A disk drive is disclosed comprising a disk and a head actuated over the disk, wherein the disk comprises a plurality of spiral tracks, each comprising a high frequency signal interrupted by a sync mark at a sync mark interval. A measurement of S at a frequency-of-interest is obtained. Further embodiments, forms, features, aspects, benefits, and advantages of the present application shall become apparent from the description and figures provided herewith. The triggering circuitry is adapted to provide the triggering signal at low RPMs on the trailing edge of a predetermined half cycle.