![]() TDRs are also very useful tools for technical surveillance counter-measures, where they help determine the existence and location of wire taps. Using a TDR, it is possible to pinpoint a fault to within centimetres. They are indispensable for preventive maintenance of telecommunication lines, as TDRs can detect resistance on joints and connectors as they corrode, and increasing insulation leakage as it degrades and absorbs moisture, long before either leads to catastrophic failures. Time domain reflectometers are commonly used for in-place testing of very long cable runs, where it is impractical to dig up or remove what may be a kilometers-long cable. This is also known as a scattering junction. Some of the energy will be reflected back to the driving source the remaining energy will be transmitted. As an example, a trace width on a printed circuit board doubled at its midsection would constitute a discontinuity. This includes abrupt changes in the characteristic impedance. Where Z o is defined as the characteristic impedance of the transmission medium and Z t is the impedance of the termination at the far end of the transmission line.Īny discontinuity can be viewed as a termination impedance and substituted as Z t. The magnitude of the step produced by the resistive load may be expressed as a fraction of the input signal as given by: If a pure resistive load is placed on the output of the reflectometer and a step signal is applied, a step signal is observed on the display, and its height is a function of the resistance. By analyzing the magnitude, duration and shape of the reflected waveform, the nature of the impedance variation in the transmission system can be determined. The TDR analysis begins with the propagation of a step or impulse of energy into a system and the subsequent observation of the energy reflected by the system. The total rise time consists of the combined rise time of the driving pulse and that of the oscilloscope or sampler that monitors the reflections. The limitation of this method is the minimum system rise time. ![]() The distance to the reflecting impedance can also be determined from the time that a pulse takes to return. ![]() The impedance of the discontinuity can be determined from the amplitude of the reflected signal. Signal (or energy) transmitted and reflected from a discontinuity Instead, if there are impedance variations, then some of the incident signal will be reflected back to the source. If the conductor is of a uniform impedance and is properly terminated, then there will be no reflections and the remaining incident signal will be absorbed at the far-end by the termination. In order to measure those reflections, the TDR will transmit an incident signal onto the conductor and listen for its reflections. It can be used to characterize and locate faults in metallic cables (for example, twisted pair wire or coaxial cable), Īnd to locate discontinuities in a connector, printed circuit board, or any other electrical path.Ī TDR measures reflections along a conductor. Electronic instrument Time-domain reflectometer for cable fault detectionĪ time-domain reflectometer ( TDR) is an electronic instrument used to determine the characteristics of electrical lines by observing reflected pulses.
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