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Demodulator Noise Reduction for Phase-Encoded Servo Pattern

IP.com Disclosure Number: IPCOM000119667D
Original Publication Date: 1991-Feb-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 88K

Publishing Venue

IBM

Related People

Elliott, PJ: AUTHOR [+2]

Abstract

Track misregistration (TMR) in a disk file which uses the phase-encoded servo pattern, as described in U.S. Patents 4,642,562 and 4,549,232, is improved by reducing noise when track following.

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Demodulator Noise Reduction for Phase-Encoded Servo Pattern

      Track misregistration (TMR) in a disk file which uses the
phase-encoded servo pattern, as described in U.S. Patents 4,642,562
and 4,549,232, is improved by reducing noise when track following.

      There are several sources of position error signal (PES) noise
in a system using a phase demodulator which detects the timing of
zero crossings in the servo pattern. One source is in the
demodulation process itself.  The amount of noise produced can be
measured by supplying the demodulator with a clean input signal of
continuously varying phase obtained from two crystal oscillators
selected to have a small frequency difference.  The demodulator
output is then stored as a function of time.  Ideally, the output
should be in the form of a linear ramp, and any deviations from the
ramp are the effects of noise.  By calculating the RMS deviation, a
measure of goodness for a particular demodulator is obtained.

      Measurements with a typical demodulator give results in the
range of 2.0 to 3.0 bits sigma (where 1 bit = 1/512 track = 1.2
microinches).  This contributes about 1.5 microinches sigma to the
non-repeatable misregistration (NRM) of the file.  It is not
one-to-one because the PES noise spectrum extends up to the sampling
frequency, but the file position loop will not respond to the higher
frequencies.  It is a small, but by no means negligible, noise value.

      Noise is generated as follows. The phase-encoded pattern
consists of three bursts of transitions, where the phase difference
between the second burst and the other two represents position.
Demodulation starts with the first zero crossing in each burst after
a window, generated by the Servo ID (SID) logic, has opened.  Since
the SID logic is clocked by an oscillator which is not synchronized
to the servo pattern, the exact time at which each window opens will
vary relative to the zero crossings.  If the two happen to be nearly
coincident in one or more of the bursts, then some of the time one
zero crossing will appear in the window, and at other times it will
just be missed, so the next zero crossing will be taken as the first.
This means that a coarse PES counter, which is c...