realizes a pure second order system. The MCSR loop filter PLL  100  may provide separate Vi and/or Vp and the Vi and/or the Vp may optionally be fed (dotted lines indicate optional) separately to the VCO  170  and/or the Vi may optionally be fed to the CPi  120  and/or the CPp  130  for specific PLL architectures (e.g., self-biased PLL) or VCO architectures (e.g., symmetric load ring oscillator). 
     FIGS. 2 and 3  in conjunction with one another illustrate the example operation of a MCSR capacitor ( 150  of  FIG. 1 ).  FIG. 2  illustrates an example timing diagram depicting the activation of the switch pairs S 1 , S 2  based on comparison of the reference signal and feedback signal and the PFD signals (UP, DN) generated therefrom.  FIG. 3  illustrates an example status of the switch pairs S 1 , S 2  at the different times identified and the operation of the Cp 1  ( 152 ) and the Cp 2  ( 154 ) based thereon. It should be noted that the generation of the UP and DOWN signals based on the clock comparison in  FIG. 2  could be changed based on implementation of the VCO generating the feedback signal without departing from the scope. 
   At point (a) the rising edge of the feedback clock is detected prior to the reference clock so the PFD (e.g.,  110  of  FIG. 1 ) initiates an UP signal. At this point, both switch signals remain inactive so that both the switch pairs S 1 , S 2  are off, and all of the charge (Qe) from the CPp (e.g.,  130 ) is injected into the Cp 1 . At point (b) the rising edge of the reference signal is detected and the PFD deactivates the UP signal and the first switch pair S 1  are activated (closed). The activation of the first switch pair S 1  provides a connection of the Cp 2  to the Cp 1 . When the Cp 2  is bridged to the Cp 1 , the Qe is shared among them so that each has half the Qe (Qe/2). It should be noted that Qe corresponds to Qp in  FIG. 1 . 
   During a first period (c) the falling edge of the reference clock is detected and the first switch S 1  signal is deactivated. Both the feedback and the reference clock signals are inactive so that the PFD is not generating any UP or DN signals and both switch signals are inactive so that both the switch pairs S 1 , S 2  are off. When both the switch pairs S 1 , S 2  are open the Cp 2  is isolated from the Cp 1  so that the charge (Qe/2) captured in each is maintained. At point (d) the rising edge of the reference clock is detected so the PFD activates a DN signal and the second switch pair S 2  is activated (closed). When the second switch pair S 2  is closed, the polarity of the Cp 2  is reversed and attached to the Cp 1 . Changing the polarity and bridging enables the charge (Qe/2) trapped in the Cp 1  and the Cp 2  to cancel out. Meantime, the Cp 1  and the Cp 2  are recharged based on the charge from the CPp. 
   During a second period (c) the falling edge of the reference clock is detected and the second switch pair S 2  is deactivated (open) so that both the switch pairs S 1 , S 2  become open. Deactivating both the switch pairs S 1 , S 2  separates the Cp 2  from the Cp 1 , and the charge (Qe/2) stored in each is maintained. It should be noted that the second period (c) is not illustrated separately in  FIG. 3 . At point (e) the rising edge of the reference clock is detected so the PFD activates a DN signal and the first switch pair S 1  is closed. When the first switch pair S 1  is closed, the polarity of the Cp 2  is reversed and attached to the Cp 1 . Changing the polarity and bridging enables the charge (Qe/2) trapped in the Cp 1  and the Cp 2  to cancel out. Meantime, the Cp 1  and the Cp 2  are recharged based on the charge from the CPp. 
   It should be noted that between the point (a) and the point (b) when both switch pairs S 1 , S 2  are open that the instantaneous proportional voltage (Vp) may become double the expected value (e.g., 2Vp). Once one of the switch pairs S 1 , S 2  turns on, the resetting and charge sharing between the Cp 1  and the Cp 2  occurs. The temporal doubling of the proportional control voltage may incur instantaneous phase error in the PLL (though its negative impact is negligible in steady state). If both switch pairs S 1 , S 2  were accidentally turned on simultaneously the PLL may incur stability issues and increase uncertainty in the loop dynamics. In order to prevent this occurrence, between sampling operations the Cp 2  may be disconnected from the Cp 1 . The isolation of the Cp 2  does not change the Vp held in the Cp 1  so the MCSR capacitor  150  is able to maintain the Vp for the entire reference clock cycle. 
     FIG. 4  illustrates an example MCSR loop filter PLL  400  having a single CP  410  and the Ci  140  in series with the MCSR  150 . The CP  410  receives the UP and DN signals from the PFD  110  and provides an injection charge (Qe) based thereon. Since both the Ci  140  and the MCSR capacitor  150  are driven by the single CP  410 , the Qe corresponds to both the Qi and the Qp of  FIG. 1 . Stacking the Ci  140  and the MCSR capacitor  150  in series realizes the voltage combining operation without a voltage combiner (e.g.,  160 ). The reduction in the number of CPs (2 to 1) and the absence of the voltage combiner save power dissipation and area. 
   The MCSR loop filter PLL  400  may optionally provide a separate Vi that may be fed (dotted lines indicate optional) to the CP  410  and/or the VCO  170  for specific PLL architectures (e.g., self-biased PLL) or certain VCO architectures (e.g., symmetric load ring oscillator). 
   The loop dynamics of the MCSR loop filter PLL  400  is mainly determined by ratio of the Vi to the Vp. The ratio of the capacitance of the Ci  140  to the total capacitance of the Cp 1   152  and the Cp 2   154  corresponds to the ratio of the Vi to the Vp. As the proportional path gain is normally much higher than the integral path gain the Cp 1   152  and the Cp 2   154  are required to be much smaller than the integral Ci  140 . Therefore the capacitors may become a design limiter. 
     FIG. 5  illustrates an example MCSR loop filter PLL  500  having a second CP  510 . The second CP  510  receives the UP and DN signals from the PFD  110  and provides a charge (Qesub) based thereon in the opposite direction of the Qe from the first CP  410 . The Qesub from the second CP  510  is a fraction of the Qe from the first CP  220 . The second CP  220  reduces the overall charge (Qe-Qesub) into the Ci  140  and reduces the integral path gain in form of an integral voltage (Vi) without increasing the size of the integral capacitor Ci  140 . The second CP  510  may be utilized to bypass some portion of the Qe from the first CP  410  and accordingly adjust the ratio of the amount of charge in (and associated size of) the Ci  140  and the Cp 1   152  and the Cp 2   154 . Utilizing the second CP  510  provides more design flexibility in the selection of the size of the capacitors Ci  140 , Cp 1   152 , Cp 2   154 . The use of the second CP  510  may result in extra power dissipation. 
   For example, the use of the second CP  510  may enable the MCSR loop filter PLL  500  to be implemented with relatively uniform size capacitors Ci  140 , Cp 1   152 , Cp 2   154 . The use of uniform size capacitors Ci  140 , Cp 1   152 , Cp 2   154  may improve device matching and result in higher yields. 
   The MCSR loop filter PLL  500  may optionally provide separate Vi that may be fed (dotted lines indicate optional) to the first CP  410 , the second CP  510  and/or the VCO  170  for specific PLL architectures (e.g., self-biased PLL) and/or VCO architectures (e.g., symmetric load ring oscillator). 
   Although the disclosure has been illustrated by reference to specific embodiments, it will be apparent that the disclosure is not limited thereto as various changes and modifications may be made thereto without departing from the scope. Reference to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described therein is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
   The various embodiments are intended to be protected broadly within the spirit and scope of the appended claims.