Channel performance with nonlinear Mr head

Abstract

The transfer curve of MR head is not linear to recorded information. Although the head and medium are designed to diminish this effect, it is unavoidable to meet the nonlinearity in head playback response. For example, improper bias current can often introduce signal asymmetry. which is from nonlinearity of the head. As PRML channels are becoming the most papular channels in magnetic recording. impact of nonlinear head to these channels need to be understood. Performance of PRML channel with nonlinear MR head is studied in this paper. The employed head transfer c w e can be obtained from the magnetic model or from experimental measurement. The nonlinear signal fmm MR head is amplified and equalized before they enter into PRML detector. If the pseudo-random sequence is used to simulate, the eye pattern of equalized signal shows the nonlinearity makes the eye open less when any other noise is not introduced. Figure 1 shows a typical eye pattern of the equalized signals for a PR4 channel with nonlinear SAUMR head. The sampled signal is not 2.0 or -2 any more, it splits into layers. Each layer comes from different combination of transition pulses. Moreover, the channel becomes very sensitive to the recording density. As the recording densities (PWSOm) grows to 2.4 or above, channel performance will be much worse than low densities because of more intersymbol interferenm. In this paper. a parameter called "deviation" is defined as the variance of the sampled equalized signal around 2, 0 or -2. When the bead operates in linear range, the deviation is always zem if no other noise is considered. If the additive noise is introduced and the head works in linear range, the deviation is the additive noise variance. The deviation is related to channel bit ermr rate. When the deviation decreases, the bit error rate will decrease. Moreover, when the deviation is small enough, the bit e m r rate would be zero. Thus, it is very useful to evaluate the channels before detector.