Cisco Time Domain Reflectometer

Megger Time Domain Reflectometer – TDR

time-domain reflectometer (TDR) is an electronic instrument that uses time-domain reflectometry to characterize and locate faults in metallic cables (for example, twisted wire pairs, coaxial cables). It can also be used to locate discontinuities in a connector,printed circuit board, or any other electrical path. The equivalent device for optical fiber is an optical time-domain reflectometer.

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A TDR transmits a short rise time pulse along the conductor. If the conductor is of a uniform impedance and is 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.

Increases in the impedance create a reflection that reinforces the original pulse whilst decreases in the impedance create 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 almost constant for a given transmission medium, can be read as a function of cable length.  Because of this sensitivity to impedance variations, a TDR may be used to verify cable impedance characteristics, splice and connector locations and associated losses, and estimate cable lengths.

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. They are indispensable for preventive maintenance of telecommunication lines, as they can reveal growing resistance levels on joints and connectors as they corrode, and increasing insulation leakage as it degrades and absorbs moisture long before either leads to catastrophic failures. Using a TDR, it is possible to pinpoint a fault to within centimetres.

TDRs are also very useful tools for technical surveillance counter-measures, where they help determine the existence and location of wire taps. The slight change in line impedance caused by the introduction of a tap or splice will show up on the screen of a TDR when connected to a phone line.

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TDR equipment is also an essential tool in the failure analysis of modern high-frequency printed circuit boards whose signal traces are carefully crafted to emulate transmission lines. By observing reflections, any unsoldered pins of a ball grid array device can be detected. Additionally, short circuited pins can also be detected in a similar fashion.

The TDR principle is used in industrial settings, in situations as diverse as the testing of integrated circuit packages to measuring liquid levels. In the former, the time domain reflectometer is used to isolate failing sites in the same. The latter is primarily limited to the process industry.