How Does the TDR Function?
A TDR is a test set used to characterize and locate faults in metallic cables. It transmits a short rise time pulse along the cable pair. If the cable pair is of a uniform admittance and properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source. This is similar in principle to radar. An increase in the impedance (an open cable pair) creates a reflection that reinforces the original pulse. A decrease in the impedance (a solid short) creates a reflection that opposes the original pulse. The resulting reflected pulse that is measured at the output input to the TDR is displayed or plotted as a function of time, and because the speed of signal propagation is relatively constant on a cable pair, the total travel time of the pulse down and back can be read as a function of cable length. Because of this sensi
