Threshold control with data receiver

Abstract

A binary data receiver data modulated light signals and applies them to a photodetector for producing electrical data signals which are amplified and applied to a data decision circuit. The data decision circuit compares the instantaneous value of the amplified data signals with a fixed threshold to determine if a logic high or low level is received. A portion of the signal path between the photodetector and the data decision circuit is ac coupled, whereby the moment-to-moment variations of duty cycle of the signal due to the information content of the data causes variations in the peak excursions of the signal which may adversely affect the data decisions or processing of the data. Open-circuit correction of the average value is accomplished by a correction signal generator which receives an ac coupled data sample, and rectifies both positive-going and negative-going excursions relative to ground to produce positive and negative average value signals. The positive and negative average value signals are summed to produce a correction signal. The correction signal is either added to the ac coupled signal or controls the output offset voltage of a signal amplifier to correct the information-depending average-value variations at the data decision circuit.