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A computer controlled time domain reflectometry multiplexer

This paper discusses the design, implementation, and experimental verification of a one to thirtytwo Time Domain Reflectometry (TDR) switch. The results of the investigation can be summarized. First, the device required a PC as a controller. Two separate programs were written for device drivers of the multiplexer. A C program permitted a TSR (Terminate Stay Resident) mode, while a Basic program allowed portable code with existing conductivity programs already written in that language. Second, the multiplexer had to be designed with a large bandwidth and flat frequency response to prevent skewing caused by harmonic distortion. This allowed resolution of closely spaced reflections. All of these were resolved by carefully constructing good matches with the circuit board and high frequency relays. Nevertheless, the device still demonstrated fairly large power losses when taking into account several reflections, resulting in error for conductivity measurements made with the Giese and Tiemann analysis. A more complete formula was derived to account for the multiplexer in the TDR sytem, thus the multiplexer reflection and loss were modeled into the Giese and Tiemann theory. Experimentation verified the new formulas for calculating conductivity with the multiplexer present.