Compensating for repeatable phase error when servo writing a disk drive from spiral tracks

1. A method of writing product servo sectors on a disk of a disk drive, the disk drive comprising control circuitry and a head disk assembly (HDA) comprising the disk, an actuator arm, a head coupled to a distal end of the actuator arm, and a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk, the disk comprising a plurality of spiral tracks, each spiral track comprises a high frequency signal interrupted periodically by a sync mark, the method comprising:

using the head internal to the disk drive to read the spiral tracks to generate a read signal;

detecting the sync marks from the read signal to generate a sync mark detect signal;

generating a timing recovery measurement in response to the sync mark detect signal, wherein the timing recovery measurement comprises a sinusoidal component;

generating a timing compensation value in response to the sync mark detect signal, wherein the timing compensation value estimates the sinusoidal component in the timing recovery measurement;

generating a frequency control signal in response to the timing recovery measurement and the timing compensation value;

generating a servo write clock in response to the frequency control signal; and

using the servo write clock and the head internal to the disk drive to write the product servo sectors to the disk.

2. The method as recited in claim 1 , wherein the spiral tracks are written on the disk using a media writer.

3. The method as recited in claim 2 , wherein the timing compensation value estimates the sinusoidal component in the timing recovery measurement due to an eccentricity of the disk when installed into the HDA.

4. The method as recited in claim 1 , wherein generating the timing recovery measurement comprises generating a coarse timing recovery measurement from the sync mark detect signal and a fine timing recovery measurement from the high frequency signal.

5. The method as recited in claim 1 , wherein generating the timing compensation value comprises adjusting coefficients {â,{circumflex over (b)}} of a sinusoid according to:

â ( k+ 1)= â ( k )− G*e ( k )*cos(2 π k/N )

{circumflex over (b)} ( k+ 1)= {circumflex over (b)} ( k )− G*e ( k )*sin(2 π k/N )

wherein G is a predetermined gain, e(k) is a timing error, and k is an index representing one of the spiral tracks.

6. The method as recited in claim 5 , wherein the timing error e(k) is estimated by:

calculating an estimated sync mark interval (SMI) according to:

EST_SMI=RI+ â *cos(2 π k/N )+ {circumflex over (b)} *sin(2 π k/N )

wherein RI is a reference interval corresponding to a nominal SMI;

detecting an actual SMI by detecting an interval between the head passing over a first and second sync mark in the spiral tracks; and

computing a difference between the estimated SMI and the detected actual SMI.

7. The method as recited in claim 5 , wherein the timing error e(k) is represented by the timing recovery measurement.

8. The method as recited in claim 1 , wherein generating the frequency control signal comprises:

combining the timing recovery measurement with the timing compensation value to generate an adjusted timing recovery measurement; and

filtering the adjusted timing recovery measurement.

9. The method as recited in claim 1 , wherein generating the frequency control signal comprises:

filtering the timing recovery measurement to generate a filtered timing recovery measurement; and

combining the filtered timing recovery measurement with the timing compensation value.

10. A disk drive comprising:

(a) a disk comprising a plurality of spiral tracks, wherein each spiral track comprises a high frequency signal interrupted periodically by a sync mark;

(b) an actuator arm;

(c) a head coupled to a distal end of the actuator arm;

(d) a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk; and

(e) control circuitry for writing product servo sectors on the disk by:

using the head internal to the disk drive to read the spiral tracks to generate a read signal;

detecting the sync marks from the read signal to generate a sync mark detect signal;

generating a timing recovery measurement in response to the sync mark detect signal, wherein the timing recovery measurement comprises a sinusoidal component;

generating a timing compensation value in response to the sync mark detect signal, wherein the timing compensation value estimates the sinusoidal component in the timing recovery measurement;

generating a frequency control signal in response to the timing recovery measurement and the timing compensation value;

generating a servo write clock in response to the frequency control signal; and

using the servo write clock and the head internal to the disk drive to write the product servo sectors to the disk.

11. The disk drive as recited in claim 10 , wherein the spiral tracks are written on the disk using a media writer.

12. The disk drive as recited in claim 11 , wherein the timing compensation value estimates the sinusoidal component in the timing recovery measurement due to an eccentricity of the disk when installed into the disk drive.

13. The disk drive as recited in claim 10 , wherein generating the timing recovery measurement comprises generating a coarse timing recovery measurement from the sync mark detect signal and a fine timing recovery measurement from the high frequency signal.

14. The disk drive as recited in claim 10 , wherein generating the timing compensation value comprises adjusting coefficients {â,{circumflex over (b)}} of a sinusoid according to:

â ( k+ 1)= â ( k )− G*e ( k )*cos(2 π k/N )

{circumflex over (b)} ( k+ 1)= {circumflex over (b)} ( k )− G*e ( k )*sin(2 π k/N )

wherein G is a predetermined gain, e(k) is a timing error, and k is an index representing one of the spiral tracks.

15. The disk drive as recited in claim 14 , wherein the timing error e(k) is estimated by:

calculating an estimated sync mark interval (SMI) according to:

EST_SMI=RI+ â *cos(2 π k/N )+ {circumflex over (b)} *sin(2 π k/N )

wherein RI is a reference interval corresponding to a nominal SMI;

detecting an actual SMI by detecting an interval between the head passing over a first and second sync mark in the spiral tracks; and

computing a difference between the estimated SMI and the detected actual SMI.

16. The disk drive as recited in claim 14 , wherein e(k) is represented by the timing recovery measurement.

17. The disk drive as recited in claim 10 , wherein generating the frequency control signal comprises:

combining the timing recovery measurement with the timing compensation value to generate an adjusted timing recovery measurement; and

filtering the adjusted timing recovery measurement.

18. The disk drive as recited in claim 10 , wherein generating the frequency control signal comprises:

filtering the timing recovery measurement to generate a filtered timing recovery measurement; and

combining the filtered timing recovery measurement with the timing compensation value.