Insertion loss

In telecommunications, insertion loss is the decrease in transmitted signal power resulting from the insertion of a device in a transmission line or optical fiber. It is usually expressed relative to the signal power delivered to that same part before insertion. Insertion loss is usually expressed in decibels (dB).

The insertion loss takes account of the signal reflection introduced by the device itself. It may also be referred to as attenuation, meaning that it reduces the transmitted signal amplitude.Line terminations play an important part in insertion loss because they reflect some of the power. Apart from this it is clear that not all of the power which is sent into the line at one end appears at the other. This is because of radiation losses, resistive losses in the conductor as well as losses in the surrounding dielectric. All of these effects can be conceptually modelled as various elements which make up the equivalent circuit of the line (see for example distributed and lumped element models). The loss which results from inserting a transmission line between a source and a load is called the insertion loss of the line.

If the power transmitted by the source is "P_T" and the power received by the load is "P_R", then the insertion loss is given by "P_R" divided by "P_T". For maximum power transfer the insertion loss should be as small as possible. In other words, the ratio "P_R"/"P_T" should be as close to 1 as possible, which in decibels means as close to 0dB as possible.

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

Insertion Loss Testing

Filters are generally specified with insertion loss performance data. 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 generally described in db according to the following equation:

Insertion loss (dB) = 20 log (V1/V2)

Filters are sensitive to source and load impedances so the exact performance of a filter in a circuit is impossible to precisely predict. Comparisons, however, of filter performance are possible if the insertion loss measurements are made with fixed source and load impedances. 50 ohms is the universally accepted measurement impedance. This data is currently specified as common-mode or differential-mode. Common-mode simply put, is a measure of the filter performance on signals that originate between the power lines and chassis ground. Differential-mode is a measure of the filter performance on signals that originate between the "hot" and neutral power lines. Common-mode insertion loss is measured by connecting the line and neutral terminals together and using the test configuration of figure 1.

Differential-mode insertion loss is measured by using the 180 degree power splitters and using the test configuration of figure 2.

References

*Federal Standard 1037C