Numerical Simulation of Measured Time Domain Reflectormetry Signatures

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Experimental Time Domain Reflectometry (TDR) data obtained from the field are compared with numerical simulations using a finite-difference solution of the transmission line equations. The model simulates transmission of a voltage pulse along a lossy coaxial cable where deformities on the cable, such as crimps and shears, are represented by capacitive discontinuities in the transmission line. Three reflection phenomena were studied in the field. The first was the increase in width and decrease in amplitude of a reflection as deformations within close proximity to each other but located at large distances from the source. These three field experiments were simulated with the model and comparisons were made to assess the model's ability to predict known behavior. Results from the field data show a linear increase in reflection width with increasing propagation distance and an exponential decrease in reflection amplitude with increasing distance. A study of multiple discontinuities shows that upstream reflection amplitudes in excess of 100mρ will begin to influence a downstream discontinuity by reducing the downstream reflection amplitude. Studies of two or three shear discontinuities with a 1.5m separation distance resulted in no measurable changes in reflection caused by addition of the upstream shear discontinuities. After the field data were collected, the model was calibrated with the measured reflection data for single discontinuities in a Comm/Scope P-3 75-875CA coaxial cable. Resistance was found by matching increasing crimp width with distance between 1 and 30m and 1 and 50m. A 12mm wide 120pF capacitive discontinuity best related modeled and measured crimps 12mm wide and 7.2mm deep at 51m and a 6mm wide 150pF capacitive discontinuity best related a 7.5mm shear displacement at 45m. With these calibrations, the model was able to predict known amplitudes to within 4mp of the measured values for single discontinuities up to distances of 94m. Simulations of the influence of an upstream crimp reflection resulted was a 72mm crimp. However, the influence of an upstream shear on a downstream crimp was detected by the code that showed a slight decrease in amplitude, yet much smaller than the measured data. These ii results suggest the model is less sensitive to multiple discontinuities than observed in the field.

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  • 08/14/2017
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