A High-Sensitivity Narrow-Band Time-Domain Reflectometer

Time-domain reflectometer (TDR) techniques have been used to test transmission line systems for over fifteen years. The technique described in this paper is an extension to present-day techniques and is designed particularly to detect small changes in the reflection coefficient of distributed systems versus time. The instrument uses some of the techniques of narrow-band TDR to provide a waveform, related to the distributed reflection coefficient of the system under test, to a small computer. The computer can then perform any of a number of manipulations to the waveform, depending on the wishes of the operator. One particular program produces a waveform profile on a storage oscilloscope which displays small changes in the distributed reflection coefficient with time. This capability of being able to display very small changes in reflection coefficients has resulted in the development of an obstacle detection scheme using so called "leaky" coaxial cable to detect objects at some distance from the cable.