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| [[File:Partial transmittance.gif|thumb|Signal (or energy) transmitted and reflected from a discontinuity]]
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| '''Time-domain reflectometry''' or '''TDR''' is a [[Measurement|measurement technique]] used to determine the characteristics of [[Transmission line|electrical lines]] by observing [[Reflection (electrical)|reflected]] [[waveform]]s.<ref>Furse, Cynthia, Lo, Chet. "[http://livewiretest.com/noise-domain-reflectometry-for-locating-wiring-faults/ Reflectometry for Locating Wiring Faults]". IEEE Transactions on Electromagnetic Compatibility. February, 2005</ref> '''Time-domain transmissometry''' ('''TDT''') is an analogous technique that measures the transmitted (rather than reflected) impulse. Together, they provide a powerful means of analysing electrical or optical transmission media such as [[coaxial cable]]s and [[optical fiber]]s. | |
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| Variations of TDR exist. For example, [[spread-spectrum time-domain reflectometry]] (SSTDR) is used to detect intermittent faults in complex and high-noise systems such as aircraft wiring.<ref>Smth,Paul, Furse, Cynthia and Gunther, Jacob. "Analysis of Spread Spectrum Time Domain Reflectometry for [http://livewiretest.com/analysis-of-spread-spectrum-time-domain-reflectometry-for-wire-fault-location/ Wire Fault Location]." IEEE Sensors Journal. December, 2005.</ref> Coherent optical time domain reflectometry (COTDR) is another variant, used in optical systems, in which the returned signal is mixed with a local oscillator and then filtered to reduce noise.<ref>José Chesnoy (ed.), ''Undersea Fiber Communication Systems'', Elsevier Science, 2002, ISBN 0-12-171408-X, p.171 (COTDR)</ref>
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| The [[Electrical impedance|impedance]] of the [[Discontinuity (transmission lines)|discontinuity]] can be determined from the [[amplitude]] of the reflected signal . The [[distance]] to the reflecting impedance can also be determined from the [[time]] that a [[Pulse (signal processing)|pulse]] takes to return. The limitation of this method is the minimum system [[rise time]]. The total rise time consists of the combined rise time of the driving pulse and that of the [[oscilloscope]] that monitors the reflections.
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| ==Method==
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| The TDR analysis begins with the propagation of a [[Step function|step]] or impulse of [[energy]] into a [[system]] and the subsequent observation of the energy reflected by the system. By analyzing the magnitude, duration and shape of the reflected waveform, the nature of the impedance variation in the transmission system can be determined.
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| ===Resistive load===
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| If a pure [[Resistor|resistive load]] is placed on the output of the [[Time-domain reflectometer|reflectometer]] and a [[Step function|step signal]] is applied, a step signal is observed on the [[Cathode ray tube|CRT]], and its height is a function of the resistance. The magnitude of the step caused by the resistive load may be expressed as a fraction of the input signal as given by:
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| :<math>\rho = \frac{R_L - Z_0}{R_L + Z_0}</math>
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| where <math>Z_0</math> is the [[characteristic impedance]] of the [[transmission line]].
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| ===Reactive load===
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| For [[electrical reactance|reactive load]]s, the observed waveform depends upon the time constant formed by the load and the characteristic impedance of the line.
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| ==See also==
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| * [[Noise-Domain Reflectometry]]
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| * [[Optical time-domain reflectometer]]
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| * [[Time-domain reflectometer]]
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| * [[Transmission line]]
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| ==References==
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| <references/>
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| {{DEFAULTSORT:Time-Domain Reflectometry}}
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| [[Category:Electronic engineering]]
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| [[fr:Réflectomètre]]
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