Abstract:
The present invention is directed to a method and apparatus for modifying a spiral profile using reference tracks written onto a disk surface of a disk drive. In one embodiment, a disk drive includes a disk surface having both a read head and a write head associated therewith. A servo track writer is provided for moving the write head and the read head relative to the disk surface. The write head, under control of the servo track writer, is used to write both a first reference track near an outer diameter of the disk surface and a second reference track near an inner diameter of the disk surface. The servo track writer is used to move the write head according to a spiral profile when writing spiral servo information onto the disk surface, and the first and second reference tracks are used to modify the spiral profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed from U.S. Provisional Patent Application Ser. No. 60/475,141 filed Jun. 2, 2003, which is incorporated herein by reference in its entirety. Priority is also claimed from U.S. Provisional Patent Application Ser. No. 60/475,113 filed Jun. 2, 2003, which is also incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to data storage devices, such as disk drives. More particularly, the present invention relates to a method and apparatus for modifying a spiral profile using reference tracks written onto a disk surface of a disk drive. 
     BACKGROUND OF THE INVENTION 
     Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk. 
     A conventional disk drive, generally designated  10 , is illustrated in  FIG. 1 . The disk drive comprises a disk  12  that is rotated by a spin motor  14 . The spin motor  14  is mounted to a base plate  16 . An actuator arm assembly  18  is also mounted to the base plate  16 . 
     The actuator arm assembly  18  includes a transducer  20  mounted to a flexure arm  22  which is attached to an actuator arm  24  that can rotate about a bearing assembly  26 . The actuator arm assembly  18  also contains a voice coil motor  28  which moves the transducer  20  relative to the disk  12 . The spin motor  14 , voice coil motor  28  and transducer  20  are coupled to a number of electronic circuits  30  mounted to a printed circuit board  32 . The electronic circuits  30  typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device. 
     The disk drive  10  typically includes a plurality of disks  12  and, therefore, a plurality of corresponding actuator arm assemblies  18 . However, it is also possible for the disk drive  10  to include a single disk  12  as shown in  FIG. 1 . 
       FIG. 2  is a functional block diagram which illustrates a conventional disk drive  10  that is coupled to a host computer  33  via an input/output port  34 . The disk drive  10  is used by the host computer  33  as a data storage device. The host  33  delivers data access requests to the disk drive  10  via port  34 . In addition, port  34  is used to transfer customer data between the disk drive  10  and the host  33  during read and write operations. 
     In addition to the components of the disk drive  10  shown and labeled in  FIG. 1 ,  FIG. 2  illustrates (in block diagram form) the disk drive&#39;s controller  36 , read/write channel  38  and interface  40 . Conventionally, data is stored on the disk  12  in substantially concentric data storage tracks on its surface. In a magnetic disk drive  10 , for example, data is stored in the form of magnetic polarity transitions within each track. Data is “read” from the disk  12  by positioning the transducer  20  above a desired track of the disk  12  and sensing the magnetic polarity transitions stored within the track, as the track moves below the transducer  20 . Similarly, data is “written” to the disk  12  by positioning the transducer  20  above a desired track and delivering a write current representative of the desired data to the transducer  20  at an appropriate time. 
     The actuator arm assembly  18  is a semi-rigid member that acts as a support structure for the transducer  20 , holding it above the surface of the disk  12 . The actuator arm assembly  18  is coupled at one end to the transducer  20  and at another end to the VCM  28 . The VCM  28  is operative for imparting controlled motion to the actuator arm  18  to appropriately position the transducer  20  with respect to the disk  12 . The VCM  28  operates in response to a control signal i control  generated by the controller  36 . The controller  36  generates the control signal i control , for example, in response to an access command received from the host computer  33  via the interface  40  or in response to servo information read from the disk surface  12 . 
     The read/write channel  38  is operative for appropriately processing the data being read from/written to the disk  12 . For example, during a read operation, the read/write channel  38  converts an analog read signal generated by the transducer  20  into a digital data signal that can be recognized by the controller  36 . The channel  38  is also generally capable of recovering timing information from the analog read signal. During a write operation, the read/write channel  38  converts customer data received from the host  33  into a write current signal that is delivered to the transducer  20  to “write” the customer data to an appropriate portion of the disk  12 . As will be discussed in greater detail, the read/write channel  38  is also operative for continually processing data read from servo information stored on the disk  12  and delivering the processed data to the controller  36  for use in, for example, transducer positioning. 
       FIG. 3  is a top view of a magnetic storage disk  12  illustrating a typical organization of data on the surface of the disk  12 . As shown, the disk  12  includes a plurality of concentric data storage tracks  42 , which are used for storing data on the disk  12 . The data storage tracks  42  are illustrated as center lines on the surface of the disk  12 ; however, it should be understood that the actual tracks will each occupy a finite width about a corresponding centerline. The data storage disk  12  also includes servo information in the form of a plurality of radially-aligned servo spokes  44  (or wedges) that each cross the tracks  42  on the disk  12 . The servo information in the servo spokes  44  is read by the transducer  20  during disk drive operation for use in positioning the transducer  20  above a desired track  42  of the disk  12 . Among other things, the servo information includes a plurality of servo bursts (e.g., A, B, C and D bursts or the like) that are used to generate a Position Error Signal (PES) to position the write head relative to a track&#39;s centerline during a track following operation. The portions of the track between servo spokes  44  are used to store customer data received from, for example, the host computer  33  and are referred to as customer data regions  46 . 
     It should be understood that, for ease of illustration, only a small number of tracks  42  and servo spokes  44  have been shown on the surface of the disk  12  of  FIG. 3 . That is, conventional disk drives include one or more disk surfaces having a considerably larger number of tracks and servo spokes. 
     During the disk drive manufacturing process, a special piece of equipment known as a servo track writer (STW) is used to write the radially-aligned servo information which forms servo spokes  44 . A STW is a very precise piece of equipment that is capable of positioning the disk drive&#39;s write head at radial positions over the disk surface, so that servo information is written on the disk surface using the disk drive&#39;s write head with a high degree of positional accuracy. 
     In general, a STW is a very expensive piece of capital equipment. Thus, it is desirable that a STW be used as efficiently as possible during manufacturing operations. Even a small reduction in the amount of data needed to be written by the STW per disk surface can result in a significant cost and time savings. 
     A STW is used to write servo information, by controlling the position of the disk drive&#39;s write head, on a disk surface in a circumferential fashion at each radius at which the disk drive&#39;s write head is positioned. During drive operation, the servo information is used to position the transducer of the disk drive over the appropriate data track and data sector of the disk. Accordingly, as the number of tracks per inch (TPI) increases, the amount of time necessary to write servo information increases. That is, the number of circumferential passes that a STW must make over a disk surface increases as TPI increases. Thus, unless more STWs are supplied, manufacturing times will continually increase as the TPI increases. 
     Instead of using a STW to write servo information in a circumferential fashion at each radius, the assignee of the present invention presently uses a STW to write servo information in a spiral fashion (in at least some of its disk drives). Specifically, the STW moves the write head in a controlled manner (e.g., at a constant velocity or along a velocity profile) from a location near the outer diameter of the disk to a location near the inner diameter of the disk (or visa-versa) as the disk spins. 
       FIG. 4  is a diagrammatic representation of a disk surface  210  having a first spiral of servo information  215  written thereon. The dashed line, identified by reference numeral  220 , represents a track. The first spiral of servo information  215  may make multiple revolutions around the disk surface  210  (roughly two revolutions as shown in  FIG. 4 ), but only crosses track  220  once. 
       FIG. 5  is a diagrammatic representation of a disk surface  210  having a first spiral of servo information  215  and a second spiral of servo information  225  written thereon. As shown in  FIG. 5 , the first and second spirals  215 ,  225  are interlaced with one another and are written approximately 180 degrees apart. Again, each spiral crosses track  220  only once. 
     Additional spirals of servo information may be written on the disk surface  210  depending upon the servo sample rate (that is, the number of servo samples required for each track  220  to keep the disk drive&#39;s transducer sufficiently on-track). For example, if a servo sample rate of 120 equally-spaced servo sectors per track was required, 120 equally-spaced spirals would be written on the disk surface  110 . Accordingly, by writing servo information in a spiral fashion, the time necessary to write servo information on disk surface  110  using the STW is a function of the servo sample rate (i.e., the number of spirals of servo information to be written) rather than the number of tracks. 
     Referring again to  FIGS. 4 and 5 , the spirals of servo information are written by moving the disk drive&#39;s write head using the STW in a generally radial direction (more accurately, in a radial direction along an arc due to the position of the bearing assembly), while both the disk is spinning and the write head is enabled. The direction of disk rotation is indicated by an arrow as shown in each of  FIGS. 4 and 5 . 
     The disk drive&#39;s write head is enabled for nearly its entire stroke (i.e., from a position near the OD to a position near the ID or visa-versa) while under the control of the STW. As a result, a continuous spiral of servo information is written. 
     Each of the spirals of servo information includes sync marks written at fixed time intervals by the disk drive&#39;s write head. As mentioned above, the STW is used to move the disk drive&#39;s write head at some fixed velocity (or velocity profile) in a generally radial direction across the disk surface. If the time interval between sync marks is known and the velocity of the disk drive&#39;s write head is known, the distance between sync marks along a spiral can be determined. Specifically, the following formula may be applied: Distance=(STW Velocity)(Time), where Distance represents the radial distance between sync marks, Velocity represents the radial velocity of the disk drive&#39;s write head (under control of the STW) and Time represents the interval between sync marks. 
     For example, the interval between sync marks may be set at 1 microsecond, while the write head may be controlled to move at a radial velocity of 10 inches per second along its stroke. Thus, the radial distance between sync marks can be calculated to be 1 microinch along each spiral. 
     Each sync mark along a given spiral corresponds to a unique radius. Accordingly, the sync marks may be used to accurately position a transducer of a disk drive over the disk surface. 
     When writing spiral servo information onto a disk surface, the STW measures the angular position of the disk drive&#39;s actuator using an optical encoder that is concentric with the actuator&#39;s axis of rotation. The STW simultaneously tracks the amount of disk rotation using a stationary head (referred to as the clock head) to sense a timing reference track (i.e., a clock track) on the disk surface. The clock track is equivalent to an encoder for disk rotation. The process of writing spirals entails sweeping the actuator through a prescribed angle θ for a given amount of disk rotation ω while a pattern (e.g., as described above) is written by the disk drive&#39;s write head as shown in  FIG. 6 . 
     The STW also includes a digital signal processor (DSP) which, during spiral write, samples the optical encoder at a rate which is locked (via the clock track) to a set amount of disk rotation ω 0 . Doing so makes the desired amount of sweep per spin angle equivalent to a desired amount of sweep per sample hit θ(k). This provides a number of advantages, the most relevant being that the position profile θ(k) can be pre-calculated as a function of sample hit for any desired spiral shape. A simple case, shown in  FIG. 6 , is where the actuator is swept in at a constant velocity: θ(k)−θ(k−1)=θ 0 . As illustrated, the position of the disk drive&#39;s write head is at sample hit k=1. This position is arrived at by moving from the initial radius R(0) to radius R(1) as the disk rotates through the angle ω 0 . 
     If, after a further rotation of ω 0 , the actuator has swept through another increment θ 0 , then the disk drive&#39;s write head should arrive at the point on the spiral labeled k=2. This is illustrated in  FIG. 7 . 
     When writing spirals, the optical encoder signal is fed back and compared with a desired spiral profile at each sample hit. The error between the measured spiral profile and desired spiral profile is used by the STW servo system to compute a torque-based correction applied to the actuator. Spiral profile tracking performance and disturbance rejection are both considered in the design of the STW servo algorithm. 
     Spiral Runout 
     There are, unfortunately, disturbances during spiral writing that are not observable by the STW optical encoder used to sense actuator position. Significant among such disturbances are dimension changes in the actuator arm, disk, and push-pin damping material that are primarily due to thermal phenomena during spiral writing. These dimension changes affect the relative geometry between the disk and actuator, and thereby distort the spiral shape away from that desired. One possible manifestation of this effect is shown in  FIG. 8  where the drive is shown writing the N th  spiral adjacent the 1 st  spiral. 
     Specifically, during the time interval between writing the 1 st  spiral and N th  spiral, the actuator pivot to write head distance has increased. The effect of this geometry-change places the N th  spiral at a distance from the 1 st  spiral that is greater than that desired even if the STW positioning system precisely executes the prescribed sweep angle per sample hit. 
     The assignee of the present invention has developed a technique for self-servo writing using the spiral servo information written onto the disk surface. In one case, the final servo patterns written by the drive appear substantially identical to traditional servo patterns. 
     At any given spiral servo track, correctly placed spirals exhibit an exact spiral-to-spiral spacing and the drive drive&#39;s servo system utilizes this as part of a technique to position the actuator. Spacing error of the spirals around the revolution, or spiral runout, can result in the degradation of drive position error while track following. If the spiral spacing error is extreme, the drive will fail to self-write. 
       FIG. 9  shows spiral runout resulting from drift in the spiral start location with respect to the disk surface. In  FIG. 9 , the disk has been “unwrapped” so that disk rotation/spin time is shown as increasing along the x-axis while actuator position/sweep angle increases along the y-axis. As illustrated, an outer diameter (OD) shift in spiral start is clearly seen to cause a spacing/timing shift in the spirals in the “downtrack” direction, which is constant across all radii. It should be noted that  FIG. 9  illustrates the effect of a constant, incremental drift of each spiral with respect to the previous spiral. 
       FIG. 10  plots the resulting spiral runout (cumulative downtrack shift) due to the shift shown in  FIG. 9 . Each spiral starts at the same amount of offset relative to the previous spiral, but this accumulates into an increasingly larger offset from the desired location. Clearly, this causes a large discontinuity in the runout between the first-written spiral and last-written spiral, which leads to a similar, undesirable discontinuity in the drive position error signal. While the spirals can be written in a different sequence to “smooth” the discontinuity (see, U.S. patent application Ser. No. 10/788,242 entitled “Method And Apparatuses For Writing Spiral Servo Patterns Onto A Disk Surface” filed Feb. 26, 2004, which is incorporated herein by reference), the spiral runout may still have a maximum magnitude equal to the maximum offset from desired. As illustrated in  FIG. 9 , this runout signature will be constant at all radii when the geometrical distortion is independent of radius. 
     If the inner diameter (ID) drift is not the same as the OD drift, then the spiral runout is not constant across all radii.  FIG. 11  illustrates this case. Here the OD and ID have drifted differently, causing both an offset and slope error in each spiral. While sequentially-written spirals will have the same discontinuous signature as before, the magnitude of the discontinuity will change from OD to ID. 
     In view of the above, it would be desirable to develop a method for reducing spiral runout due to, e.g., the aforementioned dimension changes. 
     SUMMARY 
     Various embodiments of the present invention are generally directed to the writing of servo reference tracks to a disk surface. 
     In accordance with some embodiments, a method generally comprises providing a read head and a write head associated therewith, and a servo track writer for moving said write head and said read head relative to the disk surface. First and second reference tracks are written to the disk surface by the write head, the first and second reference tracks each comprising a periodically phase shifted tone frequency pattern. 
     In accordance with other embodiments, a method generally comprises providing a disk surface having a read head and a write head associated therewith, and a servo track writer for moving said write head and said read head relative to the disk surface. First and second reference tracks are written to the disk surface using the write head. The servo track writer is used to move the write head according to a spiral profile to write spiral servo information across the disk surface, the spiral profile adjusted in relation to the first and second reference tracks. 
     In accordance with still other embodiments, an apparatus generally comprises a disk surface having a read head and a write head associated therewith. The disk surface comprises a first reference track, a second reference track, and spiral servo information written in accordance with a spiral profile adjusted in relation to the first and second reference tracks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation illustrating a conventional disk drive with its top cover removed; 
         FIG. 2  is a functional block diagram which illustrates a conventional disk drive that is coupled to a host computer via an input/output port; 
         FIG. 3  is a diagrammatic representation of a top view of a magnetic storage disk illustrating a typical organization of data on a disk surface; 
         FIG. 4  is a diagrammatic representation of a disk surface having a spiral of servo information written thereon, along with a circular data storage track; 
         FIG. 5  is a diagrammatic representation of a disk surface having two spirals of servo information written thereon, along with a circular data storage track; 
         FIG. 6  is a diagrammatic representation of a spiral prescribed as a function of sample hit with the disk drive&#39;s write head at sample k=1; 
         FIG. 7  is a diagrammatic representation of a spiral prescribed as a function of sample hit with the disk drive&#39;s write head at sample k=2; 
         FIG. 8  is a diagrammatic representation of a spiral spacing error due to geometric change in actuator pivot to write head distance; 
         FIG. 9  is a diagrammatic representation of spiral position error due to OD misalignment of spirals; 
         FIG. 10  is a diagrammatic representation of spiral runout due to OD accumulation of alignment error; 
         FIG. 11  is a diagrammatic representation of spiral position error due to OD and ID misalignment of spirals; 
         FIG. 12  is a block diagram representation of a reference track pattern demodulation circuit; 
         FIG. 13  is a diagrammatic representation of reference track distortion due to geometric change in actuator pivot to write head distance; 
         FIG. 14  is a diagrammatic representation of a square wave compared to a reference track pattern that is used in connection with one embodiment of the present invention; 
         FIG. 15  is a graphical representation of detected reference track counts showing both raw counts and filtered counts; 
         FIG. 16  is a graphical representation illustrating exemplary OD reference track motion and exemplary ID reference track motion; 
         FIG. 17  is a graphical representation of an RLS reference track filter acting on a noisy data set; 
         FIG. 18  is a simplified block diagram illustrating reference track correction applied to a spiral servo loop in accordance with one embodiment of the present invention; and, 
         FIG. 19  is a graphical representation illustrating spiral position error both with and without reference track correction. 
         FIG. 20  shows respective frequencies of reference tracks and spirals of servo information. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiments in many different forms, there are shown in the drawings and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
     The present invention includes a method and apparatus for mitigating spiral runout through the use of data head reference tracks against which spiral placement can be tracked. Among other things, the technique is to “pin” the same points on all written spirals to the locations where reference tracks are detected. Spiral profiles are defined as a function of the STW positioner location θ(k), but ultimately the profile should be determined as a function of the disk drive&#39;s head radius versus disk spin. Reference tracks allow the position of the disk drive&#39;s head to be charted as a function of the STW optical encoder. This, in turn, allows the desired profile to be shifted to match the observed change in location where the disk drive&#39;s head intercepts the reference track. Doing so pins specific points on each spiral so that they are placed consistently relative to the reference tracks and, consequently, the disk surface. 
     Reference Track Pattern and Detector 
     Each reference track is nearly a simple square wave. The playback of this pattern is detected using a standard receiver topology as shown in  FIG. 12 . The read head playback is compared against an expected pattern and, when a large enough section matches the expected pattern, a threshold is crossed, triggering a counter to be incremented. A DSP is able to read the number of detections that occur in the interval between reading, since each reading resets the counter. Note that the clock playback is used to sample the read head playback, rather than doing some sort of sample clock recovery from the data playback. This affords a simple implementation of the detector, but may leave the detection susceptible to problems caused by the dimension changes described earlier. Their effect is depicted in  FIG. 13 . 
     As the actuator pivot to write head distance changes, the read head will intercept reference track transitions at points that “slide” with respect to the clock track. If the reference track transitions are intercepted so that they are coincident at the detector with the clock track transitions, then noise can make the playback appear random, arriving on either side of the clock edge, causing a precipitous drop in successful detections. To mitigate this effect, the simple square wave reference pattern is modified so that sections of constant period are separated by special elongated periods, as illustrated in  FIG. 14 . 
     This allows only a small percentage of reference cycles to be coincident with the clock track at any given time. The maximum achievable number of detections is decreased slightly, but precipitous drops in successful detections no longer occur, due to relative shifts between clock and reference track reception. 
     Reference Track Based Profile Correction 
     During the interval between each DSP servo interrupt, the number of reference pattern detections is counted. When the read head is over the reference track, the number of detections increases significantly compared with the number of false detections occurring outside the reference track. 
       FIG. 15  plots the raw counts (curve  1502 ) as a function of servo interrupt. The raw count sequence is low-pass filtered (curve  1504 ) and the maximum filter count value is used as the reference track “center.” Due to the phase-lag of the low pass filter, this “center” is shifted from the true center. This shift is unimportant because it is consistent for each pass over the reference track and the correction of the spiral profile uses only the relative change in reference location from one pass to the next. The change in reference track “center” observed as spirals are written is a measure of the shift in disk surface relative to the external STW optical encoder. The spiral profile, which is controlled as a function of optical encoder counts, can be adjusted by the detected reference track shift, so that the spiral profile remains consistent relative to the reference tracks and, thus, the disk surface. 
     Reference Track Trajectories 
     In one embodiment of the present invention, the spiral position profile is adjusted using both an OD reference track and ID reference track. It should be understood that the present invention is not limited to using an OD reference track and an ID reference track. That is, more reference tracks can be used. However, it is believed that two reference tracks are sufficient to reduce spiral runout at all radii. 
       FIG. 16  shows the detected shift of both the OD reference track and ID reference track versus each spiral written. Both the OD reference track and ID reference track drift close to 100 STW tracks with respect to the STW optical encoder. Without correction, this amount of drift can result in spiral runout so large that the drive may not be able to track-follow on spirals well enough to perform self-servo write operations (e.g., to write final servo patterns, so that this disk surface is formatted in a fashion similar to the disk surface shown in  FIG. 3 ). It is also clear from this figure that the OD reference track and the ID reference track drift differently, confirming the need for at least two reference tracks. 
     The consistency of reference track detection can be severely degraded by read head playback amplitude, electronics noise, detector clock phase, etc. In volume production, it is typical to see “noisy” reference track trajectories. In order to keep this detection noise form degrading the spiral profile correction, a special filtering scheme is used on the sequence of reference track shifts. 
       FIG. 17  shows an example of reference track motion with a noise spike. The filtering scheme uses Recursive Least Squares (RLS) on the change in reference track position in order to predict the slope of the trajectory. The estimate of trajectory slope is used as a rate limiter on the unfiltered trajectory, and any change in reference location larger than the RLS estimated change is clamped at the RLS limit. Because an RLS algorithm with forgetting factor is used, the bound is large at both the beginning of the spiral write and also after noise spikes. The bounds tighten as more samples with consistent slope are recorded. The convergence rate of the RLS filter has been tuned to provide for expected reference track motion while filtering out obvious noise. Finally, the sequence of detected and clamped reference track shifts is also low-pass filtered to smooth the spiral profile correction (curve D). 
     Spiral Profile Correction 
     Once filtered versions of both the OD reference track shift and the ID reference track shift have been obtained, the spiral position profile can be modified.  FIG. 18  is a simplified block diagram representation of the reference track correction to the spiral servo loop. 
     The reference track demodulation block and RLS/Low pass filter block have been discussed above. The profile correction generator uses the filtered reference track shifts to generate a vector of correction values, which are then added to a table of spiral profile positions before being passed to the compensator. In one embodiment, a simple offset and slope correction generator is used. The OD reference track provides the profile correction vector offset, while the relative change between the ID reference track and the OD reference track forms the profile correction vector slope. It is believed that this simple linear fit is sufficient for reducing spiral runout at all radii. It should be understood that the present invention is not limited to the above-described profile correction generator. For example, a more sophisticated correction generator, such as a polynomial fit, may also be used. 
       FIG. 19  shows spiral runout for sequentially-written spirals around 1 revolution both with and without reference track correction. The reference tracks correct for the spiral drift, removing the spiral position error around the revolution and canceling the large discontinuity. 
     In one embodiment, the reference tracks are circular tracks written onto the disk surface by the disk drive&#39;s write head, wherein a first reference track is written near the inner diameter of the disk surface and the second reference track is written near the outer diameter of the disk surface. Furthermore, the servo track writer is used to position the write head when writing the first and second reference tracks. It should be understood, however, that one or more reference tracks may be written onto the disk surface by the disk drive&#39;s write head without being positioned by the servo track writer. It should also be understood that reference tracks may be provided on the disk surface via other means, e.g., printed media or etching processes. It should also be understood that the reference tracks do not necessarily have to be circular. 
     It should be noted that initial positions of the reference tracks relative to the servo track writer may be stored in memory at the time of writing the reference tracks. However, if delays occur between the time of writing the reference tracks and the time of writing the first spiral of servo information, initial positions of the reference tracks relative to the servo track writer may be determined just prior to writing the first spiral of servo information onto the disk surface. In another embodiment, the initial positions of the reference tracks relative to the servo track writer are determined just after writing the first spiral (or other spirals) of servo information. 
     In one embodiment, spiral servo information is written by the write head as the servo track writer moves the write head in a first direction (i.e., either from OD to ID or visa-versa) and the read head reads reference tracks as the servo track writer moves the read head in a second direction (i.e., from ID to OD or visa versa). Since spiral servo information is generally written in one direction from a radial starting point either at the OD or the ID, reading reference tracks when returning to the radial starting point is considered to be efficient because one or more extra cycles across the stroke do not have to be provided to read the reference tracks. Furthermore, there is less of a delay (and, hence, less opportunity for thermal changes and the like) between the time of reading the reference tracks to adjust the spiral profile and the time of writing the next spiral of servo information. 
     It should be noted that the frequency of the pattern in the reference tracks is preferably different than the frequency of the spirals of servo information as shown in  FIG. 20 . 
     U.S. patent application Ser. No. 10/860,063 entitled “Method And Apparatus For Performing Best Head Detection In A Disk Drive Using Reference Tracks” filed on even date herewith is incorporated by reference. U.S. Provisional Patent Application Ser. No. 60/475,113 entitled “Best Head Detection Using Reference Tracks For Spiral Servo Track Write” filed Jun. 2, 2003 (from which the above-identified patent application claims priority) is also incorporated by reference. 
     While an effort has been made to describe some alternatives to the preferred embodiment, other alternatives will readily come to mind to those skilled in the art. Therefore, it should be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not intended to be limited to the details given herein.