Limitations to track following imposed by position error signal SNR using a multi-tapped magnetoresistive servo head

Abstract

A multi-tapped magnetoresistive (MR) stripe has been commercially fabricated as a tape head for evaluation as a potential tracking sensor for magnetic tape systems with dedicated servo. In this approach, two adjacent MR elements sharing a common bias current are each positioned halfway over a single 17 um wide servo track. The head leads provide signals from each of the MR elements which are separated 1.25 um by permanent magnets that provide longitudinal bias. Since this servo configuration uses a differential sensor with a simple square wave tracking pattern, the system can allow self-servo formatting. The evaluation of these heads on a Metrum test stand with 1550 Oe Sony MP media have allowed an estimate of the finest track pitch servo system that could be supported by such a sensor. Two sources of PES error, media noise and PES non-linearity, have been quantified, and projections of system performance at high tape speeds and narrow trackwidths have been made. With this scheme it seems feasible to reach a 2 um track pitch, but to do so, a non-linear PES fit must be used. Furthermore, to maintain high SNR, the sensor should not be operated in a manner such that it needs to be scaled down in size as the trackwidth shrinks, favoring a so-called multiposition operation scheme.