CE101 calls for using a current probe to measure low frequency RF emissions with testing on both the phase and neutral (positive and negative) power leads. If emissions are measured at a level that indicates resolution is needed, the designer may need to have insight on the emission mode. Because the measurements are accomplished on each line separately, the mode is not obvious. However a quick check with both power leads in the current probe will provide a good indication on the mode.
If the measurements with both leads show a significant reduction, then differential (normal mode) emissions are present. Since normal current enters on one line and returns on the other with a 180-degree phase shift the current will cancel, showing the reduction. If the emission measurements do not change with both leads together, then common mode emissions are indicated.
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Posted in: Did You Know? MIL-STD-461G