Abstract:
A low spur phase-locked loop (PLL) architecture is provided. A frequency-synthesizing PLL that includes a differential Kvco gain linearization circuit with adjustable DC offset is used to reduce clock jitter. The free-running oscillation frequency of the VCO of the PLL is centered near the desired frequency using programmable loads to minimize the required control voltage range. The PLL uses a differential architecture that includes a charge pump that compensates for variations in Kvco and a LC tank oscillator with differential controlled varactor. The differential PLL architecture demonstrates that the reference spur can be well controlled to below −80 dBc.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/012,909, filed on Feb. 5, 2008 now abandoned. Application Ser. No. 12/012,909 claims the benefit of U.S. provisional patent application No. 60/900,180 filed Feb. 7, 2007, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to phase-locked loops. More particularly, the invention is directed to a low spur PLL using differential architecture. 
     BACKGROUND 
     Clock jitter or phase noise is a very common and important specification for many applications. Phase-locked loops (PLLs) are commonly employed to reduce clock jitter. However, the reference spur in a phase locked loop can result in a large jitter. Many techniques have been proposed to reduce reference spur in PLL, like reducing the gain (Kvco) of the voltage-control oscillator (VCO), or using a higher order filter to reduce VCO control voltage ripple. Previous works have demonstrated that the reference spur level is controlled as low as −65 dBc using third-order loop filter. One recent work controls the reference spur below −74 dBc through reducing Kvco. With previous techniques the PLL architecture is complicated and the added extra PLL blocks may burn more power or introduce new noise sources. 
     SUMMARY 
     In this invention, the PLL uses a differential architecture which consists of a charge pump configured to compensate for variations in Kvco, and a LC tank oscillator with differential controlled varactor. Through use of the invention, the PLL architecture demonstrates that the reference spur can be well controlled to below −80 dBc. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of a phase-locked loop (PLL). 
         FIG. 2  is a schematic illustration of a charge pump circuit. 
         FIG. 3  is a schematic illustration of an LC tank voltage-controlled oscillator (VCO). 
         FIG. 4  is a graph illustrating VCO gain versus control voltage at four different process corners. 
         FIG. 5  is a schematic illustration of a Kvco gain correction circuit. 
         FIG. 6  is a graph illustrating VCO gain, KVCO correction, and a resulting product. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a PLL may include a phase detector coupled to a charge pump. The charge pump may be coupled to a loop filter, which in turn is coupled to a voltage-controlled oscillator (VCO). The VCO may be coupled to a frequency divider, which may be coupled back to the phase detector. The phase detector compares the relative phase of the incoming reference clock, Fret and the (possibly divided down) output clock, Fdiv. An output pulse is generated based on the difference between arrival times of the two clock edges. The output pulse may be provided to the charge pump. The charge pump itself is a well understood in the art pair of NMOS/PMOS differential pairs used to steer mirrored values of the input current, lcp to either side of the loop filter depending on the values from the phase detector, as shown in  FIG. 2 . 
     As shown in  FIG. 2 , differential signals DNP and DNN as well as UPP and UPN received from the phase detector. The differential signals may be used (along with their inverses, inverse DNP, DNN, UPP, UPN) to control differential pair transistors as shown and provide output currents IDN and/or IUP to the loop filter. The output currents are generated based on an input current Icp. 
     The generation of the charge pump current, lcp, will be detail below and is a key to the invention. 
     The concept of adjusting the charge pump gain to compensate for variation in the VCO gain (Kvco) is known in the art for single ended VCOs. We improve and extend this basic idea to include correction for differential controlled VCOs and further use this technique to tune the VCO fixed load capacitors to center the VCO at it&#39;s optimal operating point. Before discussing the adjustment of charge pump gain to compensate for VCO gain, the remaining components of the  FIG. 1  PLL will be briefly described. 
     The loop filter takes the current output from the charge pump (IDN/IUP from  FIG. 2 ) and converts it to a voltage and applies a continuous time filter function. The loop filter is the standard lead-lag differential architecture that is well-known in the art. 
     The voltage-controlled oscillator, shown in  FIG. 3 , uses differentially controlled (floating) variable capacitor (varactor) to reduce noise coupling (the varactors are shown in  FIG. 3  labeled Cv) as well as switchable fixed capacitors to adjust frequency range (switchable fixed capacitors are shown in  FIG. 3  labeled Tune 0  through Tune&lt;n&gt;). Some additional fixed capacitance, labeled Cf in  FIG. 3 , is also provided. The tank bias current, shown in  FIG. 3  directed into the inductor labeled L, is also adjustable to compensate for variations over process, voltage, temperature or operating frequency. Resistances shown as R and R/  2  in  FIG. 3  are also provided. In this manner, the VCO generates the differential output voltage shown as VCON and VCOP in  FIG. 3 . 
     The PLL controls the oscillation frequency of the VCO of  FIG. 3  by adjusting the differential control voltage, Vcp/Vcn. The voltage-to-frequency gain of the VCO is a critical parameter in the jitter performance of the PLL and is generally noted as Kvco and is expressed in Hz/V. There is an optimum value for Kvco to minimize jitter, however, there is a wide range of values for Kvco over process and temperature corners, as shown in the graph of  FIG. 4 . 
     As shown in  FIG. 4 , the VCO gain may vary with voltage and over process and temperature conditions. 
     Previous work has shown how this variation may be somewhat compensated for by modifying the charge pump current, Icp inversely to that the change in Kvco. However, in extending this process to a differential structure, such as the differential VCO of  FIG. 3 , there are several deficiencies in the prior art that the current invention overcomes. 
       FIG. 5  shows a schematic illustration of a Kvco gain correction circuit that may be used to alter Icp, the current used in the charge pump shown in  FIG. 2 , to compensate for changes in the VCO gain. The Kvco gain correction circuit utilizes a bias current, Ibias to generate the charge pump current Icp or a correction to the charge pump current Icp. The amount of current provided to Icp is determined in part by the transistors shown in  FIG. 5 . The transistors shown in  FIG. 5  include transistors  505 ,  510 ,  515 ,  520 ,  530 , and  535 . VCN and VCP—the differential control voltage generated by the loop filter and applied to the VCO of FIG.  3 —are used to control the gates of two transistors each. For example, the transistors  505  and  510  are coupled to VCN at their gate terminals. The two transistors may be sized differently, as shown in  FIG. 5  with the designations M=2 and M=12. The transistor  505  has a size M=2. The transistor  510  has a size M=12. Adjustable offset voltages VCMP and VCMN are applied to the gates of two transistors each. For example, the transistors  515  and  520  are coupled to VCMN at their gate terminals. Again, the two transistors may be sized differently, as shown in  FIG. 5  with the designations M=2 and M=12. The transistor  515  has a size M=2, and the transistor  520  has a size M=12. As shown above in  FIG. 4 , the shape of the Kvco gain curve is not centered about 0.0V (differential loop filter voltage). The invention incorporates an adjustable offset via VCMP/VCMN into the charge pump Kvco gain correction circuitry (shown in  FIG. 5 ) that allows adjusting the optimal center voltage. 
     Further, the product of the Kvco gain curve and the Kvco gain correction response is ideally a constant. The problem with this is that the Kvco gain is dependent on fixed capacitors, varactors, inductors and various parasitics whereas the Kvco gain correction response is primarily dependent on NMOS devices. Variations in process, temperature and voltage will result in a final product that is not constant, as shown in  FIG. 6 .  FIG. 6  illustrates a VCO gain, a KVCO correction response, and a product of those two. As can be seen, when the VCO gain and KVCO correction are multiplied, the overall response is closer to constant than the VCO gain alone, but it is not completely constant. 
     The invention seeks to minimize this source of error by switching in sufficient numbers of fixed capacitors (TUNE 0  through TUNE&lt;n&gt; as shown in the LC Tank VCO Schematic of  FIG. 3 ). The current invention seeks to achieve this by holding the VCO control voltages equal to the required offset as described above (i.e. VCN=VCMN and VCP=VCMP). The frequency of oscillation is measured and compared to the reference and fixed capacitors are added or removed to bring the oscillation frequency as close as possible to the reference frequency. This allows the VCO and Kvco gain correction circuit to operate in a small range (shown above) reducing the effect of mismatches between the two. 
     As one of ordinary skill in the art will appreciate, various changes, substitutions, and alternations could be made or otherwise implemented without departing from the principles of the present invention. Accordingly, the examples and drawings disclosed are for purposes of illustrating the preferred embodiments of the present invention and are not to be construed as limiting the invention.