Magnetic media tester which includes a filter that filters the product

Abstract

A disk certifier which analyzes a read signal generated from a disk of a hard disk drive. The read signal typically has a sinusoidal waveform with modulated peak amplitudes. The waveform is generally defined by the equation A.multidot.sin(wt).multidot.(1-m(t)), where the term m(t) is the amplitude modulation of the signal. The peak signal is provided to a phase lock loop circuit which generates a multiplier peak signal generally of the form sin(wt). The certifier includes a multiplier which multiplies the read signal by the multiplier read signal to provide a multiplied read signal described by the equation A/2.multidot.(1-cos 2(wt)).multidot.(1-m(t)). The multiplied read signal is filtered by a low pass filter which has a cut-off frequency Wc below w. The low pass filter provides a filtered multiplied read signal which is described by the equation A.multidot.(1-g(t)), wherein the term g(t) is the portion of m(t) with frequencies below Wc. The filtered multiplied read signal is then compared to a threshold value(s) to determine a pass/fail error condition of the read signal. The certifier also has a histogram circuit which detects, counts and stores the occurrence of each peak amplitude value of the read signal. The peak amplitude data can be processed to plot a histogram which shows how many times the read signal produced various peak amplitudes. The histogram can be used to determine the actual performance of the disk certifier.