Patent ID: 7529055

Claim:
An apparatus for writing reference patterns on a data storage disk rotated by a spindle motor in a disk drive, the apparatus comprising: a back electromagnetic force (EMF) sense circuit configured to generate a sequence of zero crossing signals from back EMF sensed on windings of the spindle motor over each revolution of the spindle motor, the sequence of zero crossing signals defining a corresponding sequence of zero crossing periods of time between successive zero crossing signals; a Phase Locked Loop (PLL) circuit configured to generate a timing signal in response to comparison of a feedback signal to the zero crossing signals, and is configured to compensate for repeatable variation among the sequence of zero crossing periods over each revolution of the spindle motor so that the timing signal is substantially independent of the repeatable variation among the sequence of zero crossing periods; and a recording pattern generator configured to write reference patterns on the disk in response to the timing signal as the disk is rotated by the spindle motor.