Adaptive equalizer suitable for use with fiber optics

A circuit for adaptively equalizing a digital signal to compensate for distortion introduced by a transmission medium, comprising:

• means for providing a first sum of the signal to be equalized and a feedback signal;
• means responsive to said first sum for generating said feedback signal, including means, responsive to a control input signal, for controlling the magnitude of said feedback signal;
• means for providing a second sum of said signal to be equalized and said feedback signal, including weighting said feedback signal in said sum by an amount which tends to overcompensate for said distortion;
• means for providing a compensation signal and for providing a third sum of the compensation signal and said signal to be equalized, so that said third sum tends to undercompensate for said distortion;
• means for detecting errors in said second and third sums;
• and
• means for detecting the difference between said errors detected in said second and said third sums and generating said control input signal dependent on said difference,
• whereby said first sum comprises an adaptively equalized output signal.

A circuit for adaptively equalizing a digital signal to compensate for distortion introduced by a transmission medium, comprising:

• summing network means for providing a first sum of the signal to be equalized and a feedback signal;
• a variable gain differential amplifier having first and second differential inputs and a gain control input, with said first differential input responsive to said first sum and said second differential input connected to a threshold bias, so that in response to a comparison of said sum and the threshold represented by said bias, said differential amplifier switches the output state thereof to provide a digital output, the magnitude of which is controlled by a level applied to said gain control input;
• a feedback network, responsive to the magnitude controlled digital output, for providing said feedback signal;
• a summing network means for providing a second sum of said signal to be equalized and said feedback signal weighted by an amount which tends to overcompensate for said distortion;
• means for providing a compensation signal and for providing a third sum of the compensation signal and said signal to be equalized, said third sum tending to undercompensate for said distortion;
• means for detecting errors in each of said second and third sums of signals, when each of the sums fails to satisfy selected threshold criteria identifying the sum as representing either a one or a zero bit, said threshold criteria being selected to ensure a substantial rate of said errors under steady state adapted conditions;
• and
• means for providing the difference between said errors detected in said second and third sums, providing the integral of said difference, and coupling said integral to said gain control input, whereby said first sum comprises an adaptively equalized output signal.