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TDR (Time Domain Reflectometer)

What is a TDR?

A Time Domain Reflectometer transmits a short rise-time pulse along a cable to check for impedance discontinuities that can represent shorts or opens within a cable run. If the cable is completely uniform and properly terminated, the entire transmitted pulse will be absorbed into the termination and no return signal will be noted. Any impedance discounuities will be sent back to the source, thus representing and reporting the condition at any point along the cable. Because of its extreme sensitivity to impedance variations, a TDR may be used to look at cable characteristics including opens, shorts, splice and connector locations, and also to estimate cable lengths. A low-voltage TDR is an appropriate method to localize faults and other impedance changes on electrical cable such as twisted pair, parallel pair, and coaxial structure. TDRs are available in small hand-held, larger portable, and rack mount configurations for a broad variety of applications. Low-voltage, high-frequency output pulses are transmitted into and travel between two conductors of the cable. When the cable impedance changes, some or all of transmitted energy is reflected back to the TDR where it is displayed. Impedance changes are caused by a variety of disturbances on the cable including low resistance faults and landmarks such as the cable end, splices, taps, and transformers.

Faults That a Low-Voltage TDR Will Display
Low resistance faults of less than 200 Ω between conductor and ground or between conductors are displayed as downward reflections on the screen. Series opens, since they represent a very high resistance, are displayed as upward going reflections.

Landmarks That a Low-Voltage TDR Will Display
A TDR can localize cable landmarks, such as splices, wye or T-taps, and transformers. The TDR helps to determine the location of faults relative to other landmarks on the cable. This is especially true on complex circuits. Traces of complex circuits are necessarily also very complex and difficult to interpret. To make sense of these complex traces, it is extremely helpful to confirm the position of landmarks relative to the faults observed. For every landmark that causes a reflection, there is slightly less transmitted pulse amplitude traveling from that point down the cable. This means on a cable run with two identical splices, the reflection from the first splice will be larger than that of the second down the cable farther. No conclusions can be drawn based on the size or height of reflections at different distances down the cable.

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