Editing Time-domain reflectometer (section)

== Usage ==
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 line]]s, as TDRs can detect resistance on joints and [[electrical connector|connectors]] as they [[corrosion|corrode]], and increasing [[Electrical insulation|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 [[telephone tapping|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.

TDR equipment is also an essential tool in the [[failure analysis]] of modern high-frequency printed circuit boards with signal traces crafted to emulate [[transmission line]]s. Observing reflections can detect any unsoldered pins of a [[ball grid array]] device. Short-circuited pins can also be detected similarly.

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.

=== In level measurement ===
In a TDR-based [[level measurement]] device, the device generates an impulse that propagates down a thin waveguide (referred to as a probe) – typically a metal rod or a steel cable. When this impulse hits the surface of the medium to be measured, part of the impulse reflects back up the waveguide.  The device determines the fluid level by measuring the time difference between when the impulse was sent and when the reflection returned. The sensors can output the analyzed level as a continuous analog signal or switch output signals. In TDR technology, the impulse velocity is primarily affected by the permittivity of the medium through which the pulse propagates, which can vary greatly by the moisture content and temperature of the medium. In many cases, this effect can be corrected without undue difficulty. In some cases, such as in boiling and/or high temperature environments, the correction can be difficult. In particular, determining the froth (foam) height and the collapsed liquid level in a frothy / boiling medium can be very difficult.

=== Used in anchor cables in dams ===
The Dam Safety Interest Group of CEA Technologies, Inc. (CEATI), a consortium of electrical power organizations, has applied [[Spread-spectrum time-domain reflectometry]] to identify potential faults in concrete dam anchor cables. The key benefit of Time Domain reflectometry over other testing methods is the non-destructive method of these tests.<ref>[[Cynthia Furse|C. Furse]], P. Smith, M. Diamond, "[http://www.elen.utah.edu/~cfurse/Center%20of%20Excellence/wiring_papers/anchors%20R1.pdf Feasibility of Reflectometry for Nondestructive Evaluation of Prestressed Concrete Anchors]," IEEE Journal of Sensors, Vol. 9. No. 11, Nov. 2009, pp. 1322–1329</ref>

=== Used in the earth and agricultural sciences ===
{{Main|Measuring moisture content using time-domain reflectometry}}
A TDR is used to determine [[moisture content]] in soil and porous media. Over the last two decades, substantial advances have been made measuring moisture in soil, grain, food stuff, and sediment. The key to TDR's success is its ability to accurately determine the permittivity (dielectric constant) of a material from wave propagation, due to the strong relationship between the permittivity of a material and its water content, as demonstrated in the pioneering works of Hoekstra and Delaney (1974) and Topp et al. (1980). Recent reviews and reference work on the subject include, Topp and Reynolds (1998), Noborio (2001), Pettinellia et al. (2002), Topp and Ferre (2002) and Robinson et al. (2003). The TDR method is a transmission line technique, and determines apparent permittivity (Ka) from the travel time of an electromagnetic wave that propagates along a transmission line, usually two or more parallel metal rods embedded in soil or sediment. The probes are typically between 10 and 30&nbsp;cm long and connected to the TDR via coaxial cable.

=== In geotechnical engineering ===
Time domain reflectometry has also been utilized to monitor slope movement in a variety of [[geotechnical engineering|geotechnical]] settings, including highway cuts, rail beds, and open pit mines (Dowding & O'Connor, 1984, 2000a, 2000b; Kane & Beck, 1999). In TDR stability monitoring applications, a coaxial cable is installed in a vertical borehole passing through the region of concern. The electrical impedance at any point along a coaxial cable changes with deformation of the insulator between the conductors. A brittle grout surrounds the cable to translate earth movement into an abrupt cable deformation that shows up as a detectable peak in the reflectance trace. Until recently, the technique was relatively insensitive to small slope movements and could not be automated because it relied on human detection of changes in the reflectance trace over time. Farrington and Sargand (2004) developed a simple signal processing technique using numerical derivatives to extract reliable indications of slope movement from the TDR data much earlier than by conventional interpretation.

Another application of TDRs in geotechnical engineering is to determine the soil moisture content. This can be done by placing the TDRs in different soil layers and measuring the time of start of precipitation and the time that TDR indicates an increase in the soil moisture content. The depth of the TDR (d) is a known factor and the other is the time it takes the drop of water to reach that depth (''t''); therefore the speed of water [[infiltration (hydrology)|infiltration]] (''v'') can be determined. This is a good method to assess the effectiveness of Best Management Practices (BMPs) in reducing [[stormwater]] [[surface runoff]].

=== In semiconductor device analysis ===
Time domain reflectometry is used in semiconductor failure analysis as a non-destructive method for the location of defects in semiconductor device packages. The TDR provides an electrical signature of individual conductive traces in the device package, and is useful for determining the location of opens and shorts.

=== In aviation wiring maintenance ===
Time domain reflectometry, specifically [[spread-spectrum time-domain reflectometry]] is used on aviation wiring for both preventive maintenance and fault location.<ref>Smith, P., [[Cynthia Furse|C. Furse]], and J. Gunther, 2005. "Analysis of spread spectrum time domain [http://livewiretest.com/analysis-of-spread-spectrum-time-domain-reflectometry-for-wire-fault-location/ reflectometry for wire fault location] {{webarchive|url=https://web.archive.org/web/20101231231446/http://livewiretest.com/analysis-of-spread-spectrum-time-domain-reflectometry-for-wire-fault-location/ |date=2010-12-31 }}". IEEE Sensors Journal 5:1469–1478.</ref> Spread spectrum time domain reflectometry has the advantage of precisely locating the fault location within thousands of miles of aviation wiring. Additionally, this technology is worth considering for real time aviation monitoring, as  spread spectrum reflectometry can be employed on live wires.

This method has been shown to be useful to locating intermittent electrical faults.<ref>[[Cynthia Furse|Furse, Cynthia]], Smith, P., Safavi, Mehdi, and M. Lo, Chet. "[http://livewiretest.com/feasibility-of-spread-spectrum-sensors-for-location-of-arcs-on-live-wires/ Feasibility of Spread Spectrum Sensors for Location of Arcs on Live Wires] {{webarchive|url=https://archive.today/20100501194849/http://livewiretest.com/feasibility-of-spread-spectrum-sensors-for-location-of-arcs-on-live-wires/ |date=2010-05-01 }}". IEEE Sensors Journal. December 2005.</ref>

Multi carrier time domain reflectometry (MCTDR) has also been identified as a promising method for embedded EWIS diagnosis or troubleshooting tools. Based on the injection of a multicarrier signal (respecting EMC and harmless for the wires), this smart technology provides information for the detection, localization and characterization of electrical defects (or mechanical defects having electrical consequences) in the wiring systems.
Hard fault (short, open circuit) or intermittent defects can be detected very quickly increasing the reliability of wiring systems and improving their maintenance.<ref>G.Millet, S.Bruillot, D.Dejardin, N.Imbert, F.Auzanneau, L.Incarbone, M.Olivas, L.Vincent, A.Cremzi, S.Poignant, 2014.[https://www.researchgate.net/publication/260075393_Aircraft_Electrical_Wiring_Monitoring_System "Aircraft Electrical Wiring Monitoring System"]</ref>