Disk drive subtracting low frequency disturbance from a servo burst signal

1. A disk drive comprising:

(a) a disk comprising a plurality of data tracks, wherein:

each data track comprises a plurality of data sectors and a plurality of servo sectors; and

each servo sector comprises a plurality of servo bursts;

(b) a head actuated over the disk operable to read the data sectors and servo sectors to generate read signals;

(c) a data path comprising a data decoder operable to decode a data read signal generated as the head passes over the data sectors; and

(d) a servo path comprising:

a low pass filter operable to extract a low frequency component from a servo read signal generated as the head passes over the servo bursts, wherein the low pass filter does not operate on the data read signal;

a subtractor operable to subtract the low frequency component from the servo read signal to generate a servo burst signal;

a servo demodulator operable to demodulate the servo burst signal; and

a servo burst window signal operable to enable the low pass filter and subtractor as the head passes over the servo bursts.

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

(a) each servo sector further comprises servo data; and

(b) at least one of the low pass filter and subtractor is disabled as the head passes over the servo data.

3. The disk drive as recited in claim 1 , wherein the data path further comprises a thermal asperity filter operable to attenuate a thermal asperity disturbance in the data read signal.

4. The disk drive as recited in claim 3 , wherein:

(a) the low pass filter has a first cutoff frequency; and

(b) the thermal asperity filter has a second cutoff frequency substantially less than the first cutoff frequency.

5. Control circuitry for use in a disk drive, the disk drive comprising a disk having a plurality of data tracks, wherein each data track comprises a plurality of data sectors and a plurality of servo sectors, and each servo sector comprises a plurality of servo bursts, and a head actuated over the disk operable to read the data sectors and servo sectors to generate read signals, the control circuitry comprising:

(a) a data path comprising a data decoder operable to decode a data read signal generated as the head passes over the data sectors; and

(b) a servo path comprising:

a low pass filter operable to extract a low frequency component from a servo read signal generated as the head passes over the servo bursts, wherein the low pass filter does not operate on the data read signal;

a subtractor operable to subtract the low frequency component from the servo read signal to generate a servo burst signal;

a servo demodulator operable to demodulate the servo burst signal; and

a servo burst window signal operable to enable the low pass filter and subtractor as the head passes over the servo bursts.

6. The control circuitry as recited in claim 5 , wherein:

(a) each servo sector further comprises servo data; and

(b) at least one of the low pass filter and subtractor is disabled as the head passes over the servo data.

7. The control circuitry as recited in claim 5 , wherein the data path further comprises a thermal asperity filter operable to attenuate a thermal asperity disturbance in the data read signal.

8. The control circuitry as recited in claim 7 , wherein:

(a) the low pass filter has a first cutoff frequency; and

(b) the thermal asperity filter has a second cutoff frequency substantially less than the first cutoff frequency.

9. A method of operating a disk drive, the disk drive comprising a disk having a plurality of data tracks, wherein each data track comprises a plurality of data sectors and a plurality of servo sectors, and each servo sector comprises a plurality of servo bursts, and a head actuated over the disk operable to read the data sectors and servo sectors to generate read signals, the method comprising the steps of:

(a) decoding a data read signal generated as the head passes over the data sectors;

(b) extracting a low frequency component from a servo read signal generated as the head passes over the servo bursts, wherein the step of extracting does not operate on the data read signal;

(c) subtracting the low frequency component from the servo read signal to generate a servo burst signal;

(d) demodulating the servo burst signal; and

(e) enabling the extracting and subtracting steps as the head passes over the servo bursts.

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

(a) each servo sector further comprises servo data; and

(b) at least one of the steps of extracting and subtracting is disabled as the head passes over the servo data.

11. The method as recited in claim 9 , further comprising the step of attenuating a thermal asperity disturbance in the data read signal.

12. The method as recited in claim 11 , wherein:

(a) the step of extracting the low frequency component from the servo read signal occurs at a first cutoff frequency; and

(b) the step of attenuating the thermal asperity disturbance in the data read signal occurs at a second cutoff frequency substantially less than the first cutoff frequency.