A magnetic recording disk drive has a disturbance sensor and a disturbance
frequency identifier that are used to adjust the frequency of a peak
filter as the disturbance frequency changes. The sensor and the frequency
identifier are separate from the servo control loop and thus do not rely
on the head position error signal (PES) to predict the disturbance
frequency. The adjustable peak filter is coupled in parallel with the
servo feedback controller. The peak filter modifies the open loop
transfer function and the error rejection function of the servo control
loop to provide a higher rejection at the identified frequency. The peak
filter may be switched out of or uncoupled from the servo feedback
controller during track-seeking or as desired, depending on the amplitude
of the sensor signal or the amplitude of the PES.