EMI Troubleshooting: The Need for Close Field Probes
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Close field electromagnetic interference (EMI) tests are a valuable tool in electromagnetic compatibility (EMC) radiated emission pre-compliance testing. Close field testing is performed at short distances. Far Field testing is performed at distances between 3 to 10 meters at test facilities using EMI receivers and antennas. The nature of the electromagnetic field is determined by the device under test (DUT) and the distance of the receivers and antennas from the DUT. A far field radiated emission measurement accurately determines if the DUT is compliant to related EMC/EMI standards.
However, far field testing has limitations. It cannot identify the source of emissions. Radiated emissions may come from a USB port, a LAN port, the seam of a shield, a cable, or even a power cord. Close field testing is the only way to locate the exact source of the emission and is typically performed using a signal analyzer and close field probe.