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 dB is given by,
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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 (V_{1}) to the signal level with the filter installed (V_{2}). This ratio is described in dB by the following equation:
Link with Scattering parameters
In case the two measurement ports use the same reference impedance, the insertion loss (IL) is defined as^{[1]}^{[2]}:
and not, as often mistakenly thought, by:
dB.
It is the extra loss produced by the introduction of the DUT between the 2 reference planes of the measurement. Notice that 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.
For more detail, see the main article on Scattering Parameters.
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