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Dedicated Servo Pattern and Low-Cost Demodulator

IP.com Disclosure Number: IPCOM000039637D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 76K

Publishing Venue

IBM

Related People

Weed, MA: AUTHOR

Abstract

An improved servo pattern and demodulator are provided for a disk storage drive. In this instance, the servo head picks up signals of two frequencies, 2.5 MHz and 1.25 MHz, whereby the ratio is two to one. The servo pattern is written whereby the phase of one frequency varies oppositely to the phase of the other frequency as the servo head moves radially over the disk. By dividing the 2.5 MHz signal by two and (Image Omitted) comparing the phase of the resulting 1.25 MHz signal to the 1.25 MHz signal of the second frequency, a position signal is obtained where the radial position is proportional to the phase difference of the two 1.25 MHz signals. The servo pattern of Fig. 1 could be used but is not practical because of clock head jitter when the pattern is written.

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Dedicated Servo Pattern and Low-Cost Demodulator

An improved servo pattern and demodulator are provided for a disk storage drive. In this instance, the servo head picks up signals of two frequencies, 2.5 MHz and 1.25 MHz, whereby the ratio is two to one. The servo pattern is written whereby the phase of one frequency varies oppositely to the phase of the other frequency as the servo head moves radially over the disk. By dividing the 2.5 MHz signal by two and

(Image Omitted)

comparing the phase of the resulting 1.25 MHz signal to the 1.25 MHz signal of the second frequency, a position signal is obtained where the radial position is proportional to the phase difference of the two 1.25 MHz signals. The servo pattern of Fig. 1 could be used but is not practical because of clock head jitter when the pattern is written. The jitter causes timing variations from track to track which appear as phase shifts between the two frequencies. Hence, the pattern of Fig. 1 could be used to measure clock head jitter but not be used as a servo pattern.

(Image Omitted)

The servo pattern of Fig. 2 is practical and differs from that of Fig. 1 by having alternating stretches of the two frequencies written on the same half track. Because only the phase difference is measured, the same phase shift in the two servo frequencies caused by jitter has no effect on the position signal. The switching frequency, however, can not be too high or undesirable effects on the phases of the two servo frequencies occur. A period of switching of 6.4 microseconds is practical. The 2.5 MHz pattern is shifted right 50 nanoseconds on successive half tracks, while the 1.25 MHz pattern is shifted left 50 nanoseconds. The 16 half-track patterns make up a complete pattern cycle (Fig.
2), where the top half-track is the next pattern after the bottom half- track; the pattern is to be extended cyclically over the entire range of the servo head. The pattern is also cyclic in the horizontal direction, the 6.4-microsecond-long blocks being replicated around the circumference of the disk. The basic demodulator circuit for the servo pattern of Fig. 2 is shown in Fig. 3. The analog servo head signal drives two "tank" circuits 10 and 20, each consisting of an inductor and a capacitor connected in parallel. The tanks act as bandpass filters, one at 2.5 MHz and the other at 1.25 MHz, and also average these frequency components over several cycles. The tank circuits 10 and 20 drive comparators 15 and 25 which detect the zero voltage crossings of the frequencies. The logic level output of the 2.5 MHz comparator 15 clocks a D-type flip- flop 30; the D input to the flip- flop 30 is the output of the 1.25 MHz comparator 25. This produces a 1.25 MHz signal at the Q output of the

(Image Omitted)

flip-flop which is in phase with the 2.5 MHz signal and with less than 180o of phase difference from the 1.25 MHz comparator 25 output. The phase difference of the two 1.25 MHz signals is found by feeding the...