Evaluation of track following servo performance for patterned servo sectors in hard disk drives

A promising approach for ultra high data storage capacities in magnetic hard disk drives is the use of bit-patterned media (BPM) that allows both higher track densities and increased linear bit spacing of data. To allow for such ultra high track densities, it is important to select an embedded servo pattern within the BPM that provides a Position Error Signal (PES) with sufficient quality to maintain (high bandwidth) head positioning. This paper discusses servo pattern performance from a closed-loop point of view for several servo patterns that could be incorporated in a BPM. A dedicated magnetic readback signal simulation and signal decoding along with a closed-loop simulation are used to evaluate tracking and PES quality. Due to PES nonlinearity in the conventional amplitude based servo pattern currently used in most drives, a timing based chevron pattern and a differential frequency pattern are considered as case studies in this paper. For the timing based servo patterns, both readback signal sampling and transition jitter were found to greatly affect PES quality. It is shown by the simulation studies in this paper that the differential frequency servo pattern can surpass the chevron and amplitude based servo patterns when subjected to sampling and jitter noise due to the averaging effects of the rotational symmetry of the pattern.