Signal reflection


Signal reflection

Signal reflection occurs when a signal is transmitted along a transmission medium, such as a 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 impedance mismatches and non-linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected. In radio frequency (RF) practice, this is often measured in a dimensionless ratio known as VSWR with a VSWR bridge. The ratio of energy bounced back depends on the impedance mismatch. Mathematically, it is defined using the reflection coefficient.

Although 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.

The most likely places for reflections are at joints in the cable, or at points of damage.

Impedance discontinuities cause attenuation, attenuation distortion, standing waves, and other effects because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an 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 method of connecting electronic components.

When a returning reflection strikes another discontinuity, some of the signal rebounds in the original signal direction, creating multiple echo effects. These forward echoes 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.

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 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 suppressors or echo cancellers, 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.

ee also

*Bergeron diagram
*Crosstalk (electronics)
*Digital subscriber line
*Ground-penetrating radar
*Impedance matching
*Signal integrity


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Reflection seismology — (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth s subsurface from reflected seismic waves. The method requires a controlled seismic source of energy,… …   Wikipedia

  • Reflection — or reflexion may refer to:Computers* in computer graphics, the techniques for simulating optical Reflection. * Reflection, a programming language feature for metaprogramming * Reflection , a piece of installation art by Shane Cooper also called… …   Wikipedia

  • Reflection coefficient — The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave… …   Wikipedia

  • Reflection loss — In telecommunications, reflection loss occurs on a line which results in part of the energy being reflected back to the source. This can occur: #At a discontinuity or impedance mismatch, e.g. in a transmission line, the ratio of the incident… …   Wikipedia

  • signal — Synonyms and related words: ALGOL, COBOL, CRT spot, DM display, Doppler signal, EDP, FORTRAN, IF signal, IM display, RF echoes, Roman candle, Teletype, Wirephoto, aid to navigation, alarm, alert, alphabetic data, alphanumeric code, amber light,… …   Moby Thesaurus

  • Signal strength — In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or… …   Wikipedia

  • reflection — Synonyms and related words: CRT spot, DM display, Doppler signal, IF signal, IM display, Parthian shot, RF echoes, S curve, action and reaction, address, adumbration, adverse criticism, advisement, affirmation, albedo, allegation, animadversion,… …   Moby Thesaurus

  • Bandwidth (signal processing) — Bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a filter, a communication channel, or a signal spectrum, and is typically measured in hertz. In case of a baseband channel or signal, the bandwidth is… …   Wikipedia

  • Total internal reflection fluorescence microscope — (TIRFM) diagram 1. Specimen 2. Evanescent wave range 3. Cover slip 4. Immersion oil 5. Objective 6. Emission beam (signal) 7. Excitation beam ] 1. Objective 2. Emission beam (signal) 3. Immersion oil 4. Cover slip 5. Specimen 6. Evanescent wave… …   Wikipedia

  • List of electronics topics — Alphabetization has been neglected in some parts of this article (the b section in particular). You can help by editing it. This is a list of communications, computers, electronic circuits, fiberoptics, microelectronics, medical electronics,… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.