Patent Publication Number: US-7715143-B2

Title: Delta-sigma PLL using fractional divider from a multiphase ring oscillator

Description:
REFERENCES TO RELATED APPLICATIONS 
   The present U.S. Utility Patent Application claims priority pursuant to 35 U.S.C. §119(e) to the following U.S. Provisional Patent Applications which are hereby incorporated herein by reference in their entirety and made part of the present U.S. Utility Patent Application for all purposes: 
   1. U.S. Provisional Application Ser. No. 60/882,963, entitled “Delta-sigma PLL using fractional divider from multi-phase ring oscillator,”, filed Dec. 31, 2006. 
   2. U.S. Provisional Application Ser. No. 60/966,555, entitled “Delta-sigma PLL using fractional divider from multi-phase ring oscillator,”, filed Apr. 3, 2007. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to hard disk drives; and, more particularly, to the synchronization of a disk drive controller to a hard disk of the hard disk drive. 
   BACKGROUND OF THE INVENTION 
   As is known, many varieties of memory storage devices (e.g. disk drives), such as magnetic disk drives are used to provide data storage for a host device, either directly, or through a network such as a storage area network (SAN) or network attached storage (NAS). Typical host devices include stand alone computer systems such as desktop and laptop computers, enterprise storage devices such as servers, storage arrays such as a redundant array of independent disks (RAID) arrays, storage routers, storage switches and storage directors, and other consumer devices such as video game systems and digital video recorders. These devices provide high storage capacity in a cost effective manner. 
   The structure and operation of hard disk drives is generally known. Hard disk drives (HDDs) include, generally, a case, a hard disk having magnetically alterable properties, and a read/write mechanism including Read/Write (RW) heads operable to write data to the hard disk by locally alerting the magnetic properties of the hard disk and to read data from the hard disk by reading local magnetic properties of the hard disk. The hard disk may include multiple platters, each platter being a planar disk. The read/write mechanism also includes a disk drive controller. 
   All information stored on the hard disk is recorded in tracks, which are concentric circles organized on the surface of a platter. Data stored on the disks may be accessed by moving RW heads radially as driven by a head actuator to the radial location of the track containing the data. To efficiently and quickly access this data, fine control of RW hard positioning and sampling is required. The track-based organization of data on the hard disk(s) allows for easy access to any part of the disk, which is why hard disk drives are called “random access” storage devices. 
   Since each track typically holds many thousands of bytes of data, the tracks are further divided into smaller units called sectors. This reduces the amount of space wasted by small files. Each sector holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction. 
   Within such HDDs, disk drive controllers control the various processes associated with the read/write of data to the physical media (hard disk). As the amount of data stored to the physical media increases, the ability to accurately read data from the physical media is adversely effected. This is further complicated as hard disk drives are forced into smaller form factor devices, the physical media or disk itself becomes smaller further increasing the need for increased storage density. Thus, smaller form factors and higher storage densities make these disks more susceptible to fluctuations of a disk clock as the disk itself may not necessarily rotate in circular fashion. The disk (i.e. physical media) itself may not necessarily rotate in a truly circular path. There may be some variations where the center of rotation may be off axis. Other variations arise from the mechanism used to drive the rotation of the disk, resulting in an elliptical path of the hard disk about the axis of rotation. This ellipse may change as the physical media shifts on the spindle or axis of rotation. These effects are most noticeable in small form factor device. In order to ensure that data is properly read from small form factor devices it is necessary to track the variations associated with the disk rotation to properly sample the data. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention are directed to systems and methods that are further described in the following description and claims. Advantages and features of embodiments of the present invention may become apparent from the description, accompanying drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numerals may indicate like features and wherein: 
       FIG. 1  is a system diagram illustrating a disk drive unit that incorporates structure and/or operations of embodiments of the present invention; 
       FIG. 2  is a block diagram illustrating an embodiment of a disk drive controller constructed and operating in accordance with embodiments of the present invention; 
       FIGS. 3A through 3E  illustrate various devices that employ hard disk drives constructed in accordance with embodiments of the present invention; 
       FIG. 4  is a diagram illustrating how the time of arrival (TOA) of the Read Write (RW) head between Servo Address Marks (SSMs) can indicate the velocity of the RW head within a read path; 
       FIG. 5  is a block diagram illustrating a Data Locking Clock (DLC) scheme constructed and operating in accordance with embodiments of the present invention; 
       FIG. 6  is a timing diagram illustrating how an instantaneous period of a hard disk (i.e. frequency or RPM associated with the hard disk of a HDD) may be determined; 
       FIG. 7  is a block diagram illustrating of a servo disk clock circuit constructed and operating in accordance with embodiments of the present invention; 
       FIG. 8  is a block diagram illustrating an embodiment of a fractional N sigma delta Phase Lock Loop (PLL) constructed and/or operating according to one or more embodiments of the present invention; 
       FIG. 9  is a block diagram illustrating another embodiment of a fractional N sigma delta Phase Lock Loop (PLL) constructed and/or operating according to one or more embodiments of the present invention; and 
       FIG. 10  is a flow chart illustrating operation according to one or more embodiments of the present invention for maintaining data PLL frequency/phase lock, and/or a server PLL frequency/phase lock. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention are illustrated in the Figures with like numerals generally used to refer to corresponding elements of the various drawings. However, elements having common numbering may have differing structure/operation in various embodiments of the present invention. 
   Embodiments of the present invention are incorporated within a Hard Disk Drive (HDD), with a disk drive controller and its various components incorporating aspects of the present invention. The disk drive controller includes a servo system operable to associate a time stamp with an arrival of a servo wedge, a firmware loop, and core PLLs associated with a read channel. The firmware loop is operable to determine a period between the arrivals of a consecutive servo wedges (Servo Marks (SSMs)) and produce a desired frequency of corresponding to read/write data of the disk based on the period between the arrivals of the consecutive SSMs of the servo wedges. Processing circuitry is operable to adjust a clock signal based upon the consecutive SSMs, wherein the clock signal itself may not be locked to the data but that may be used to produce a fine control signal for the core PLLs in the read channel. These core PLLs are operable to produce output signals that are used to sample and/or write data to the hard disk. In some embodiments of the present invention, the core PLLs are Sigma Delta PLLs, which may include fractional N dividers. The structure and operation of these core PLLs support increased accuracy in both the read and write access of the hard disk, thereby supporting greater storage capacity and smaller dimensions of the hard disk. 
   The Delta Sigma PLLs generate a frequency F VCO =F ref *N eff , where N eff  is the average N coming from a Multi-stAge noise SHaping (MASH) modulator that generates a time varying sequence of integers. Jitter of the output of the Delta Sigma PLL can be further reduced by using multiple phases of a ring oscillator and switching between different ‘fractional N’ values produced by the ring oscillator. 
   Embodiments of the present invention may employ two Sigma Delta PLLs in a Disk Clock system, which may be implemented in hardware. The Sigma Delta PLLs keep the Data PLL frequency locked (while the servo PLL can be phase and/or frequency locked). Generally, F servonom =F ref *N servonom  where N servonom  is the nominal fractional divider. The disk clock produces a time varying N servo (k) that moves around N servonom . Hardware/software algorithms calculate and apply updates to the Data PLL(s) to cause the Data PLL(s) to maintain a near exact frequency lock to the hard disk. 
     FIG. 1  is a system diagram illustrating a disk drive unit  100  that incorporates structure and/or operations of embodiments of the present invention. In particular, disk drive unit  100  includes a disk  102  that is rotated by a servo motor (not specifically shown) at a velocity such as 3600 revolutions per minute (RPM), 4200 RPM, 4800 RPM, 5,400 RPM, 7,200 RPM, 10,000 RPM, 15,000 RPM; however, other velocities including greater or lesser velocities may likewise be used, depending on the particular application and implementation in a host device. In one possible embodiment, disk  102  can be a magnetic disk that stores information as magnetic field changes on some type of magnetic medium. The medium can be a rigid (hard disk), a non-rigid, removable disk, or a non-removable disk, that consists of or is coated with magnetic material. 
   Disk drive unit  100  further includes one or more read/write heads  104  that are coupled to arm  106  that is moved by actuator  108  over the surface of the disk  102  either by translation, rotation or both. A disk drive controller  130  is included for controlling the read and write operations to and from the drive, for controlling the speed of the servo motor and the motion of actuator  108 , and for providing an interface to and from the host device. 
     FIG. 2  is a block diagram illustrating an embodiment of a disk drive controller  130  constructed and operating in accordance with embodiments of the present invention. The disk drive controller operates as part of a system  200  that includes read/write heads  104 , drive devices  109 , and a host device  50 . In particular, disk drive controller  130  includes a read/write channel  140  for reading and writing data to and from disk  102  through read/write heads  104 . Disk formatter  125  controls the formatting of data and provides clock signals and other timing signals that control the flow of data written to, and data read from disk  102  via the read/write heads  104  and the read/write channel  140 . Servo formatter  120  provides clock signals and other timing signals based on servo control data read from disk  102  via the read/write heads  104  and the read/write channel  140 . Device controllers  105  control the operation of drive devices  109  such as actuator  108  and the servo motor, etc. Host interface module  150  receives read and write commands from host device  50  and transmits data read from disk  102  along with other control information in accordance with a host interface protocol. In one embodiment, the host interface protocol can include, SCSI, SATA, enhanced integrated drive electronics (EIDE), or any number of other host interface protocols, either open or proprietary that can be used for this purpose. 
   Disk drive controller  130  further includes a processing module  132 , memory module  134 , bus  136 , and bus  137 . Processing module  132  can be implemented using one or more microprocessors, micro-controllers, digital signal processors, microcomputers, central processing units, field programmable gate arrays, programmable logic devices, state machines, logic circuits, analog circuits, digital circuits, and/or any devices that manipulates signal (analog and/or digital) based on operational instructions that are stored in memory module  134 . When processing module  132  is implemented with two or more devices, each device can perform the same steps, processes or functions in order to provide fault tolerance or redundancy. Alternatively, the function, steps and processes performed by processing module  132  can be split between different devices to provide greater computational speed and/or efficiency. 
   Memory module  134  may be a single memory device or a plurality of memory devices. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static random access memory (SRAM), dynamic random access memory (DRAM), flash memory, cache memory, and/or any device that stores digital information. Note that when the processing module  132  implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory module  134  storing the corresponding operational instructions may be embedded within, or external to, the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. Further note that, the memory module  134  stores, and the processing module  132  executes, operational instructions that can correspond to one or more of the steps or a process, method and/or function illustrated herein. 
   The device controllers  105 , the processing module  132 , the memory module  134 , the read/write channel  140 , the servo formatter  120 , the disk formatter  125 , and host interface module  150  are interconnected via bus  136  and bus  137 . The host interface module  150  may be connected to only bus  137 . However, in other embodiments, such connectivity may differ. Each of these modules can be implemented in hardware, firmware, software or a combination thereof, in accordance with the broad scope of the present invention. While a particular bus architecture is shown in  FIG. 2  with buses  136  and  137 , alternative bus architectures that include either a single bus configuration or additional data buses, further connectivity, such as direct connectivity between the various modules, are likewise possible to implement the features and functions included in various embodiments. 
   In one possible embodiment, one or more modules of disk drive controller  130  are implemented as part of a system on a chip (SoC) integrated circuit. In an embodiment, this SoC integrated circuit includes a digital portion that may include additional modules such as protocol converters, linear block code encoding and decoding modules, etc., and an analog portion that includes device controllers  105  and optionally additional modules, such as a power supply, etc. In a further embodiment, the various functions and features of disk drive controller  130  may be implemented in a plurality of integrated circuit devices that communicate and combine to perform the functionality of disk drive controller  130 . 
   When the drive unit  100  is manufactured, disk formatter  125  writes a plurality of servo wedges along with a corresponding plurality of servo address marks (SSMs) at equal radial distance along the disk  102 . The SSMs are used by a timing generator for triggering the “start time” for various events employed when accessing the media of the disk  102  through read/write heads  104 . 
     FIGS. 3A through 3E  illustrate various devices that employ hard disk drives constructed in accordance with embodiments of the present invention.  FIG. 3A  illustrates an embodiment of a handheld audio unit  51 . In particular, disk drive unit  100  can be implemented in the handheld audio unit  51 . In one possible embodiment, the disk drive unit  100  can include a small form factor magnetic hard disk whose disk  102  has a diameter 1.8″ or smaller that is incorporated into or otherwise used by handheld audio unit  51  to provide general storage or storage of audio content such as motion picture expert group (MPEG) audio layer 3 (MP3) files or Windows Media Architecture (WMA) files, video content such as MPEG4 files for playback to a user, and/or any other type of information that may be stored in a digital format. 
     FIG. 3B  illustrates an embodiment of a computer  52 . In particular, disk drive unit  100  can be implemented in the computer  52 . In one possible embodiment, disk drive unit  100  can include a small form factor magnetic hard disk whose disk  102  has a diameter 1.8″ or smaller, a 2.5″ or 3.5″ drive or larger drive for applications such as enterprise storage applications. Disk drive  100  is incorporated into or otherwise used by computer  52  to provide general purpose storage for any type of information in digital format. Computer  52  can be a desktop computer, or an enterprise storage devices such a server, of a host computer that is attached to a storage array such as a redundant array of independent disks (RAID) array, storage router, edge router, storage switch and/or storage director. 
     FIG. 3C  illustrates an embodiment of a wireless communication device  53 . In particular, disk drive unit  100  can be implemented in the wireless communication device  53 . In one possible embodiment, disk drive unit  100  can include a small form factor magnetic hard disk whose disk  102  has a diameter 1.8″ or smaller that is incorporated into or otherwise used by wireless communication device  53  to provide general storage or storage of audio content such as motion picture expert group (MPEG) audio layer 3 (MP3) files or Windows Media Architecture (WMA) files, video content such as MPEG4 files, JPEG point photographic expert group) files, bitmap files and files stored in other graphics formats that may be captured by an integrated camera or downloaded to the wireless communication device  53 , emails, webpage information and other information downloaded from the Internet, address book information, and/or any other type of information that may be stored in a digital format. 
   In a possible embodiment, wireless communication device  53  is capable of communicating via a wireless telephone network such as a cellular, personal communications service (PCS), general packet radio service (GPRS), global system for mobile communications (GSM), and integrated digital enhanced network (iDEN) or other wireless communications network capable of sending and receiving telephone calls. Further, wireless communication device  53  is capable of communicating via the Internet to access email, download content, access websites, and provide steaming audio and/or video programming. In this fashion, wireless communication device  53  can place and receive telephone calls, text messages such as emails, short message service (SMS) messages, pages and other data messages that can include attachments such as documents, audio files, video files, images and other graphics. 
     FIG. 3D  illustrates an embodiment of a personal digital assistant (PDA)  54 . In particular, disk drive unit  100  can be implemented in the personal digital assistant (PDA)  54 . In one possible embodiment, disk drive unit  100  can include a small form factor magnetic hard disk whose disk  102  has a diameter 1.8″ or smaller that is incorporated into or otherwise used by personal digital assistant  54  to provide general storage or storage of audio content such as motion picture expert group (MPEG) audio layer 3 (MP3) files or Windows Media Architecture (WMA) files, video content such as MPEG4 files, JPEG (joint photographic expert group) files, bitmap files and files stored in other graphics formats, emails, webpage information and other information downloaded from the Internet, address book information, and/or any other type of information that may be stored in a digital format. 
     FIG. 3E  illustrates an embodiment of a laptop computer  55 . In particular, disk drive unit  100  can be implemented in the laptop computer  55 . In one possible embodiment, disk drive unit  100  can include a small form factor magnetic hard disk whose disk  102  has a diameter 1.8″ or smaller, or a 2.5″ drive. Disk drive  100  is incorporated into or otherwise used by laptop computer  52  to provide general purpose storage for any type of information in digital format. 
     FIG. 4  is a diagram illustrating how the time of arrival (TOA) of the Read Write (RW) head between Servo Address Marks (SSMs) can indicate the velocity of the RW head with respect to the disk within a read path.  FIG. 4  depicts how the time of arrival (TOA) of the RW head between SSMs  402  can indicate the velocity of the RW head within read path  404 . Additionally, this information can be used to determine frequency and phase that is used to sample an analog waveform read from the disk. 
   In  FIG. 4 , the TOA actual  should equal TOA expected . This equality indicates that the read path is radially aligned and the RW head maintains a constant linear velocity during a read operation. A TOA error  between TOA actual  and TOA expected  indicates a non constant linear velocity during a read operation, which may be due to errors associated with the servo motor used to rotate the disk or the alignment of the axis or rotation of the disk. Both of these sources of error may be magnified by the small geometries demanded of small form factor disks. Such errors may result in an improper sampling of the analog waveform read from the disk, degrading the integrity of the data read from the disk. For proper radial alignment and operation with a constant linear velocity, the TOA actual  should equal TOA expected . 
   As each SSM  402  is encountered by the read-write head the instantaneous velocity of the read-write head relative to the physical media may be measured very accurately. With knowledge of the position of the SSM the instantaneous rotation of the physical media may be easily computed. This information may be used to very accurately determine when to sample the disk for analog wave form data. Embodiments of the present invention may apply this information in order to lock data sampling of an Analog to Digital (ADC) converter to an analog wave form read from the disk (physical media). Likewise, this information may be employed to lock data writing to the disk. 
     FIG. 5  is a block diagram illustrating a Data Locking Clock (DLC) scheme  500  constructed and operating in accordance with embodiments of the present invention. DLC clocking scheme  500  is partitioned between a controller portion and a read channel portion. The controller portion of the DLC clocking scheme  500  includes oscillator  502 , PLL  504 , control dividers  506 , phase mixer  508 , divider  510 , frequency integrator  512 , firmware loop  514 , and disk formatter  516 . The read channel portion of the DLC clocking scheme  500  includes Data PLL (DPLL)  518 , read PLL  520 , phase mixer  522 , divider  524 , servo PLL  526 , multiplexer  528 , and counter  530 . 
   The read channel portion reads/detects the time of arrival of SSMs (or other timing information in differing embodiments). A time stamp is recorded with the arrival of each new servo wedge (i.e. SSM). Embodiments of the present invention accurately measure this time, which can be compared to adjacent the TOA or SSM&#39;s time stamp. A period between the adjacent TOA or SSM is measured by counter  530  and then provided to firmware loop  514 . This allows the instantaneous RPM (frequency) of the HDD to be determined. This information is used by the disk drive controller/read channel to adjust the core PLLs of the read channel. The core PLLs of prior device read channels have been analog PLLs that did not allow the fine control required by increasing data density and smaller form factors that exacerbate problems associated with a HDD that is not perfectly stable, at a constant RPM and perfectly circular. Thus the clock signals previously used are not synchronous to the data read/written from/to the disk. This resulted in a multitude of problems including format problems associated with reading the disk as well as reading writing data to the disk. 
   Thus, embodiments of the present invention may employ core PLLs (data PLL, Servo PLL, Read PLL) in the read channel operable to determine a phase and/or frequency associated with when an analog signal is sampled and/or written to disk, wherein the core PLLs are Sigma Delta PLLs that may employ fractional N constructs. Finer control and further improved results may be derived from using a multiphase ring oscillator in combination with the sigma delta PLLs. 
   Frequency integrator  512  supports the selection of desired phase. During normal drive operations for reading and writing the disk it is important to have the correct frequency for sampling with the read PLL (data PLL). Similarly during a self servo right it is important to have both phase and frequency properly controlled. Frequency integrator  512  is similar to a digital VCO and allows the desired phase and or frequency to be selected. The phase mixer  508  and the analog phase select  722  of  FIG. 7  allow one of numerous phases to be selected. For example this may be one of 64 phases that may be selected in certain embodiments of the present invention. These multiple phases may be generated using a multiple ring oscillator. For example a four gate ring oscillator will create eight clocks at the same frequency having different phases. These phases are equally staggered. Additional circuitry may be used to interpolate between the eight phases in order to produce any number of phases. 
     FIG. 6  is a timing diagram illustrating how an instantaneous period of a hard disk (i.e. frequency or RPM associated with the hard disk of a HDD) may be determined. This timing diagram shows first that the servo marks (SSM) are detected, a time stamp may be associated with this detection. The detection may also involve performing a bit where a half value based on a full bit count is identified. These counts roll over when a full count is reached. The half values found during these bid counts are then identified and have an associated time stamp. This allows a time period (shown as SSM to SSM Count  1 , SSM to SSM Count  1 , Count  2 , etc.) to be determined which directly relates to the instantaneous RPM of the disk drive. 
     FIG. 7  is a block diagram illustrating of a servo disk clock circuit constructed and operating in accordance with embodiments of the present invention. Servo disk clock  700  includes Variable Gain Amplifier (VGA)  702 , Analog to Digital Converter (ADC)  704 , Finite Impulse Response (FIR) filter  706 , servo digital interpolator  708 , Servo Address Mark Detector SAMD block  710 , servo TOA block  712 , combiner  714 , disk clock loop filter  716 , servo PLL  718 , data PLL  720 , analog phase selector  722 , and synchro-timing recovery module  724 . 
   An analog signal is provided from a pre-amplifier to VGA  702 , which amplifies the signal. An ADC  704  then samples the analog signal to produce a digital signal, which is provided to FIR filter  706 , which filters the digital signal. The output of FIR filter  706  may also be provided as an input to Synchro Timing Recovery Module  724 . The output of the FIR filter  706  is provided to the Servo Digital Interpolator  708 , which produces several outputs. These include a servo gray data output, an output provided to the servo SAMD block  710 , and an output provided to the servo wedge Time of Arrival (TOA) block  712 . A time stamp associated with the servo wedge time of arrival may be used to determine an instantaneous RPM frequency associated with the disk drive. The measured time of arrival can be provided to a register in firmware or hardware wherein the time of arrival may be compared with a desired time of arrival. Other embodiments may compare consecutive time of arrivals to determine a period associated with the consecutive time of arrivals. This period may then be compared with a desired period. In either case the result of Combiner  714  is a timing error. Whether the timing error is a time of arrival error or a period error depends on the specific embodiment of the present invention. The timing error is provided to a firmware loop or Disc Clock Loop Filter  716  in order to produce a digital output (fractional number) to the core PLLs, which include a Data PLL  720  and a Servo PLL  718 . The digital output provided to the core PLLs (Data PLL  720  and Servo PLL  718 ) is a fractional number in the embodiment of  FIG. 7 . The digital outputs of the core PLLs are used to perform an analog phase select using analog phase selector Module  722 . The Synchro Timing Recovery Module  724  provides another input to the analog phase select selector module  724 . This allows the proper frequency and/or phase to be selected for sampling by ADC  704 . 
   Referring to both  FIGS. 5 and 7 , embodiments of the present invention help address problems exacerbated by small form factors, higher data storage densities, perpendicular versus longitudinal recording, the disk not being a perfectly stable, constant velocity, perfectly circular rotational platform, and shock and vibration associated with portable HDDs. These problems result in oscillator  502  no longer being synchronous with the analog signal being read from the disk. Read PLL  520  and servo PLL  526  of  FIG. 5  and data PLL  720  and servo PLL  718  of  FIG. 7  may now be delta sigma PLLs. These PLLs receive a digital word as an input shown on  FIG. 7  as reg_n_frac_data and reg_n frac_servo. Where 
   
     
       
         
           
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               data 
             
           
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               N 
               
                 frac 
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                 servo 
               
             
             * 
             
               
                 
                   F 
                   data 
                 
                 
                   F 
                   servo 
                 
               
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     FIG. 8  is a block diagram illustrating a first embodiment of a fractional N sigma delta Phase Lock Loop (PLL) constructed and/or operating according to one or more embodiments of the present invention. The illustrated portion of the sigma delta PLL  800  receives a crystal oscillation input F XTAL  (from crystal oscillator) and also the input N frac . The sigma delta PLL  800  includes a divide by M Module  802 , a phase detector  804 , a low pass filter  806 , and a delta sigma modulator/MASH modulator/Divider  808 . The divide by M module  802  receives the input F xtal  and divides the input by M. The output of the divide by M module  802  is received by phase detector  804 , which also receives an output from the delta sigma modulator/MASH modulator/Divider  808 . An input N frac  being a fractional number is provided to delta sigma modulator/MASH modulator/Divider  808 . 
   The delta sigma modulator/MASH modulator/Divider  808  receives the fractional number N frac  input as produced by the disk clock loop filter  716  of  FIG. 7 , which is a function of time. N frac  (a function of time) is used by the delta sigma modulator/MASH modulator/Divider  808  to produce a sequence of dividers, N, that is uses to divide the input received from VCO  807  and to thereby produce an input to the phase detector  804 . 
   For example, based upon an N frac  input of 100.5, the delta sigma modulator/MASH modulator/Divider  808  may produce a series of dividers, N, of 100, 101, 100, 101 that are used to divide the output of the VCO  807  to produce input to the phase detector  804 . The delta sigma modulator/MASH modulator/Divider  808  dithers N at a high enough rate that, when operated on by the low pass filter  806 , the input to the VCO  807  is sufficiently smoothed. 
   The output of the delta sigma modulator/MASH modulator/Divider  808  is received by the phase detector  800 , which compares the phase of its two inputs. The output of the phase detector  804 , which represents a phase difference between its two inputs, is received by LPF  806 , which filters the input. The filtered output of the LPF  806  is provided to VCO  807 , which is an oscillation received by the delta sigma modulator/MASH modulator/Divider  808 . 
     FIG. 9  is a block diagram illustrating another embodiment of a fractional N sigma delta Phase Lock Loop (PLL) constructed and/or operating according to one or more embodiments of the present invention. The fractional N sigma delta PLL  900  receives a first input from a crystal oscillator having a crystal frequency F XTAL  and includes a multiple phase VCO  902 , a divide by M Module  910 , a phase detector  904 , a low pass filter  906 , a delta sigma modulator/MASH modulator/Divider  908 , and a phase selector  912 . The phase selector  912  may be the same as the phase selector  722  of  FIG. 7  in some embodiments. 
   As compared to the structure of  FIG. 8 , the fractional N sigma delta PLL  900  includes the multiple phase VCO  902  and the phase selector  912 . The multiphase aspect provided by VCO  902  allows even finer control than that provided by the fractional N sigma delta PLL  800  of  FIG. 8 . 
     FIG. 10  is a flow chart illustrating operation according to one or more embodiments of the present invention for maintaining data PLL frequency/phase lock, and/or a server PLL frequency/phase lock. Operations  1000  begin in step  1002  where a period between server marks on a disk beneath a read write head are determined. This may be done as described previously with reference to  FIG. 6 . In step  1004  an instantaneous RPM of the disk based on the determined period from step  1002  may be determined. This allows in step  1006  a comparison between the determined period (frequency) to be made in order to produce a period (frequency) error. Then in step  1008  at least one corp. PLL associated with the read write path may be adjusted based on the period (frequency) error associated with the instantaneous RPM of the disk. As stated before, improved results may be derived from fractional processing that is made possible with the use of a sigma delta PLL. Finer control and further improved results may be derived from using a multiphase ring oscillator in combination with the sigma delta PLLs. 
   This allows errors associated with a noncircular, nonstable rotational platform to be addressed as well as other errors associated where these errors are exacerbated by small form factor high data density hard disk drives. The core PLLs of the read write channel may be a servo PLL and a read (data) PLL. Circuitry within the disk lock DLC clocking scheme of the present invention allow adjusting when the analog wave form is to be read or written to the disk. 
   In summary, embodiments of the present invention provide a disk drive controller is provided. The drive controller includes a servo system operable to associate a time stamp with an arrival of a servo wedge, a firmware loop and core PLLs in the read channel. The firmware loop is operable to determine a period between the arrival of a pair of consecutive servo wedges and produce a desired frequency of when to read/write data to disk based on the period between the arrival of a pair of consecutive servo wedges. Processing circuitry is operable to adjust a clock signal, wherein the clock signal itself is not locked to the data and produce a fine control signal for the core PLLs in the read channel. These core PLLs are operable to determine a phase and/or frequency associated with when an analog signal is sampled and/or written to disk, wherein these core PLLs comprises Sigma Delta PLLs. 
   As one of average skill in the art will appreciate, the term “substantially” or “approximately”, as may be used herein, provides an industry-accepted tolerance to its corresponding term. Such an industry-accepted tolerance ranges from less than one percent to twenty percent and corresponds to, but is not limited to, component values, integrated circuit process variations, temperature variations, rise and fall times, and/or thermal noise. As one of average skill in the art will further appreciate, the term “operably coupled”, as may be used herein, includes direct coupling and indirect coupling via another component, element, circuit, or module where, for indirect coupling, the intervening component, element, circuit, or module does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. As one of average skill in the art will also appreciate, inferred coupling (i.e., where one element is coupled to another element by inference) includes direct and indirect coupling between two elements in the same manner as “operably coupled”. As one of average skill in the art will further appreciate, the term “compares favorably”, as may be used herein, indicates that a comparison between two or more elements, items, signals, etc., provides a desired relationship. For example, when the desired relationship is that signal  1  has a greater magnitude than signal  2 , a favorable comparison may be achieved when the magnitude of signal  1  is greater than that of signal  2  or when the magnitude of signal  2  is less than that of signal  1 . 
   Although the present invention is described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as described by the appended claims.