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,

## 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:

For passive filters, |*V*_{2}| will be smaller than |*V*_{1}|. 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 () is defined as:^{[1]}^{[2]}

- .

Here 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

## References

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