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 PT and the power received by the load after insertion is PR, then the insertion loss in decibels is given by,


 * $$IL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm T \over P_\mathrm R}$$

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 ($$\left\vert V_1 \right\vert$$) to the signal level with the filter installed ($$\left\vert V_2 \right\vert$$). This ratio is described in decibels by the following equation:


 * $$\mbox{Insertion loss (dB)} = 10 \log_{10} {\frac{ \left\vert V_1 \right\vert ^2 }{ \left\vert V_2 \right\vert ^2 }} = 20 \log_{10} { \frac{\left\vert V_1 \right\vert}{\left\vert V_2 \right\vert}}$$

For passive filters, $$\left\vert V_2 \right\vert$$ will be smaller than $$\left\vert V_1 \right\vert$$. 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 ($$IL$$) is defined as:


 * $$ IL = -20\log_{10}\left|S_{21}\right| \,\text{dB}$$.

Here $$S_{21}$$ 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.