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[[File:Partial transmittance.gif|thumb|Signal (or energy) transmitted and reflected from a discontinuity]] | |||
'''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. | |||
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> | |||
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. | |||
==Method== | |||
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. | |||
===Resistive load=== | |||
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: | |||
:<math>\rho = \frac{R_L - Z_0}{R_L + Z_0}</math> | |||
where <math>Z_0</math> is the [[characteristic impedance]] of the [[transmission line]]. | |||
===Reactive load=== | |||
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. | |||
==See also== | |||
* [[Noise-Domain Reflectometry]] | |||
* [[Optical time-domain reflectometer]] | |||
* [[Time-domain reflectometer]] | |||
* [[Transmission line]] | |||
==References== | |||
<references/> | |||
{{DEFAULTSORT:Time-Domain Reflectometry}} | |||
[[Category:Electronic engineering]] | |||
[[fr:Réflectomètre]] | |||
Revision as of 19:35, 18 June 2013
Time-domain reflectometry or TDR is a measurement technique used to determine the characteristics of electrical lines by observing reflected waveforms.[1] 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 cables and optical fibers.
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.[2] 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.[3]
The impedance of the 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 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.
Method
The TDR analysis begins with the propagation of a 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.
Resistive load
If a pure resistive load is placed on the output of the reflectometer and a step signal is applied, a step signal is observed on the 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:
where is the characteristic impedance of the transmission line.
Reactive load
For reactive loads, the observed waveform depends upon the time constant formed by the load and the characteristic impedance of the line.
See also
- Noise-Domain Reflectometry
- Optical time-domain reflectometer
- Time-domain reflectometer
- Transmission line
References
- ↑ Furse, Cynthia, Lo, Chet. "Reflectometry for Locating Wiring Faults". IEEE Transactions on Electromagnetic Compatibility. February, 2005
- ↑ Smth,Paul, Furse, Cynthia and Gunther, Jacob. "Analysis of Spread Spectrum Time Domain Reflectometry for Wire Fault Location." IEEE Sensors Journal. December, 2005.
- ↑ José Chesnoy (ed.), Undersea Fiber Communication Systems, Elsevier Science, 2002, ISBN 0-12-171408-X, p.171 (COTDR)