Switched threshold comparator for a fiber-optic receiver

Abstract

A fiber-optic receiver invention includes an improved edge detector which receives the input current pulse from a photo diode located at the output of an optical fiber link. The edge detector differentiates the input waveform and provides a sequential positive going and negative going pulse combination at its output. The output of the edge detector is applied to the input of a switched threshold comparator circuit. The switched threshold comparator senses the differentiated waveform signal and provides complementary output signals which are the desired data output waveforms. At the same time, the switched threshold comparator applies the output waveform to an emitter-coupled differential amplifier having an unbalanced collector circuit. The emitter-coupled differential amplifier has a first transistor connected between a current and the reference input to a differential comparator. The differential amplifier has a second transistor connected in emitter-coupled logic fashion to the first transistor, with its collector connected to ground potential. The respective inputs to the bases of the first and second transistors are the true and complement outputs of the differential comparator. When the output of the differential comparator goes positive, the unbalanced differential amplifier applies a negative going reference potential to the reference input to the differential comparator which switches the effective reference potential to a more negative value in anticipation of the negative going differentiated waveform which will next be output from the edge detector. After the negative going differential waveform from the edge detector is applied to the signal input of the differential comparator, the data output waveform goes to its less positive value. That less positive value is also applied to the unbalanced differential amplifier, thereby changing the reference potential applied to the reference input of the differential comparator, to a more positive value in anticipation of the positive going waveform which will next be output from the edge detector circuit. In this manner, the effective threshold of the differential comparator is switched between two predetermined values in response to the output of the differential comparator.