Editing Time-domain reflectometer (section)

== Description ==
A TDR measures reflections along a conductor. In order to measure those reflections, the TDR will transmit an incident signal onto the conductor and listen for its [[signal reflection|reflections]]. If the conductor is of a uniform [[Electrical impedance|impedance]] and is properly [[Electrical termination|terminated]], then there will be no reflections and the remaining incident signal will be absorbed at the far-end by the termination. Instead, if there are impedance variations, then some of the incident signal will be reflected back to the source. A TDR is similar in principle to [[radar]].

[[File:Partial transmittance.gif|thumb|Signal (or energy) transmitted and reflected from a discontinuity]]

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]] or sampler 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.

If a pure [[Resistor|resistive load]] is placed on the output of the reflectometer and a [[Step function|step signal]] is applied, a step signal is observed on the display, and its height is a function of the resistance. The magnitude of the step produced by the resistive load may be expressed as a fraction of the input signal as given by:

<math display="block">\rho = \frac{R_L - Z_0}{R_L + Z_0}</math>

where <math>Z_0</math> is the [[characteristic impedance]] of the [[transmission line]].

=== Reflection ===
Generally, the reflections will have the same shape as the incident signal, but their sign and magnitude depend on the change in impedance level.  If there is a step increase in the impedance, then the reflection will have the same sign as the incident signal; if there is a step decrease in impedance, the reflection will have the opposite sign.  The magnitude of the reflection depends not only on the amount of the impedance change, but also upon the loss in the conductor.

The reflections are measured at the [[Input/output|output/input]] to the TDR and displayed or plotted as a function of time.  Alternatively, the display can be read as a function of [[Electrical cable|cable]] length because the speed of signal propagation is almost constant for a given transmission medium.

Because of its sensitivity to impedance variations, a TDR may be used to verify cable impedance characteristics, [[Fusion splicing|splice]] and [[electrical connector|connector]] locations and associated losses, and estimate cable lengths.

=== Incident signal ===
TDRs use different incident signals. Some TDRs transmit a [[pulse]] along the conductor; the resolution of such instruments is often the width of the pulse. Narrow pulses can offer good resolution, but they have high frequency signal components that are attenuated in long cables.  The shape of the pulse is often a half cycle sinusoid.<ref>1983 Tektronix Catalog, pages 140–141, the 1503 uses "1/2-sine-shaped pulses" and has a 3-foot resolution and a range of 50,000 feet.</ref> For longer cables, wider pulse widths are used.

Fast [[rise time]] steps are also used.  Instead of looking for the reflection of a complete pulse, the instrument is concerned with the rising edge, which can be very fast.<ref>1983 Tektronix Catalog, pages 140–141, the 1502 uses a step (system rise time less than 140&nbsp;ps), has a resolution of 0.6&nbsp;inch and a range of 2,000&nbsp;feet.</ref>  A 1970s technology TDR used steps with a rise time of 25&nbsp;ps.<ref>1983 Tektronix Catalog, page 289, S-52 pulse generator has a 25-ps risetime.</ref><ref>{{citation |title=S-6 Sampling Head |series=Instruction Manual |date=September 1982 |publisher=Tektronix |location=Beaverton, OR}}  First printing is 1982, but copyright notice includes 1971.</ref><ref>{{citation |title=7S12 TDR/Sampler |series=Instruction Manual |date=November 1971 |publisher=Tektronix |location=Beaverton, OR}}</ref>

Still other TDRs transmit complex signals and detect reflections with correlation techniques. See [[spread-spectrum time-domain reflectometry]].

=== Variations and extensions ===
The equivalent device for [[optical fiber]] is an [[optical time-domain reflectometer]].

{{anchor|TDT}}'''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]] and [[optical fiber]].

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>Smith, Paul, [[Cynthia Furse|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] {{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. 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>