Disk drive rotating phase based servo bursts based on radial location of head

1. A disk drive comprising:

a disk comprising a plurality of servo tracks defined by a plurality of servo sectors, wherein

each servo sector comprises a plurality of servo bursts; and

each servo burst comprises a predetermined phase;

a head actuated over the disk; and

control circuitry operable to:

position the head over one of the servo tracks;

read at least one servo burst in one of the servo sectors to generate a read signal;

extract a sine and cosine component from the read signal;

rotate at least one of the sine and cosine components by an angle based on a radial location of the head; and

generate a position error signal for the head in response to the rotated component.

2. The disk drive as recited in claim 1 , wherein:

a first one of the servo bursts in a first servo sector comprises a N servo burst;

a second one of the servo bursts in the first servo sector comprises a Q servo burst; and

the N servo burst is offset radially from the Q servo burst by approximately a half of a servo track.

3. The disk drive as recited in claim 2 , wherein the control circuitry is operable to:

rotate the N servo burst read signal by a N_angle; and

rotate the Q servo burst read signal by a Q_angle.

4. The disk drive as recited in claim 3 , wherein the control circuitry is operable to generate the N_angle and the Q_angle at the radial location of the head:

N_angle in response to a tan(N_cos/N_sin);

Q_angle in response to a tan(Q_cos/Q_sin);

wherein:

N_sin is a sine component of the N servo burst;

N_cos is a cosine component of the N servo burst;

Q_sin is a sine component of the Q servo burst; and

Q_cos is a cosine component of the Q servo burst.

5. The disk drive as recited in claim 4 , wherein the control circuitry is operable to generate the N_angle and the Q_angle at the radial location of the head:

N_angle in response to (a tan — 1 (N_cos/N_sin)+a tan — 2(N_cos/N_sin))/2;

Q_angle in response to (a tan — 1 (Q_cos/Q_sin)+a tan — 2(Q_cos/Q_sin))/2;

wherein:

a tan — 1 is computed at a first offset of the head relative to a servo track;

a tan — 2 is computed at a second offset of the head relative to the servo track;

N_sin is a sine component of the N servo burst;

N_cos is a cosine component of the N servo burst;

Q_sin is a sine component of the Q servo burst; and

Q_cos is a cosine component of the Q servo burst.

6. The disk drive as recited in claim 4 , wherein the control circuitry is operable to rotate the sine components of the N and Q servo bursts to generate rotated sine components by computing:

N′_sin=N_cos·sin(N_angle)+N_sin·cos(N_angle);

Q′_sin=Q_cos·sin(Q_angle)+Q_sin·cos(Q_angle);

wherein:

N′_sin is the rotated sine component of the N servo burst; and

Q′_sin is the rotated sine component of the Q servo burst.

7. The disk drive as recited in claim 6 , wherein the control circuitry generates the position error signal in response to N′_sin and Q′_sin.

8. A method of operating a disk drive, the disk drive comprising a head actuated over a disk comprising a plurality of servo tracks defined by a plurality of servo sectors, wherein each servo sector comprises a plurality of servo bursts, and each servo burst comprises a predetermined phase, the method comprising:

positioning the head over one of the servo tracks;

reading at least one servo burst in one of the servo sectors to generate a read signal;

extracting a sine and cosine component from the read signal;

rotating at least one of the sine and cosine components by an angle based on a radial location of the head; and

generating a position error signal for the head in response to the rotated component.

9. The method as recited in claim 8 , wherein:

a first one of the servo bursts in a first servo sector comprises a N servo burst;

a second one of the servo bursts in the first servo sector comprises a Q servo burst; and

the N servo burst is offset radially from the Q servo burst by approximately a half of a servo track.

10. The method as recited in claim 9 , further comprising:

rotating the N servo burst read signal by a N_angle; and

rotating the Q servo burst read signal by a Q_angle.

11. The method as recited in claim 10 , further comprising generating the N_angle and the Q_angle at the radial location of the head:

N_angle in response to a tan(N_cos/N_sin);

Q_angle in response to a tan(Q_cos/Q_sin);

wherein:

N_sin is a sine component of the N servo burst;

N_cos is a cosine component of the N servo burst;

Q_sin is a sine component of the Q servo burst; and

Q_cos is a cosine component of the Q servo burst.

12. The method as recited in claim 11 , further comprising generating the N_angle and the Q_angle at the radial location of the head:

N_angle in response to (a tan — 1(N_cos/N_sin)+a tan — 2(N_cos/N_sin))/2;

Q_angle in response to (a tan — 1(Q_cos/Q_sin)+a tan — 2(Q_cos/Q_sin))/2;

wherein:

a tan — 1 is computed at a first offset of the head relative to a servo track;

a tan — 2 is computed at a second offset of the head relative to the servo track;

N_sin is a sine component of the N servo burst;

N_cos is a cosine component of the N servo burst;

Q_sin is a sine component of the Q servo burst; and

Q_cos is a cosine component of the Q servo burst.

13. The method as recited in claim 11 , further comprising rotating the sine components of the N and Q servo bursts to generate rotated sine components by computing:

N′_sin=N_cos·sin(N_angle)+N_sin·cos(N_angle);

Q′_sin=Q_cos·sin(Q_angle)+Q_sin·cos(Q_angle);

wherein:

N′_sin is the rotated sine component of the N servo burst; and

Q′_sin is the rotated sine component of the Q servo burst.

14. The method as recited in claim 13 , further comprising generating the position error signal in response to N′_sin and Q′_sin.