Servo writing a disk drive by synchronizing a servo write clock to a reference pattern on the disk and compensating for repeatable phase error

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 reference pattern having a plurality of sync marks written around the disk, the method comprising the steps of:

• (a) using the head internal to the disk drive to read the reference pattern to generate a read signal;
• (b) detecting the sync marks from the read signal to generate a sync mark detect signal;
• (c) generating a timing recovery measurement in response to the sync mark detect signal;
• (d) generating a frequency control signal in response to the timing recovery measurement;
• (e) generating a servo write clock in response to the frequency control signal;
• (f) generating a feed-forward compensation value in response to the timing recovery measurement and the frequency control signal;
• (g) adjusting the timing recovery measurement in response to the feed-forward compensation value to compensate for a repeatable phase error in the timing recovery measurement;
• and
• (h) using the servo write clock and the head internal to the disk drive to write the product servo sectors to the disk.