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Wide Bandwidth Focus Error Generation Circuit

IP.com Disclosure Number: IPCOM000119625D
Original Publication Date: 1991-Feb-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 97K

Publishing Venue

IBM

Related People

Bui, NX: AUTHOR [+3]

Abstract

Optical disks employ focus error mechanisms which are used to correct focus errors in the optical path. In Fig. 1, a four-element detector "Quad" detects light for indicating focus errors. It supplies four signals to an algebra circuit which generates a focus error signal. This article teaches how to broaden the bandwidth of decoder, to enhance focus control. Two circuits are shown.

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Wide Bandwidth Focus Error Generation Circuit

      Optical disks employ focus error mechanisms which are
used to correct focus errors in the optical path.  In Fig. 1, a
four-element detector "Quad" detects light for indicating focus
errors.  It supplies four signals to an algebra circuit which
generates a focus error signal.  This article teaches how to broaden
the bandwidth of decoder, to enhance focus control.  Two circuits are
shown.

      The focus error signal is generated using one of two equations
solved by the decoder.  The symbols A-D respectively refer to the
four independent photodetectors of the Quad.  Either equation can be
used.  Focus Error Signal (FES) = (A-D)/(A+D) + (C-B)/(C+B) or FES =
(A-B)/ A+B) + (C-D)/(C+D).

      The bandwidth of the decoder is limited by the combination of
relatively low input signal currents and high detector capacitance.
One way to widen the bandwidth is to increase the light beam power so
as to maintain the same signal while reducing the area of the
detector and, thus, its capacitance.  A better approach is to use
circuit means to correct for both low input currents and high
detector capacitance.  In this approach, circuits are used to amplify
the input signal currents and buffer the algebra circuit from the
detector capacitance.  In Fig. 2, transimpedance amplifiers TAA and
TAB amplify the input signal currents from two photo-detectors of the
Quad to signal voltages at nodes A0 and B0.  Typically, the input
resistance of a TA is low so that, in parallel with the detector
capacitance, bandwidths in the megahertz range are obtained.

      The current mode is regained with voltage-to-current converters
VICA and VICB between nodes A0 and B0 and the inputs to the deco...