Abstract:
A method and apparatus for linearizing a phase locked loop (PLL) are provided. To accomplish this, three separate signal (two feedback/one reference or two reference/one feedback) are applied to two phase/frequency detectors (PFDs). Either an edge of the one reference signal or one feedback signal is approximately equidistant between corresponding edges of the two feedback or two reference signals so that the PFDs can properly apply actuation signals to a charge pump that account for jitter. Thus, a more linear PLL is provided.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to phase locked loops (PLLs) and, more particularly, to charge pump linearization for delta-sigma fractional-N PLLs. 
       BACKGROUND 
       [0002]    For phase detectors used in PLLs, linearity can be important. Nonlinearities can lead to large reference spurs or increased in-band reference noise. Linearization techniques for PLLs, though, have been developed over the years. Two examples are: Temporiti, E. et al., “A 700-kHz Bandwidth ΣΔ Fractional Synthesizer With Spurs Compensation and Linearization Techniques for WCDMA Applications,”  IEEE Journal of Solid-State Circuits,  Vol. 39, September 2004 (“Temporiti”); and Pamarti, S. et al., “A Wideband 2.4-GHz Delta-Sigma Fractional-N PLL With 1-Mb/s In-Loop Modulation”,  IEEE Journal of Solid-State Circuits,  Vol. 39, January 2004 (“Pamarti”). Some other additional example of conventional PLLs can be found in U.S. Pat. Nos. 6,236,703; 6,960,947; 7,005,928; 7,142,025; U.S. Patent Pre-Grant Publ. No. 2004/0223576, and European Patent Appl. Nos. EP1458099; EP1458099; EP1458100; and EP 1458101. However, each technique and PLL design has drawbacks. 
       SUMMARY 
       [0003]    An embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises a first phase/frequency detector (PFD) that receives a first signal and a second signal; a second PFD that receives the second signal and a third signal wherein an edge of the second signal occurs about equidistantly between corresponding edges of the first and third signals; a charge pump having an up actuator and a down actuator, wherein the up actuator is coupled to the first PFD, and wherein the down actuator is coupled to the second PFD; a filter that is coupled to the charge pump; a voltage controlled oscillator (VCO) that is coupled to the filter, wherein the VCO generates an output signal; and a feedback loop that is coupled to the VCO, the first PFD, and the second PFD, wherein the feedback loop provides at least one of the third signal to the first and second PFDs or the first and third signals to the first and second PFDs. 
         [0004]    In accordance with another embodiment of the present invention, the first and third signals are reference signals and the second signal is a feedback signal from the feedback loop. 
         [0005]    In accordance with another embodiment of the present invention, the first and third signals are feedback signals from the feedback loop and the second signal is a reference signal. 
         [0006]    In accordance with another embodiment of the present invention, the feedback loop further comprises: a divider that is coupled to the VCO, the first PFD, and the second PFD; and an error compensator that is coupled to the divider. 
         [0007]    In accordance with another embodiment of the present invention, the error compensator further comprises a sigma-delta modulator. 
         [0008]    In accordance with another embodiment of the present invention, the divider further comprises: a prescaler that is coupled to the VCO and the second PFD; and a plurality of D flip-flops coupled in series with one another, wherein the first D flip-flop of the plurality of D flip-flops coupled in series with one another is coupled to the prescaler, and wherein the last D flip-flop of the plurality of D flip-flops coupled in series with one another is coupled to the first PFD. 
         [0009]    In accordance with another embodiment of the present invention, an apparatus is provided. The apparatus comprises a first PFD having an up output terminal, a down output terminal, a feedback input terminal, and a reference input terminal, wherein the feedback terminal of the first PFD receives a first signal, and wherein the reference terminal of the first PFD receives a reference signal; a second PFD having an up output terminal, a down output terminal, a feedback input terminal, and a reference input terminal, wherein the feedback terminal of the second PFD receives a second signal, and wherein the reference terminal of the second PFD receives the reference signal, and wherein an edge of the reference signal occurs about equidistantly between corresponding edges of the first and second signals; a charge pump having an up actuator and a down actuator, wherein the up actuator is coupled to up output terminal of the first PFD, and wherein the down actuator is coupled to the down output terminal of the second PFD; a filter that is coupled to the charge pump; a VCO that is coupled to the filter, wherein the VCO generates an output signal; and a feedback loop that is coupled to the VCO, the first PFD, and the second PFD, wherein the feedback loop provides at least one of the third signal to the first and second PFDs or the first and third signals to the first and second PFDs. 
         [0010]    In accordance with another embodiment of the present invention, A method for linearizing a charge pump in a phase locked loop (PLL) is provided. The method comprising the steps of generating a first signal and a second signal, wherein the first signal and the second signal are out of phase; receiving a third signal and the first signal by a first PFD; receiving the third signal and the second signal by a second PFD, wherein an edge of the third signal occurs about equidistantly between corresponding edges of the first and second signals; outputting an up signal from the first PFD to an up actuator of a charge pump; and outputting a down signal from the second PFD to a down actuator of the charge pump. 
         [0011]    In accordance with another embodiment of the present invention, the step of generating further comprises: generating the first signal from an output signal; and delaying the first divided signal to generate the second divided signal. 
         [0012]    In accordance with another embodiment of the present invention, the method further comprises the steps of: filtering a signal output from the charge pump; and generating the output signal with a VCO from the filtered signal. 
         [0013]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0015]      FIGS. 1A and 1B  are examples of block diagrams depicting a PLL in accordance with an embodiment of the present invention; 
           [0016]      FIG. 2  is a block diagram of the divider of  FIG. 1A ; and 
           [0017]      FIGS. 3A and 3B  are timing diagrams for the PLLs of  FIGS. 1A and 1B . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0019]    Referring to  FIGS. 1A and 1B  of the drawings, the reference numerals  100 - 1  and  100 - 2  generally designate PLLs in accordance with an embodiment of the present invention. The PLLs  100 - 1  and  100 - 2  generally comprise a phase/frequency detectors (PFD)  102  and  104 , a charge pump  106 , a filter  108 , and a voltage controlled oscillator (VCO)  110 . A different between PLLs  100 - 1  and  100 - 2  exists in the feedback loop  111 - 1  and  111 - 2 , where divider  112 - 1  provides two feedback signals and divider  112 - 1  provides one feedback signal. 
         [0020]    Turning first to  FIG. 1A , the VCO  110  generates an output signal OUT, which is output to other external circuitry and to the feedback loop  111 - 1 . The feedback loop  111 - 1  is generally comprised of a divider  112 - 1  and a phase loop error compensator or sigma-delta modulator  114 . Divider  112 - 1  can be implemented using a conventional programmable divider having a multi-modulus prescaler followed by an A/M counter  124  (as can be seen in  FIG. 2 ) for generating signal F 1  and a delay chain  125  for generating signal F 2 . Preferably, the delay chain  125  is comprised of a plurality of D-type flip-flops  126  arranged in series and clocked on the output signal OUT from the VCO  110 , so that signal F 2  is essentially a delayed copy of signal F 1 . Additionally, for reduced noise/jitter, signal F 1  can be retimed by a flip-flop that is clocked by the VCO  110  directly or by a signal in the prescaler  124 . Moreover, the compensator  114  receives signal F 1  and provides a correction signal K to the precaler  124 . 
         [0021]    These signals F 1  and F 2  are then, preferably, used to achieve a lock condition in the PLL  100 - 1 . Signals F 1  and F 2  are compared to a reference signal REF in PFDs  104  and  102  (respectively). Ideally, when the PLL  100 - 1  is in a locked condition, an edge of the reference voltage REF will occur in the about in middle or approximately equidistantly between the two corresponding edges of signals F 1  and F 2  (as shown in  FIG. 3A ). In other words and as seen in  FIG. 3A , the time between T 1  and T 2  is approximately equal to the time between T 2  and T 3 . An up signal can then be provided to the up actuator or switch  118  of the charge pump  106  from the up output terminal of PFD  102 , while a down signal can be provided to the down actuator or switch  120  of charge pump  106  from the down output terminal of PFD  104 . By having the signal REF being between signal F 1  and F 2 , the charge pump  106  can output a positive current from current source  116  during the time from an edge of signal F 1  to an edge of the reference signal REF and a negative current from the current source  122  during the time from an edge of the reference signal REF to an edge of the signal F 2 . The charge pump  106  can then output a signal to filter  108 , which provides a filtered signal to the VCO  110 . 
         [0022]    Under these circumstances, if the effective positive and negative currents differ in magnitude, the edge of the reference signal REF will be offset from the middle. Any phase error or jitter from the VCO  110  will cause corresponding edges of signals F 1  and F 2  to move by about the same amount in the same direction. In other words, if pulse width for the up signal changes by ΔT, then the pulse width of the down signal changes by −ΔT. Assuming a generally constant positive and negative current, a charge contribution would be: 
         [0000]      | I   U   |·ΔT−|I   DOWN |·(−Δ T )=(| I   UP   |+|I   DOWN |)·Δ T    (1)
 
         [0000]    Thus, the inequalities between positive and negative current can be cancelled or significantly reduced. Additionally, the PLL  100 - 1  allows the charge pump current to effectively be doubled compared to traditional designs. 
         [0023]    It should also noted that the linearization and doubled effective charge pump current only occurs when the phase error is small enough to maintain an edge of the reference signal REF between the corresponding edges of the signals F 1  and F 2 . Outside this phase error range, the effective charge pump current is either |I UP | or |I DOWN |. Therefore, the delay between signals F 1  and F 2  should be chosen to be larger than the largest or smallest value of the accumulated jitter that can be caused by the delta-sigma modulator  114 . 
         [0024]    Alternatively, as can be seen in  FIG. 1B , divider  112 - 2  can provide a single feedback signal F PLL  to each of PFDs  102  and  104 , and two separate reference signals REF 1  and REF 2  can be provided to PFDs  102  and  104 . In this configuration, an edge of signal F PLL  is approximately equidistantly between corresponding edges of signals REF 1  and REF 2  (as can be seen in  FIG. 3B  where the span between times T 4  and T 5  is about the same as the span between times T 5  and T 6 ) to achieve substantially the same result as in PLL  100 - 1 . 
         [0025]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.