Abstract:
A disk drive is disclosed having a disk comprising a plurality of spiral tracks, a head actuated over the disk, and control circuitry operable to process the spiral tracks in an interleaved manner to write a plurality of product servo sectors to the disk to define a plurality of radially spaced, concentric data tracks.

Description:
This application is a divisional of patent application Ser. No. 10/769,387 filed on Jan. 31, 2004 and issued as U.S. Pat. No. 6,965,489. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to disk drives for computer systems. More particularly, the present invention relates to servo writing a disk drive by processing spiral tracks in an interleaved manner. 
   2. Description of the Prior Art 
   When manufacturing a disk drive, product servo sectors  2   0 – 2   N  are written to a disk  4  which define a plurality of radially-spaced, concentric data tracks  6  as shown in the prior art disk format of  FIG. 1 . Each product servo sector (e.g., servo sector  2   4 ) comprises a preamble  8  for synchronizing gain control and timing recovery, a sync mark  10  for synchronizing to a data field  12  comprising coarse head positioning information such as a track number, and servo bursts  14  which provide fine head positioning information. During normal operation the servo bursts  14  are processed by the disk drive in order to maintain a head over a centerline of a target track while writing or reading data. In the past, external servo writers have been used to write the product servo sectors  2   0 – 2   N  to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the product servo sectors  2   0 – 2   N  are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the head disk assembly (HDA) without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process. 
   The prior art has suggested various “self-servo” writing methods wherein the internal electronics of the disk drive are used to write the product servo sectors independent of an external servo writer. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral tracks to the disk which are then processed to write the product servo sectors along a circular path. The spiral tracks are written “open loop” by seeking the head from an outer diameter of the disk to an inner diameter of the disk. The disk drive calibrates acceleration/deceleration impulses to seek the head from the outer to inner diameter in a desired amount of time. Accurate radial positioning of the spiral tracks assumes the calibration process is accurate and that the calibrated acceleration/deceleration impulses will generate a repeatable response over multiple seeks. However, the calibration process will inevitably exhibit some degree of error and the dynamics of the disk drive will change between seeks inducing errors in the radial position of the spiral tracks. Dynamic errors which degrade the spiral tracks written during an open loop seek include vibration of the HDA, flutter and non-repeatable run-out of the disk and spindle bearings, stiction and non-repeatable run-out of the pivot bearings, windage on the head and arm, and flex circuit bias, windage and vibration. Errors in writing the spiral tracks will propagate to the product servo sectors, thereby degrading the operating performance of the disk drive and reducing the manufacturing yield. 
   In the &#39;679 patent, each spiral track is written to the disk as a high frequency signal (with missing bits), wherein the position error signal (PES) for tracking is generated relative to time shifts in the detected location of the spiral tracks. In addition, the &#39;679 patent generates a servo write clock by synchronizing a phase-locked loop (PLL) to the missing bits in the spiral tracks. In order to initially synchronize the PLL to the missing bits the head must servo accurately in a circular path since PLL phase error can occur due to actual timing errors or radial tracking errors. Conversely, PLL phase errors cause radial tracking errors making it difficult to simultaneously maintain the head in a circular path by servoing on the spiral tracks while attempting to synchronize the PLL to the missing bits. 
   There is, therefore, a need to improve the servo writing process for a disk drive by reducing the bottleneck and expense of external servo writers while maintaining adequate operating performance and manufacturing yield. 
   SUMMARY OF THE INVENTION 
   An embodiment of the present invention comprises a disk drive having a disk comprising a plurality of spiral tracks, a head actuated over the disk, and control circuitry operable to process the spiral tracks in an interleaved manner to write a plurality of product servo sectors to the disk to define a plurality of radially spaced, concentric data tracks. 
   In one embodiment, each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark. 
   In another embodiment, the spiral tracks are written to the disk using an external spiral servo writer. 
   In yet anther embodiment, the spiral tracks comprise a first subset and a second subset, the control circuitry operable to process the first subset of spiral tracks when writing the product servo sectors over the second subset of spiral tracks, and the control circuitry operable to process the second subset of spiral tracks when writing the product servo sectors over the first subset of spiral tracks. 
   Another embodiment of the present invention comprises a method of writing a plurality of product servo sectors to a disk of a disk drive to define a plurality of radially spaced, concentric data tracks, wherein the disk drive comprises a head actuated over the disk, and the disk comprises a first subset of spiral tracks and a second subset of spiral tracks. The first subset of spiral tracks are processed when writing the product servo sectors over the second subset of spiral tracks, and the second subset of spiral tracks are processed when writing the product servo sectors over the first subset of spiral tracks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a prior art disk format comprising a plurality of radially spaced, concentric tracks defined by a plurality of product servo sectors. 
       FIGS. 2A and 2B  illustrate an embodiment of the present invention wherein an external spiral servo writer is used to write a plurality of reference servo sectors and a plurality of spiral tracks to the disk for use in writing product servo sectors to the disk. 
       FIG. 3A  shows an embodiment of the present invention wherein a servo write clock is synchronized by clocking a modulo-N counter so that it reaches terminal count at the frequency of sync marks in the reference servo sectors. 
       FIG. 3B  shows an eye pattern generated by reading the spiral track, including the sync marks in the spiral track. 
       FIG. 4  illustrates an embodiment of the present invention wherein synchronization of the servo write clock is maintained from a coarse timing recovery measurement generated in response to the sync marks recorded in the spiral tracks and a fine timing recovery measurement generated in response to the high frequency signal in the spiral tracks. 
       FIGS. 5A–5B  illustrate how in one embodiment the control circuitry for demodulating the servo bursts in product servo sectors is also used to demodulate the high frequency signal in the spiral tracks as servo bursts to generate the PES for tracking. 
       FIGS. 6A–6B  show an embodiment of the present invention for calibrating the correlation between the PES generated from reading the spiral tracks and off-track displacement. 
       FIG. 7  shows an embodiment of the present invention wherein an external product servo writer is used to process the spiral tracks in order to write the product servo sectors to the disk. 
       FIG. 8  shows an embodiment of the present invention wherein an external spiral servo writer is used to write the reference servo sectors and the spiral tracks, and a plurality of external product servo writers write the product servo sectors for the HDAs output by the external spiral servo writer. 
       FIG. 9  shows an embodiment of the present invention wherein an external spiral servo writer is used to write the reference servo sectors and the spiral tracks, and the control circuitry within each product disk drive is used to write the product servo sectors. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2A and 2B  illustrate an embodiment of the present invention for writing product servo sectors to a disk  16  of a disk drive  18 . The disk drive  18  comprises control circuitry  20  and a head disk assembly (HDA)  22  comprising the disk  16 , an actuator arm  24 , a head  26  connected to a distal end of the actuator arm  24 , and a voice coil motor  28  for rotating the actuator arm  24  about a pivot to position the head  26  radially over the disk  16 . A head positioning pin  30  of an external spiral servo writer  32  is inserted into the HDA  22 , wherein the head positioning pin  30  for engaging the actuator arm  24 . The external spiral servo writer  32  derives a radial location of the head  26  and actuates the head positioning pin  30  in response to the radial location of the head  26  in a closed loop system to rotate the actuator arm about the pivot in order to position the head radially over the disk  16  while writing a plurality of spiral tracks  36   0 – 36   N , wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark. 
   The head positioning pin  30  is then removed from the HDA  22 , and the head  26  internal to the disk drive  18  is used to read the high frequency signal  38  in the spiral tracks  36   0 – 36   N . A position error signal is generated to maintain the head  26  along a substantially circular target path while reading the sync marks  40  in the spiral tracks  36   0 – 36   N  to generate a spiral sync mark detect signal, wherein a servo write clock is synchronized in response to the spiral sync mark detect signal. The servo write clock and the head  26  internal to the disk drive  18  are then used to write the product servo sectors along the circular target path. 
   In one embodiment, the external spiral servo writer  32  also writes a plurality of reference servo sectors  34  ( FIG. 2B ) in a substantially circular reference path, each reference servo sector  34  comprising a sync mark  10  and a plurality of servo bursts  14  ( FIG. 1 ). The servo bursts  14  in the reference servo sectors  34  are then read using the head  26  internal to the disk drive  18  to generate a position error signal used to maintain the head  26  along the circular reference path while reading the sync marks  10  in the reference servo sectors  34  to generate a reference sync mark detect signal which is used to initially synchronize the servo write clock. 
   In the embodiment of  FIG. 2A , the external spiral servo writer  32  comprises a head positioner  42  for actuating the head positioning pin  30  using sensitive positioning circuitry, such as a laser interferometer. Pattern circuitry  44  generates the data sequence written to the disk  16  for the reference servo sectors  34  and the spiral tracks  36   0 – 36   N . The external spiral servo writer  32  writes a clock track  46  ( FIG. 2B ) at an outer diameter of the disk  16 , and a clock head  48  is inserted into the HDA  22  for reading the clock track  46  to generate a clock signal  50 . Timing circuitry  52  in the external spiral servo writer  32  processes the clock signal  50  to enable the pattern circuitry  44  at the appropriate time so that the reference servo sectors  34  and spiral tracks  36   0 – 36   N  are written at the appropriate circumferential location. The clock signal  50  also enables the pattern circuitry  44  to write the sync marks  40  ( FIG. 3B ) within the spiral tracks  36   0 – 36   N  at the same circumferential location from the outer diameter to the inner diameter of the disk  16 . As described below with reference to  FIG. 4 , the constant interval between sync marks  40  (independent of the radial location of the head  26 ) enables the servo write clock to maintain synchronization. 
   In the embodiment of  FIG. 2A , the entire disk drive  18  is shown as being inserted into the external spiral servo writer  32 . In an alternative embodiment, only the HDA  22  is inserted into the external spiral servo writer  32 . 
   After the external spiral servo writer  32  writes the reference servo sectors  34  and the spiral tracks  36   0 – 36   N  to the disk  16 , the head positioning pin  30  and clock head  48  are removed from the HDA  22  and the product servo sectors are written to the disk  16 . In one embodiment, the control circuitry  20  within the disk drive  18  is used to process the reference servo sectors  34  and spiral tracks  36   0 – 36   N  in order to write the product servo sectors to the disk  16 . In an alternative embodiment described below with reference to  FIGS. 7 and 8 , an external product servo writer is used to process the reference servo sectors  34  and spiral tracks  36   0 – 36   N  in order to write the product servo sectors to the disk  16  during a “fill operation”. 
   At the beginning of the fill operation, the reference servo sectors  34  are processed in order to synchronize the servo write clock. The reference servo sectors  34  are processed similar to conventional product servo sectors. The circumferential location of the reference servo sectors  34  is first determined by searching for the sync mark  10  asynchronously. Once a sync mark  10  is detected, the reference servo sectors  34  are detected synchronously by synchronizing a read clock to the preamble  8  ( FIG. 1 ) preceding the sync mark  10 . The servo bursts  14  in the reference servo sectors  34  are also demodulated in a conventional manner to generate a position error signal (PES) for use in maintaining (tracking) the head  26  along a circumferential path while reading the reference servo sectors  34 . In one embodiment, several tracks of reference servo sectors  34  are written to the disk  16  to facilitate finding and tracking the reference servo sectors  34 . 
     FIG. 3A  shows an embodiment of the present invention wherein two spiral tracks are written between each reference servo sector (e.g., spiral tracks  36   0  and  36   1  written between reference servo sectors  34   0  and  34   1 ). Also shown in  FIG. 3A  is a saw-tooth waveform  54  representing the value of a modulo-N counter. The modulo-N counter is clocked by the servo write clock, and the frequency of the servo write clock is adjusted until the modulo-N counter reaches terminal count synchronous with detecting the sync mark  10  in the reference servo sector  34 . The servo write clock may be generated using any suitable circuitry. In one embodiment, the servo write clock is generated using a phase locked loop (PLL). As each sync mark  10  in the reference servo sectors  34  is detected, the value of the modulo-N counter represents the phase error for adjusting the PLL. Once the modulo-N counter reaches terminal count substantially synchronous with detecting the sync marks  10  in the reference servo sectors  34 , the servo write clock is coarsely locked to the desired frequency for writing the product servo sectors to the disk. 
   After synchronizing the servo write clock in response to the reference servo sectors, the spiral tracks  36   0 – 36   N  are read to generate the PES signal for tracking as well as to maintain synchronization of the servo write clock.  FIG. 3B  illustrates an “eye” pattern in the read signal that is generated when the head  26  passes over a spiral track  36 . The read signal representing the spiral track comprises high frequency transitions  38  interrupted by sync marks  40 . When the head  26  moves in the radial direction, the eye pattern will shift (left or right) while the sync marks  40  remain fixed. The shift in the eye pattern (detected from the high frequency signal  38 ) relative to the sync marks  40  provides the off-track information for servoing the head  26 . 
   The sync marks  40  in the spiral tracks  36   0 – 36   N  may comprise any suitable pattern, and in one embodiment, a pattern that is substantially shorter than the sync mark  10  in the reference servo sectors  34 . Referring again to  FIG. 3A , when the sync marks  40  in the spiral tracks  36   0 – 36   N  are detected, the value of the modulo-N counter is compared to an expected value, and the resulting error represents the phase error for adjusting the PLL that generates the servo write clock. In one embodiment, the PLL is updated when any one of the sync marks  40  within the eye pattern is detected. In this manner the multiple sync marks  40  in each eye pattern (each spiral track crossing) provides redundancy so that the PLL is still updated if one or more of the sync marks  40  are missed due to noise in the read signal. 
   In one embodiment, the servo write clock is further synchronized by generating a timing recovery measurement from the high frequency signal  38  between the sync marks  40  in the spiral tracks  36   0 – 36   N . Synchronizing the servo write clock to the high frequency signal  38  helps maintain proper radial alignment (phase coherency) of the Gray coded track addresses in the product servo sectors. The timing recovery measurement may be generated in any suitable manner. In one embodiment, the servo write clock is used to sample the high frequency signal  38  and the signal sample values are processed to generate the timing recovery measurement. The timing recovery measurement adjusts the phase of the servo write clock (PLL) so that the high frequency signal  38  is sampled synchronously. In this manner, the sync marks  40  provide a coarse timing recovery measurement and the high frequency signal  38  provides a fine timing recovery measurement for maintaining synchronization of the servo write clock. 
     FIG. 4  illustrates how the product servo sectors  56   0 – 56   N  are written to the disk  16  after synchronizing the servo write clock using the reference servo sectors  34   0 – 34   N . In the embodiment of  FIG. 4 , the dashed lines represent the centerlines of the data tracks. The sync marks in the spiral tracks  36   0 – 36   N  are written so that there is a shift of two sync marks in the eye pattern ( FIG. 3B ) between data tracks. In an alternative embodiment, the sync marks in the spiral tracks  36   0 – 36   N  are written so that there is a shift of N sync marks in the eye pattern between data tracks. In practice the width of the spiral tracks  36   0 – 36   N  in the embodiment of  FIG. 4  will be proximate the width of a data track. The spiral tracks  36   0 – 36   N  are shown in  FIG. 4  as being wider than the width of a data track for illustration purposes. 
   The PES for maintaining the head  26  along a servo track (tracking) may be generated from the spiral tracks  36   0 – 36   N  in any suitable manner. In one embodiment, the PES is generated by detecting the eye pattern in  FIG. 3B  using an envelope detector and detecting a shift in the envelope relative to the sync marks  40 . In one embodiment, the envelope is detected by integrating the high frequency signal  38  and detecting a shift in the resulting ramp signal. In an alternative embodiment disclosed below with reference to  FIG. 5B , the high frequency signal  38  between the sync marks  40  in the spiral tracks are demodulated as servo bursts and the PES generated by comparing the servo bursts in a similar manner as with the servo bursts  14  in the reference servo sectors  34   0 – 34   N . 
   Once the head  26  is tracking on a servo track, the product servo sectors  56   0 – 56   N  are written to the disk using the servo write clock. Write circuitry is enabled when the modulo-N counter reaches a predetermined value, wherein the servo write clock clocks the write circuitry to write the product servo sector  56  to the disk. The spiral tracks  36   0 – 36   N  on the disk are processed in an interleaved manner to account for the product servo sectors  56   0 – 56   N  overwriting a spiral track. For example, when writing the product servo sectors  56   1  to the disk, spiral track  36   2  is processed initially to generate the PES tracking error and the timing recovery measurement. When the product servo sectors  56   1  begin to overwrite spiral track  36   2 , spiral track  36   3  is processed to generate the PES tracking error and the timing recovery measurement. 
     FIGS. 5A–5B  illustrate an embodiment of the present invention wherein control circuitry for demodulating the servo bursts in prior art product servo sectors is also used to demodulate the high frequency signal  38  in the spiral tracks as servo bursts to generate the PES for tracking.  FIG. 5A  shows the eye pattern of  FIG. 3B  which is processed similar to the prior art product servo sector shown in  FIG. 1 . The first segment  38 A of the high frequency signal in the eye pattern of  FIG. 5A  is processed as a preamble similar to the preamble  8  in  FIG. 1  for synchronizing a read clock  58  generated by a read voltage controlled oscillator (VCO)  60 . The first sync mark  40 A in the eye pattern is processed similar to the sync mark  10  in  FIG. 1 . The following segments  38 B– 38 E of the high frequency signal in the eye pattern are demodulated as servo bursts used to generate the PES for tracking. 
     FIG. 5B  shows example control circuitry for demodulating the prior art product servo sector of  FIG. 1  as well as the eye pattern ( FIG. 5A ) of the spiral tracks  36 . The embodiment employs a read VCO  60  and a write VCO  62 . The read VCO  60  generates a read clock  58  for sampling the read signal  64  during normal operation when demodulating the product servo sectors  54  and user data recorded on the disk. The write VCO  62  generates the servo write clock  66  used to write the product servo sectors  54  to the disk during the fill operation. The write VCO  62  is also used to sample the read signal  64  when demodulating the servo bursts from the high frequency signal  38  in the spiral tracks  36 . 
   In one embodiment, the read clock  58  is also used to sample the read signal  64  when reading the first segment  38 A of the high frequency signal representing the preamble as well as the first sync mark  40 A in the eye pattern ( FIG. 5A ) of the spiral tracks  36 . The read clock  58  is selected by multiplexer  68  as the sampling clock  70  for sampling  72  the read signal  64 . The read signal sample values  74  are processed by a first timing recovery circuit  76  which generates a timing recovery signal used to adjust the read VCO  60  until the read clock  58  is sampling the preamble  38 A synchronously. Once locked onto the preamble  38 A, a sync detector  78  is enabled for detecting the sync mark  40 A in the eye pattern. When the sync detector  78  detects the sync mark  40 A, it activates a sync detect signal  80 . The first timing recovery circuit  76  responds to the sync detect signal  80  by configuring the multiplexer  68  over line  82  to select the servo write clock  66  as the sampling clock  70 . The first timing recovery circuit  76  enables a timer for timing an interval between the sync mark  40 A and the beginning of the A servo burst  38 B in the eye pattern. When the timer expires, the first timing recovery circuit  76  enables a burst demodulator  84  over line  86  for demodulating the A, B, C and D servo bursts in the eye pattern from the read signal sample values  74 . 
   In one embodiment, the burst demodulator  84  rectifies and integrates the rectified read signal sample values  74  representing the respective A, B, C and D servo bursts to generate respective servo burst signals  88  which correspond to integrating the A, B, C and D servo bursts  14  in the prior art product servo sector of  FIG. 1 . A PES generator  90  processes the servo burst signals  88  to generate a PES signal  92  used for tracking. The PES generator  90  may compare the servo burst signals  88  to generate the PES signal  92  using any suitable algorithm when demodulating the servo bursts in either the prior art product servo sectors of  FIG. 1  or the eye pattern of  FIG. 5A . In one embodiment, the PES signal  92  when reading the eye pattern of  FIG. 5A  is generated according to (A−D)/(A+D). In this embodiment, evaluating the servo bursts near the edges of the eye pattern increases the sensitivity of the PES measurement. This is because deviations in the radial location of the head  26  cause a more precipitous change in the servo burst values at the edges of the eye pattern as compared to the servo burst values near the center of the eye pattern. 
   In the embodiment of  FIG. 5B , a control signal C/S  94  configures the first timing recovery circuit  76 , the sync detector  78 , and the PES generator  90  depending on whether the control circuitry is configured for demodulating the product servo sector (prior art product servo sector of  FIG. 1 ) or the spiral tracks. The first timing recovery circuit  76  adjusts the timing between the detection of the sync mark ( 10  in  FIGS. 1 and 40A  in  FIG. 5A ) and the beginning of the A servo burst ( 14  in  FIGS. 1 and 38B  in  FIG. 5A ). The sync detector  78  adjusts the target sync pattern depending on whether the sync mark  10  in the product servo sector is being detected or the sync mark  40 A in the eye pattern of the spiral track. The PES generator  90  adjusts the algorithm for comparing the servo burst signals  88  depending on whether the servo bursts  14  in the product servo sectors are being demodulated or the servo bursts  38 B– 38 E in the eye pattern of the spiral track are being demodulated. 
   The control circuitry in the embodiment of  FIG. 5B  further comprises a second timing recovery circuit  96  for generating a timing recovery measurement that controls the write VCO  62  for generating the servo write clock  66 . The second timing recovery circuit  96  comprises the modulo-N counter which is synchronized to the sync marks in the reference servo sectors  34  as shown in  FIG. 3A . When servoing on the spiral tracks  36 , the second timing recovery circuit  96  enables a sync mark detection window over line  98  commensurate with the modulo-N counter approaching a value corresponding to the expected occurrence of a sync mark  40  in a spiral track. When a sync mark  40  is actually detected over line  80 , the second timing recovery circuit  96  generates a coarse timing recovery measurement as the difference between the expected value of the module-N counter and the actual value. When reading the high frequency signal  38  in the spiral tracks, the second timing recovery circuit  96  generates a fine timing recovery measurement using any suitable timing recovery algorithm. For example, the fine timing recovery measurement can be generated using a suitable timing gradient, a suitable trigonometric identity, or a suitable digital signal processing algorithm such as the Discrete Fourier Transform (DFT). The coarse and fine timing recovery measurements are combined and used to adjust the write VCO  62  in order to maintain synchronization of the servo write clock  66 . 
   The servo write clock  66  is applied to write circuitry  100  used to write the product servo sectors  56  to the disk during the fill operation. The second timing recovery circuit  96  generates a control signal  102  for enabling the write circuitry  100  at the appropriate time so that the product servo sectors  56  are written at the appropriate circumferential location from the outer diameter of the disk to the inner diameter of the disk. In one embodiment, the control signal  102  enables the write circuitry  100  each time the module-N counter reaches a predetermined value so that the product servo sectors  56  form servo wedges as illustrated in  FIG. 1  and  FIG. 4 . 
   Although the first timing recovery circuit  76  shown in  FIG. 5B  adjusts the frequency of the read clock  58 , any suitable timing recovery technique may be employed. In an alternative embodiment, interpolated timing recovery is employed. With interpolated timing recovery the read signal  64  is sampled asynchronously and interpolated to generate the synchronous sample values  74 . In addition, the reference servo sectors  34  may comprise any suitable sync mark recorded at any suitable location within the reference servo sector. In one embodiment, the reference servo sector comprises multiple sync marks to provide redundancy similar to the eye pattern of  FIG. 5A . 
     FIGS. 6A and 6B  illustrate an embodiment of the present invention for calibrating the correlation between the PES generated from demodulating the spiral tracks  36  and the off-track displacement of the head  26 . The segments  38 B– 38 E of the high frequency signal in the spiral tracks  36  are demodulated as servo bursts to generate corresponding servo burst signals A, B, C and D. A PES is generated by comparing the servo burst signals according to any suitable algorithm, such as (A−D)/(A+D). As shown in  FIG. 6A , when the head  26  is on track a predetermined relationship between the servo burst signals (e.g., A=D) generates a predetermined value for the PES (e.g., zero). The head  26  is then moved away from the center of the track until the servo burst signals reach a second predetermined relationship (e.g., B=D) as shown in  FIG. 6B . When the servo burst signals reach the second predetermined relationship, the shift in the eye pattern relative to the sync marks  40 A– 40 D is known and therefore the amount of off-track displacement is known. Measuring the PES when the servo burst signals reach the second predetermined relationship provides the correlation (assuming a linear relationship) between the PES and the amount of off-track displacement. 
     FIG. 7  shows an embodiment of the present invention wherein after writing the reference servo sectors  34  and spiral tracks  36   0 – 36   N  to the disk  16  ( FIGS. 2A–2B ), the HDA  22  is inserted into an external product servo writer  104  comprising suitable circuitry for reading and processing the reference servo sectors  34  and the spiral tracks  36   0 – 36   N  in order to write the product servo sectors  56   0 – 56   N  to the disk  16 . The external product servo writer  104  comprises a read/write channel  106  for interfacing with a preamp  108  in the HDA  22 . The preamp  108  amplifies a read signal emanating from the head  26  over line  110  to generate an amplified read signal applied to the read/write channel  106  over line  112 . The read/write channel  106  comprises the circuitry of  FIG. 5B  for generating the servo burst signals  88  applied to a servo controller  114 . The servo controller  114  processes the servo burst signals  88  to generate the PES  92 . The PES  92  is processed to generate a VCM control signal applied to the VCM  28  over line  116  in order to maintain the head  26  along a circular path while writing the product servo sectors  56   0 – 56   N . The servo controller  114  also generates a spindle motor control signal applied to a spindle motor  118  over line  120  to maintain the disk  16  at a desired angular velocity. Control circuitry  122  processes information received from the read/write channel  106  over line  124  associated with the spiral tracks (e.g., timing information) and provides the product servo sector data to the read/write channel  106  at the appropriate time. The product servo sector data is provided to the preamp  108  which modulates a current in the head  26  in order to write the product servo sectors  56   0 – 56   N  to the disk  16 . The control circuitry  122  also transmits control information over line  126  to the servo controller  114  such as the target servo track to be written. After writing the product servo sectors  56   0 – 56   N  to the disk  16 , the HDA  22  is removed from the external product servo writer  104  and a printed circuit board assembly (PCBA) comprising the control circuitry  20  ( FIG. 2A ) is mounted to the HDA  22 . 
   In one embodiment, the external product servo writer  104  of  FIG. 7  interfaces with the HDA  22  over the same connections as the control circuitry  20  to minimize the modifications needed to facilitate the external product servo writer  104 . The external product servo writer  104  is less expensive than a conventional servo writer because it does not require a clean room or sophisticated head positioning mechanics. In an embodiment shown in  FIG. 8 , a plurality of external product servo writers  104   0 – 104   N  process the HDAs  22   i – 22   i+N  output by an external spiral servo writer  32  in order to write the product servo sectors less expensively and more efficiently than a conventional servo writer. In an alternative embodiment shown in  FIG. 9 , an external spiral servo writer  32  is used to write the reference servo sectors and the spiral tracks, and the control circuitry  20  within each product disk drive  18   i – 18   i+N  is used to write the product servo sectors.