Timing discriminator for pulsed time-of-flight laser rangefinding measurements

Abstract

A time-pickoff circuit based on the constant fraction discriminator (CFD) timing principle has been developed for pulsed time-of-flight laser rangefinding with pulse lengths of 5–10 ns. It is based on detection of the crossing point of the trailing edge of the original timing pulse and the leading edge of its delayed replica with a fast emitter coupled logic comparator. A simplified theory is presented here for its walk error. Three types of comparator were tested in the CFD. It is shown that the dominant source of walk error is that produced by the limited gain-bandwidth product of the comparator and that walk error can be reduced to ±1 mm in a 1:10 dynamic range of input pulses by adding an external offset voltage between the input nodes of the comparator.