Abstract:
Aspects for reducing jitter in a PLL of a high speed serial link include examining at least one parameter related to performance of a voltage controlled oscillator (VCO) in the PLL, and controlling adjustment of a supply voltage to the VCO based on the examining. A regulator control circuit performs the examining and controlling.

Description:
FIELD OF THE INVENTION 
   The present invention relates to jitter reduction in phase-locked loops of high speed serial links. 
   BACKGROUND OF THE INVENTION 
   The ability to perform and achieve high speed transmissions of digital data has become expected in today&#39;s computing environment. In most cases, the transmission of digital data over longer distances is accomplished by sending the data in a high-speed serial format (i.e., one single bit after another) over a communication link designed to handle computer communications. In this fashion, data can be transferred from one computer system to another, even if the computer systems are geographically remote. 
   In order for high-speed serial transmission to occur, the digital data signal from inside the computer must be transformed from the parallel format into a serial format prior to transmission of the data over the serial communication link. This transformation is generally accomplished by processing the computer&#39;s internal data signal through a piece of computer equipment known as a serial link transmitter or “serializer”. The function of the serializer is to receive a parallel data stream as input and, by manipulating the parallel data stream, output a serial form of the data capable of high-speed transmission over a suitable communication link. Once the serialized data has arrived at the desired destination, a piece of computer equipment known as a “deserializer” is employed to convert the incoming data from the serial format to a parallel format for use within the destination computer system. 
   For high speed serializer/deserializer (HSS) link pairs, a phase-locked loop (PLL) is used to get a phase lock based on the incoming signal. A basic block diagram of a typical PLL is illustrated in  FIG. 1 . The PLL includes a phase/frequency detector (PD)  10  coupled to a charge pump (CP)  12 , which is coupled to a voltage controlled oscillator (VCO)  14 . A regulator (REG)  16  is included for the PLL circuitry to supply a filtered/regulated version of the supply voltage (Vcc) to the VCO  14 . The PD  10  compares the phase (FREQout) of the VCO  14  signal filtered through a frequency divider (DIV)  18  with that of the incoming signal (FREQin) and adjusts the control voltage (Vcntrl) to keep the VCO  14  in phase with the incoming signal. 
   In general, maintaining the control voltage of the VCO within a certain range helps in reducing jitter. Prior art approaches have designed the VCO/control voltage with a fixed high gain to cover the range. In an alternative design, the regulator output voltage can be set to set the power supply voltage that the VCO sees. While such approaches do address the jitter problem to a certain extent, they are limited against having the flexibility to more readily address jitter across a broad range of variations in the incoming signal frequency. 
   Accordingly, a need exists for a manner of reducing jitter in PLLs of high speed serial links that accommodates variations in the incoming signal frequency. The present invention addresses such a need. 
   SUMMARY OF THE INVENTION 
   Aspects for reducing jitter in a PLL of a high speed serial link are described. The aspects include examining at least one parameter related to performance of a voltage controlled oscillator (VCO) in the PLL, and controlling adjustment of a supply voltage to the VCO based on the results. A regulator control circuit performs this examination and controls the resultant supply voltage to the PLL. 
   Through the present invention, jitter in a PLL is successfully reduced by examination of the parameters directly related to PLL performance without knowledge of absolutes in frequency from the reference clock or in operating frequency. Thus, a flexible and efficient approach to accommodating variations in the frequency of the incoming signal for a PLL of a high speed serial link is achieved. These and other advantages will become readily apparent from the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a block diagram of a phase locked loop (PLL) of the prior art. 
       FIG. 2  illustrates a circuit diagram for a regulator control circuit in accordance with the present invention. 
       FIG. 3  illustrates a logic table and pseudo-code for the decision logic of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   The present invention relates to jitter reduction in phase-locked loops of high speed serial links. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
   In accordance with the present invention, a regulator control circuit for utilization with a PLL is provided. In general, the regulator control circuit maintains the control voltage of a VCO within certain boundaries deemed preferable for VCO operation while adjusting the regulator output seen by the VCO to mitigate certain noise effects. With the regulator control circuit, there are multiple levels of regulator output voltages under logic control, and the range of Vp and Vn control voltages for the VCO are limited while still achieving the required range of frequency operation. 
   Referring to the circuit diagram of  FIG. 2 , a regulator control circuit in accordance with the present invention is illustrated. As shown, the regulator control circuit includes a band gap-based voltage reference generator  20  having two voltage settings. It should be appreciated that although the following description refers to utilization of two regulator voltage settings, this is meant as illustrative and not restrictive of the number of regulator voltage settings that could be included. The output of the reference generator  20  is input to dual comparators  22 ,  24 . The comparators  22 ,  24  further receive the control voltage of the PLL  26 . With the control voltage being a differential signal, either the positive (Vp) or negative (Vn) version is suitable for comparison purposes. In the example embodiment, the negative version, Vn, is used. 
   The measurement logic  28  examines whether the control voltage (Vn) is within a predetermined allowable range. In a preferred embodiment, the predetermined range is based on limits deemed preferable for VCO operation, and as indicated by REF, is one-half of the regulator voltage plus/minus 200 millivolts (0.2 v). It has been found by the inventors that if the control voltage is within these limits, the VCO is operating in the portion of the gain curve in which jitter is lower than if this control voltage is allowed to be in excess of the limits. The level of Vctlok (control voltage okay) from the measurement logic  28  indicates whether the control voltage is within the limits. 
   The Vctlok signal is received by decision logic  30 . Decision logic  30  performs two main functions. One function of the decision logic  30  is to examine the level of the PLL lock signal (PLL_LOCK). The PLL_LOCK signal is generated based on a reference clock and a clock from the PLL which should have the same frequency as the reference clock. Each clock is made to run a two bit counter, and the two counters are started at different values. The PLL_LOCK signal will be asserted if the two counters values are never equal to each other during a period of time determined by a time-out counter. 
   The second function of the decision logic  30  is to control the regulator voltage selection signals (REGOVR_OUT and REGHI) which go to the regulator  32 . Although two selection signals are illustrated, this is meant as illustrative and not restrictive of the number of regulator voltage selection signals that can be used. The levels of the selection signals are determined based on the inputs of Vctlok and PLL_LOCK.  FIG. 3  illustrates a table that indicates how the REGOVR_OUT and REGHI signals of the decision logic  30  correspond to the regulator  32  voltage setting and illustrates pseudo-code of the implementation steps performed by the decision logic  30  in producing the REGHI and REGOVR_OUT signals. In addition to those signals already indicated in  FIG. 2 , in the pseudo-code of  FIG. 3 , REGOVR_IN represents an override signal input (e.g., via a switch), BLOCK_PLLLOCK represents an internal logic signal that can block external knowledge of the PLL_LOCK signal information as needed during the performance of the process, and VCTLOK_SAMPLE represents the result of the “Sample” step shown. 
   As indicated by the psuedo-code, the decision logic  30  examines whether the control voltage is in the predetermined allowable range and determines if the PLL is locked. If both conditions are met, the regulator  32  is considered to be operating appropriately and is left in the startup condition. If either condition fails, the regulator  32  is moved to another output voltage value and again a check is made for appropriate operation. If the desired operation is achieved, then the outputs are kept the same until the next reset. If in all regulator positions, a fail still exists, the PLL should experience increased jitter and be rejected. 
   Thus, jitter in a PLL is successfully reduced by examination of the parameters directly related to PLL performance without knowledge of absolutes in frequency from the reference clock or in operating frequency; i.e., jitter is managed based on the differential VCO control voltage&#39;s distance from its optimum. By managing jitter in this manner, performance is readily improved and design variations readily accommodated in a straightforward and efficient manner. 
   From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the novel concept of the invention. It is to be understood that no limitation with respect to the specific methods and apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.