View source for Signal reflection

{{Short description|When a transmitted signal reflects back through the medium it was transmitted over}}In [[telecommunications]], '''signal reflection''' occurs 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]]. Some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable cause [[Electrical impedance|impedance]] mismatches and non-linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected.<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> In [[radio frequency]] (RF) practice this is often measured in a dimensionless ratio known as [[voltage standing wave ratio]] (VSWR) with a VSWR bridge.  The ratio of energy bounced back depends on the [[Impedance matching|impedance mismatch]].  Mathematically, it is defined using 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: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}}
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[[Category:Radio electronics]]
[[Category:Electricity]]
[[Category:Geometrical optics]]
[[Category:Electronic design]]
[[Category:Electrical engineering]]
[[Category:Physical optics]]