Disk drive subtracting low frequency disturbance from a servo burst signal

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.

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.