Exponential pulse generator for a time domain reflectometer

Abstract

An exponential pulse generator for use in a Time-Domain Reflectometer is described. This pulse has the property that its leading edge does not suffer distortion while propagating in dispersive lossy transmission lines. It thus enables accurate measurement of time of flight of a single pulse reflected by an impedance-discontinuity in a faulty transmission line. The design of a precision pulse generator that enables the control of pulse-amplitude, pulse-rate and the time constant of its exponentially rising edge is described. A key feature of the design is a compensation scheme to eliminate the temperature variation arising from the reverse saturation current as well as the thermal voltage in the exponential v-i characteristics of a p-n junction used to generate the pulse. Here we present simulation results as well as a circuit using discrete components interconnected on a printed circuit board. Experimental results from a calibrated prototype show that a fault 100 meters away along a cable can be located with a precision of a fraction of a millimeter.