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Quadrature Servo Signal Utilizing a PLO Time Detection System

IP.com Disclosure Number: IPCOM000080409D
Original Publication Date: 1973-Dec-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 4 page(s) / 62K

Publishing Venue

IBM

Related People

Comstock, RL: AUTHOR [+2]

Abstract

This servo concept is based on the principle that the sum of two sinusoids (single harmonic) of the same frequency, but of different phase relative to some reference, produce a sinusoid of the same frequency. The phase of the resultant sinusoid is a function of the ratio of amplitudes of the two sinusoids and their initial phase difference. A single frequency sinusoidal signal is recorded in quadrature, i.e., 90 degrees or pi/4 out of phase, on alternate tracks as shown in Fig. 1. Half of each track is read, in the "on-track" condition. Any misregistration produces a time shift in the zero crossing of the resultant sinusoid. The zero crossing time can be measured against a reference and a servo error signal generated by the difference.

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Quadrature Servo Signal Utilizing a PLO Time Detection System

This servo concept is based on the principle that the sum of two sinusoids (single harmonic) of the same frequency, but of different phase relative to some reference, produce a sinusoid of the same frequency. The phase of the resultant sinusoid is a function of the ratio of amplitudes of the two sinusoids and their initial phase difference. A single frequency sinusoidal signal is recorded in quadrature, i.e., 90 degrees or pi/4 out of phase, on alternate tracks as shown in Fig. 1. Half of each track is read, in the "on-track" condition. Any misregistration produces a time shift in the zero crossing of the resultant sinusoid. The zero crossing time can be measured against a reference and a servo error signal generated by the difference.

Time difierence detection can readily be achieved with simple phase-locked oscillator (PLO) systems. It is utilization of this detection technique which makes this concept independent of signal amplitude and resolution, except in the more generalized terms of raw signal-to-noise ratio.

The zero crossing for the off track condition is found, analytically, to be: t(shift) = (1/omega(s)) Tan/-1/ (A(2)/A(1)) t(shift) would appear to be a highly nonlinear function of the misregistration which might cause significant problems. The ratio, -A(2)/A(1), however, is also a nonlinear function of the misregistration.

The time shift as a function of head position is given by: t(act) = (1/2 pi f(s)) Tan/-1/ [(1-2x)/(1+2x)] where x = misregistration/head width.

The result is an almost linear timing variation with increasing (magnitude) misregistration.

A reference signal at frequency f(s) would be recorded in synchronization sectors on the disk, to establish a servo crossing time reference for a PLO. The signal in these regions would be written at the servo frequency with the transitions for each track coherently written along radial lines to provide a single reference sinusoid regardless of head position. The two head positioning servo signals would be written +/- pi/4 (1/8 omega(s)) out-of-phase with the reference signal and by definition in quadrature with one another.

When head positioning, the time difference between the PLO output and the actual time crossing would be converted to a DC level error signal to drive the servomechanism. Since only the zero crossing of a single sinusoid out of the head is involved, the system is insensitive to changes in amplitude or resolution due to changing flying heights, etc. Additionally, because all the information is contained in a single sinusoid, there is only one common channel and amplitude imbalance problems do not occur.

The time shift range for complete misregistration from one side to the other is 1/4 the time period of the servo frequency. Typically, for a 1.0 MHz signal this is 250 nsec. This large value would tend to minimize the effects of tolerances in the clocking circuits.

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