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 insertion is P_T and the power received by the load after insertion is P_R, then the insertion loss in decibels is given by,

${\displaystyle IL(\mathrm {dB} )=10\log _{10}{P_{\mathrm {T} } \over P_{\mathrm {R} }}}$

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 (${\displaystyle \left\vert V_{1}\right\vert }$) to the signal level with the filter installed (${\displaystyle \left\vert V_{2}\right\vert }$). This ratio is described in decibels by the following equation:

${\displaystyle {\mbox{Insertion loss (dB)}}=10\log _{10}{\frac {\left\vert V_{1}\right\vert ^{2}}{\left\vert V_{2}\right\vert ^{2}}}=20\log _{10}{\frac {\left\vert V_{1}\right\vert }{\left\vert V_{2}\right\vert }}}$

For passive filters, ${\displaystyle \left\vert V_{2}\right\vert }$ will be smaller than ${\displaystyle \left\vert V_{1}\right\vert }$. In this case, the insertion loss is positive and measures how much smaller the signal is after adding the filter.

Link with scattering parameters[]

In case the two measurement ports use the same reference impedance, the insertion loss (${\displaystyle IL}$) is defined as:^[1][2]

${\displaystyle IL=-20\log _{10}\left|S_{21}\right|\,{\text{dB}}}$.

Here ${\displaystyle S_{21}}$ is one of the scattering parameters. Insertion loss is the extra loss produced by the introduction of the DUT between the 2 reference planes of the measurement. The extra loss can be introduced by intrinsic loss in the DUT and/or mismatch. In case of extra loss the insertion loss is defined to be positive.

See also[]

• Mismatch loss
• Return loss

References[]

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