Return Loss
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In
telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, return loss is a measure in relative terms of the
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
of the
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
reflected by a discontinuity in a
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
or
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...
. This discontinuity can be caused by a mismatch between the termination or load connected to the line and the characteristic impedance of the line. It is usually expressed as a ratio in decibels (dB); :RL(\mathrm) = 10 \log_ :where ''RL''(dB) is the return loss in dB, ''P''i is the incident power and ''P''r is the reflected power. Return loss is related to both
standing wave ratio In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the trans ...
(SWR) and
reflection coefficient In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected w ...
(Γ). Increasing return loss corresponds to lower SWR. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower
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 (dB). If the power transmitted to the load before inserti ...
. From a certain perspective 'Return Loss' is a misnomer. The usual function of a transmission line is to convey power from a source to a load with minimal loss. If a transmission line is correctly matched to a load, the reflected power will be zero, no power will be lost due to reflection, and 'Return Loss' will be infinite. Conversely if the line is terminated in an open circuit, the reflected power will be equal to the incident power; all of the incident power will be lost in the sense that none of it will be transferred to a load, and RL will be zero. Thus the numerical values of RL tend in the opposite sense to that expected of a 'loss'.


Sign

As defined above, RL will always be positive, since ''P''r can never exceed ''P''i . However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature.Trevor S. Bird
"Definition and Misuse of Return Loss"
''IEEE Antennas & Propagation Magazine'', vol.51, iss.2, pp. 166–167, April 2009.
Strictly speaking, if a negative sign is ascribed to RL, the ratio of reflected to incident power is implied; :RL'(\mathrm) = 10 \log_ :where ''RL(dB) is the negative of ''RL''(dB). In practice, the sign ascribed to RL is largely immaterial. If a transmission line includes several discontinuities along its length, the total return loss will be the sum of the RLs caused by each discontinuity, and provided all RLs are given the same sign, no error or ambiguity will result. Whichever convention is used, it will always be understood that ''P''r can never exceed ''P''i .


Electrical

In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. The ratio of the amplitude of the reflected wave ''Vr'' to the amplitude of the incident wave ''Vi'' is known as the
reflection coefficient In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected w ...
\Gamma. :\mathit \Gamma = Return loss is the negative of the magnitude of the reflection coefficient in dB. Since power is proportional to the square of the voltage, return loss is given by, :RL(\mathrm) = -20 \log_ \left, \mathit \Gamma \ where the
vertical Vertical is a geometric term of location which may refer to: * Vertical direction, the direction aligned with the direction of the force of gravity, up or down * Vertical (angles), a pair of angles opposite each other, formed by two intersecting s ...
bars indicate
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
. Thus, a large positive return loss indicates the reflected power is small relative to the incident power, which indicates good impedance match between transmission line and load. If the incident power and the reflected power are expressed in 'absolute' decibel units, (e.g.,
dBm DBM or dbm may refer to: Science and technology * dBm, a unit for power measurement * DBM (computing), family of key-value database engines including dbm, ndbm, gdbm, and Berkeley DB * Database Manager (DBM), a component of 1987's ''Extended Edi ...
), then the return loss in dB can be calculated as the difference between the incident power ''P''i (in absolute decibel units) and the reflected power ''P''r (also in absolute decibel units), :RL(\mathrm) = P_\mathrm i(\mathrm) - P_\mathrm r(\mathrm)\,


Optical

In
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
(particularly in fiber optics) a loss that takes place at discontinuities of
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
, especially at an air-
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
such as a fiber endface. At those interfaces, a fraction of the optical
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
is reflected back toward the source. This reflection phenomenon is also called "
Fresnel reflection The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media. They were deduced by Augustin-Jean Fres ...
loss," or simply "Fresnel loss." Fiber optic transmission systems use
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
s to transmit signals over optical fiber, and a high optical return loss (ORL) can cause the laser to stop transmitting correctly. The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelength-division multiplexing increases. These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser. :\text(\mathrm) = 10 \log_ where \scriptstyle P_\mathrm r is the reflected power and \scriptstyle P_\mathrm i is the incident, or input, power.


See also

* Hybrid balance *
Mismatch loss Mismatch loss in transmission line theory is the amount of power expressed in decibels that will not be available on the output due to impedance mismatches and signal reflections. A transmission line that is properly terminated, that is, terminate ...
*
Signal reflection In telecommunications, 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 ...
*
Time-domain reflectometer A time-domain reflectometer (TDR) is an electronic instrument used to determine the characteristics of electrical lines by observing reflected waveforms. It can be used to characterize and locate faults in metallic cables (for example, twisted pa ...
**
Optical time domain reflectometer An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. It is the optical equivalent of an electronic time domain reflectometer which measures the impedance of the cable or transmissio ...


Notes


References

;Notes ;Bibliography *
Federal Standard 1037C Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms, is a United States Federal Standard issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, a ...
and from
MIL-STD-188 MIL-STD-188 is a series of U.S. military standards relating to telecommunications. Purpose Faced with "past technical deficiencies in telecommunications systems and equipment and software…that were traced to basic inadequacies in the applicat ...

Optical Return Loss Testing—Ensuring High-Quality Transmission
EXFO Application note #044 {{DEFAULTSORT:Return Loss Wave mechanics Radio electronics Engineering ratios Electrical parameters Fiber optics de:Rückflussdämpfung