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:
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.
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