PERFORMANCE DEGRADATION OF PRML CHANNELS DUE TO NONLINEAR DISTORTIONS

The effects of nonlinear distortions on the performance of a PRML channel are presented. The nonlinear distortions are introduced during the process of writing (transition shift) and reading (head distortion) data from a magnetic medium. Measurements were taken on the bench using simulated waveforms based on the Lorentzian wave shape, digital equalizer, and experimental PRML channel hardware. Head asymmetry and saturation, typical of experimental magneto resistive heads, were modeled by adding quadratic and cubic distortions, respectively. Noise was injected to the unequalized signal, presenting a colored spectrum to the input of the detector. The errors were detected by comparing the output of the detector and a reference NRZ pattern. The system SNR was computed using a sample distribution method. Noisy samples were collected at the input of the detector and compared to the three levels given by the ideal PR-IV signal. The effect of various kinds of nonlinearities on the eye diagram was studied, and templates which are useful in identifying the type of nonlinearity present in a real system were generated.