Calibrating correlation between PES and off-track displacement by processing high frequency signal in spiral track

1. A method associated with a disk drive, the disk drive comprising control circuitry and a head disk assembly (HDA) comprising a disk, an actuator arm, a head connected 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 comprising a high frequency signal interrupted at a predetermined interval by a sync mark, the method comprising the steps of:

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

(b) using burst demodulation circuitry to demodulate the read signal representing the high frequency signal between the sync marks in one of the spiral tracks into a plurality of respective servo burst signals;

(c) processing the servo burst signals generated from reading the spiral track to generate a position error signal (PES) used to maintain the head along a substantially circular target path; and

(d) calibrating a correlation between the position error signal and an off-track displacement of the head.

2. The method as recited in claim 1 , further comprising the step of using the head internal to the disk drive to write product servo sectors along the circular target path, wherein each product servo sector comprises a plurality of servo bursts.

3. The method as recited in claim 1 , wherein the step of calibrating the correlation between the position error signal and an off-track displacement of the head comprises the steps of:

moving the head radially over the disk until the servo burst signals generated from reading the spiral tracks attain a first predetermined relationship;

generating a first position error signal from the servo burst signals; and

calibrating the correlation in response to the first position error signal.

4. The method as recited in claim 1 , wherein the burst demodulation circuitry comprises an integrator.

5. The method as recited in claim 1 , wherein the step of calibrating the correlation between the position error signal and the off-track displacement of the head further comprises the steps of:

moving the head radially over the disk until the servo burst signals generated from reading the spiral tracks attain a second predetermined relationship;

generating a second position error signal from the servo burst signals; and

calibrating the correlation in response to the first and second position error signals.

6. The method as recited in claim 5 , wherein the step of calibrating the correlation between the position error signal and the off-track displacement of the head further comprises the steps of:

computing a first normalized scaling factor as a ratio between a first expected head displacement and the first position error signal;

computing a second normalized scaling factor as a ratio between a second expected head displacement and the second position error signal; and

calibrating the correlation in response to the first and second normalized scaling factors.

7. The method as recited in claim 6 , wherein the step of calibrating the correlation between the position error signal and the off-track displacement of the head further comprises the steps of:

weighting the first normalized scaling factor using a first weight to generate a first weighted normalized scaling factor;

weighting the second normalized scaling factor using a second weight to generate a second weighted normalized scaling factor, wherein the second weight is different than the first weight; and

calibrating the correlation in response to the first and second weighted normalized scaling factors.

8. The method as recited in claim 1 , wherein the control circuitry within the disk drive comprises the burst demodulation circuitry.

9. The method as recited in claim 1 , wherein an external product servo writer comprises the burst demodulation circuitry.

10. A disk drive comprising:

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

(b) an actuator arm;

(c) a head connected 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;

(e) burst demodulation circuitry; and

(f) control circuitry for:

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

using the burst demodulation circuitry to demodulate the read signal representing the high frequency signal between the sync marks in one of the spiral tracks into a plurality of respective servo burst signals;

processing the servo burst signals generated from reading the spiral track to generate a position error signal (PES) used to maintain the head along a substantially circular target path;

calibrating a correlation between the position error signal and an off-track displacement of the head.

11. The disk drive as recited in claim 10 , wherein the control circuitry for using the head internal to the disk drive to write product servo sectors along the circular target path, wherein each product servo sector comprises a plurality of servo bursts.

12. The disk drive as recited in claim 10 , wherein the control circuitry for calibrating the correlation between the position error signal and an off-track displacement of the head by:

moving the head radially over the disk until the servo burst signals generated from reading the spiral tracks attain a first predetermined relationship;

generating a first position error signal from the servo burst signals; and

calibrating the correlation in response to the first position error signal.

13. The disk drive as recited in claim 10 , wherein the burst demodulation circuitry comprises an integrator.

14. The disk drive as recited in claim 10 , wherein the control circuitry calibrates the correlation between the position error signal and the off-track displacement of the head by further:

moving the head radially over the disk until the servo burst signals generated from reading the spiral tracks attain a second predetermined relationship;

generating a second position error signal from the servo burst signals; and

calibrating the correlation in response to the first and second position error signals.

15. The disk drive as recited in claim 14 , wherein the control circuitry calibrates the correlation between the position error signal and the off-track displacement of the head by further:

computing a first normalized scaling factor as a ratio between a first expected head displacement and the first position error signal;

computing a second normalized scaling factor as a ratio between a second expected head displacement and the second position error signal; and

calibrating the correlation in response to the first and second normalized scaling factors.

16. The disk drive as recited in claim 15 , wherein the control circuitry calibrates the correlation between the position error signal and the off-track displacement of the head by further:

weighting the first normalized scaling factor using a first weight to generate a first weighted normalized scaling factor;

weighting the second normalized scaling factor using a second weight to generate a second weighted normalized scaling factor, wherein the second weight is different than the first weight; and

calibrating the correlation in response to the first and second weighted normalized scaling factors.