Non-linear center-error generator for dvd servo control

Abstract

An optical-disk play-back system has a lens that receives a light beam reflected from a data surface of an optical disk. The lens focuses the light on four quadrant photodiodes. The position of the lens relative to the photodiodes is adjusted by a standard tracking control loop. In addition, the sledge position is controlled by a sledge-center-error signal. Since the sledge position changes only infrequently, the center error is a low-frequency signal. A non-linear center-error-generating circuit uses two op-amp stages. Signals from an inner pair of photodiodes are summed and applied to one input of the first-stage op amp, while signals from an outer pair of photodiodes are summed and applied to the other input of the first-stage op amp. The first-stage op amp output an overall error signal that includes high-frequency errors. A high-pass filter removes low-frequency components output from the first-stage op amp. The high-pass filter drives an inverting input of the second-stage op amp. The non-inverting input of the second-stage op amp receives the unfiltered output of the first-stage op amp. The second-stage op amp suppresses the high-frequency components from the overall error signal, producing a low-frequency center-error signal.