Signal reflection
Encyclopedia
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
.
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.
Impedance discontinuities cause attenuation
, attenuation distortion
, standing waves, ringing
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 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
.
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 suppressor
s 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.
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, 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
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(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
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
. Mathematically, it is defined using the reflection coefficient
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 relative to an incident wave...
.
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.
Impedance discontinuities cause attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
, attenuation distortion
Attenuation Distortion
Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal....
, standing waves, ringing
Ringing (signal)
In electronics, signal processing, and video, ringing is unwanted oscillation of a signal, particularly in the step response...
and other effects because a portion of a transmitted signal will be reflected back to the transmitting
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
device rather than continuing to the receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
, 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
Daisy chain
Daisy chain may refer to a daisy garland created from daisy flowers, the original meaning and the one from which the following derive by analogy:*Daisy chain *Daisy chain *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 echo
Forward echo
Forward echo: In a transmission line, a reflection propagating in the same direction as the original wave and consisting of energy reflected back by one discontinuity and then forward again by another discontinuity. Forward echoes can be supported by reflections caused by splices or other...
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
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
.
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
Optical time domain reflectometer
An optical time-domain reflectometer is an optoelectronic instrument used to characterize an optical fiber. An OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered or reflected back from points along the fiber...
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
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss
Insertion loss
In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels ....
. Proper network operation depends on constant characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
in all cables and connectors, with no impedance discontinuities in the entire cable system. When a sufficient degree of impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
is not practical, echo suppressor
Echo suppressor
An echo suppressor is a telecommunications device used to reduce the echo heard on long telephone circuits, particularly circuits that traverse satellite links...
s or echo cancellers, or both, can sometimes reduce the problems.
The Bergeron Diagram
Bergeron diagram
Bergeron diagram method is a method to value the reflection's effects on an electric signal. This graphic method—based on the real line's characteristic—is valid both for linear and non linear models and helps to calculate the delay of an electromagnetic signal on an electric line.Using Bergeron...
method, valid for both linear and non-linear models, evaluates the reflection's effects in an electric line.
See also
- Bergeron diagramBergeron diagramBergeron diagram method is a method to value the reflection's effects on an electric signal. This graphic method—based on the real line's characteristic—is valid both for linear and non linear models and helps to calculate the delay of an electromagnetic signal on an electric line.Using Bergeron...
- Crosstalk (electronics)Crosstalk (electronics)In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel...
- Digital subscriber lineDigital Subscriber LineDigital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
- Echo (phenomenon)Echo (phenomenon)In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...
- Ground-penetrating radarGround-penetrating radarGround-penetrating radar is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band of the radio spectrum, and detects the reflected signals from subsurface structures...
- Impedance matchingImpedance matchingIn electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
- Signal integritySignal integritySignal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise,...
- Reflections of signals on conducting linesReflections of signals on conducting linesA signal travelling along an electrical transmission line will be partly, or wholly, reflected back in the opposite direction when the travelling signal encounters a discontinuity in the transmission parameters of the line, or at the far end of the line if the line is not correctly terminated in...