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{{Short description|When a transmitted signal reflects back through the medium it was transmitted over}}
In [[telecommunications]], '''signal reflection''' happens when a [[signal]] is [[Signal transmission|transmitted]] along a [[transmission medium]] (such as a [[reflections of signals on conducting lines|copper cable]] or an [[optical fiber]]) and part of it is reflected back toward the source instead of reaching the end. This reflection is caused by imperfections or physical variations in the cable (such as abrupt changes in its geometry) that lead to [[Electrical impedance|impedance]] mismatches.<ref>{{Cite web |title=Physics behind signal reflections and series termination |url=https://electronics.stackexchange.com/questions/484269/physics-behind-signal-reflections-and-series-termination |access-date=2023-06-03 |website=Electrical Engineering Stack Exchange |language=en}}</ref> These mismatches disrupt the signal and cause some of it to bounce back. In [[radio frequency]] (RF) systems, this is typically measured using the [[voltage standing wave ratio]] (VSWR), with a device called a VSWR bridge. The amount of reflected energy depends on the degree of [[Impedance matching|impedance mismatch]] and is mathematically described by the [[reflection coefficient]].<ref>{{Cite web |date=2023-04-22 |title=What Is Signal Reflection? (with pictures) |url=http://www.allthescience.org/what-is-signal-reflection.htm |access-date=2023-06-03 |website=All the Science |language=en-US}}</ref>

Because the principles are the same, this concept is perhaps easiest to understand when considering an optical fiber. Imperfections in the glass create mirrors that reflect the light back along the fiber.<ref>{{Cite web |title=Why Reflections Happen |url=http://www.signalintegrity.com/Pubs/edn/whyreflectionshappen.htm |access-date=2023-06-03 |website=www.signalintegrity.com}}</ref>

Impedance discontinuities cause [[attenuation]], [[attenuation distortion]], [[standing waves]], [[ringing (signal)|ringing]] and other effects because a portion of a transmitted signal will be reflected back to the [[transmitter|transmitting]] device rather than continuing to the [[receiver (radio)|receiver]], much like an [[Echo (phenomenon)|echo]]. This effect is compounded if multiple discontinuities cause additional portions of the remaining signal to be reflected back to the transmitter. This is a fundamental problem with the [[daisy chain (electrical engineering)|daisy chain]] method of connecting electronic components.<ref>{{Cite book |url=https://www.ti.com/lit/an/snla027b/snla027b.pdf |title=AN-807 Reflections: Computations and Waveforms |access-date=2023-06-03}}</ref>

When a returning reflection strikes another discontinuity, some of the signal rebounds in the original signal direction, creating multiple echo effects. These [[forward echo]]es strike the receiver at different intervals making it difficult for the receiver to accurately detect data values on the signal. The effects can resemble those of [[jitter]].

[[File:Locale_RS6_Megger Time-Domain Reflectometer MTDR1.jpg|thumb|[[Time-domain reflectometer]] for [[electrical cable]] fault detection]]
Because damage to the cable can cause reflections, an instrument called an electrical [[time-domain reflectometer]] (ETDR; for electrical cables) or an [[optical time-domain reflectometer]] (OTDR; for optical cables) can be used to locate the damaged part of a cable. These instruments work by sending a short pulsed signal into the cable and measuring how long the reflection takes to return. If only reflection magnitudes are desired, however, and exact fault locations are not required, VSWR bridges perform a similar but lesser function for [[Radio frequency|RF cables]].

The combination of the effects of signal attenuation and impedance discontinuities on a [[communications link]] is called [[insertion loss]]. Proper network operation depends on constant [[characteristic impedance]] in all cables and connectors, with no impedance discontinuities in the entire cable system. When a sufficient degree of [[impedance matching]] is not practical, [[echo suppressor]]s or [[echo canceller]]s, or both, can sometimes reduce the problems.

The [[Bergeron diagram]] method, valid for both linear and non-linear models, evaluates the reflection's effects in an electric line.

==See also==
*[[Crosstalk (electronics)]]
*[[Digital subscriber line]]
*[[Project Echo]]
*[[Fresnel reflection]]
*[[Ground-penetrating radar]]
*[[Impedance matching]]
*[[Signal integrity]]
*[[Reflections of signals on conducting lines]]
*[[Reflection phase change]]

== References ==
{{Reflist}}
{{DEFAULTSORT:Signal Reflection}}
[[Category:Radio electronics]]
[[Category:Electricity]]
[[Category:Geometrical optics]]
[[Category:Electronic design]]
[[Category:Electrical engineering]]
[[Category:Physical optics]]

Signal reflection - Revision history

RS-485: Imported from Wikipedia (overwrite)