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Phase Compensator for Digital Servo of Optical Disk Drive

IP.com Disclosure Number: IPCOM000102389D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 77K

Publishing Venue

IBM

Related People

Semba, T: AUTHOR

Abstract

Disclosed is a method for achieving lower digital servo sampling frequency for optical disk drives. By simply adding the filter into the servo loop without changing the servo scheme, the phase margin of the servo loop can be improved. This method can reduce the sampling frequency more than the case with using a non-recursive filter [1,2].

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Phase Compensator for Digital Servo of Optical Disk Drive

       Disclosed is a method for achieving lower digital servo
sampling frequency for optical disk drives. By simply adding the
filter into the servo loop without changing the servo scheme, the
phase margin of the servo loop can be improved. This method can
reduce the sampling frequency more than the case with using a
non-recursive filter [1,2].

      One of the major factors that makes the sampling frequency
higher in digital servo systems is the phase delay caused by the
sample-and- hold circuit. Lower sampling frequency will cause poor
servo performance. Fig. 1 shows the Bode plots and step response of a
tracking servo on changing the sampling frequency from 15 to 100 kHz
without phase compensation. The minimum sampling frequency is
estimated as 45 kHz.

      The phase of zero-order-hold circuit changes as a constant time
delay and is converted into discrete-time domain. It is approximated
by the first-order term of a Taylor's expansion:

                            (Image Omitted)

      The transfer function of the compensator is given by the
inverse of this formula. Using the terms up to first-order of Z, it
is approximated to be:

                            (Image Omitted)

      This compensator can improve the performance as shown in Fig.
3.  The sampling frequency of the system can be reduced to 21 kHz.
The hardware representation...