Insertion loss: Definition from Answers.com

In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber. Usually expressed as a ratio in dB relative to the transmitted signal power, it can also be referred to as attenuation.

If the power transmitted by the source is P_T and the power received by the load is P_R, then the insertion loss in dB is given by

$10 \log_{10} {P_R \over P_T}$

In metallic conductor systems, radiation losses, resistive losses in the conductor as well as losses in the surrounding dielectric all reduce the power. Line terminations play an important part in insertion loss because they reflect some of the power. All of these effects can be conceptually modelled as various elements which make up the equivalent circuit of the line (see distributed and lumped element models).

In an optical fiber system, insertion loss is introduced by things such as connectors, splices, and couplers.

1 Electronic filters
2 Link with Scattering parameters
3 See also
4 References

Electronic filters

Insertion loss is a figure of merit for an electronic filter and this data is generally specified with a filter. Insertion loss is defined as a ratio of the signal level in a test configuration without the filter installed (V₁) to the signal level with the filter installed (V₂). This ratio is described in dB by the following equation:

$\mbox{Insertion loss (dB)} = 10 \log_{10} {V_1^2 \over V_2^2} = 20 \log_{10} {V_1 \over V_2}$

Filters are sensitive to source and load impedances so the exact performance of a filter in a circuit is difficult to precisely predict. Comparisons, however, of filter performance are possible if the insertion loss measurements are made with fixed source and load impedances and 50 Ω is the typical impedance to do this with. This data is specified as common-mode or differential-mode. Common-mode is a measure of the filter performance on signals that originate between the power lines and chassis ground, whereas differential-mode is a measure of the filter performance on signals that originate between the two power lines.

Link with Scattering parameters

Assuming the impedances are matched and real, Insertion Loss ( $IL\,$ ) is as follows:

$IL = 10\log_{10}\left|S_{21}\right|^2 \,\text{dB}$

or alternatively

$IL= 10\log_{10} [1- \left|S_{11}\right|^2] \,\text{dB}$

the previous equation would result in a negative number, however it is more convenient to drop the minus sign as it is indicated by term loss in "Insertion Loss".

References

This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

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Insertion loss

Contents

Electronic filters

Link with Scattering parameters

See also

References