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time domain reflectometry (TDR)

Timedomain reflectometry (TDR) is a measurement method for fault location and fault analysis in transmission cables. In principle, time domain reflectometry uses the propagation time in physical media to determine the fault location; the fault analysis can be derived from the shape of the reflection

. According to the method, a step pulse or a single pulse is sent into the transmission line. The pulse travels through the cable

and is partially reflected off any irregularities in the cable, such as changes in impedance or reactance. The reflected portion of the pulse travels back down the cable to the feed point where it is evaluated.

Oscilloscope display of a TDR measurement

Oscilloscope display of a TDR measurement

The temporal resolution of the TDR measurement, i.e. the ability to distinguish between closely spaced junctions, can be increased by shorter rise times. TDR methods with higher resolution are called High Definition Time Domain Reflectometry (HDTDR). One speaks of HDTDR when the test impluses have a pulse width of 2 ns. However, the resolving power also depends on the inherent rise time of the TDR measurement device. For example, if an oscilloscope

has an intrinsic rise time of 0.7 ns, then it can resolve two reflected pulses 5.2 cm apart. Thebasis of this calculation is the approximation formula distance = speed of light x intrinsic rise time/4. This resolution behavior is usually sufficient for measurements on cables, but investigations on RF connectors

areonly possible with HDTDR, since they can only be investigated with intrinsic rise times in the pico-second range.

In optical transmission technology, optical waveguides, FO connectors and cabling are measured with optical time domain reflectometry (OTDR).

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Englisch: time domain reflectometry - TDR
Updated at: 03.09.2019
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