Whitepaper
EMI Troubleshooting: The Need for Close Field Probes
This Keysight Technologies white paper addresses the practical challenges of electromagnetic interference (EMI) troubleshooting and explains how close field probes provide an efficient, targeted solution. The document centers on the Keysight N9311X-100 close field probe set, a four-probe system covering 30 MHz to 3 GHz, designed to help engineers quickly locate and diagnose EMI emissions at the component and circuit-board level before full compliance testing. Each probe in the set serves a distinct purpose: the largest (25 mm diameter) offers maximum sensitivity for measurements up to 10 cm from a unit; a medium probe (10 mm) balances sensitivity and resolution for measurements up to 3 cm; a fine-tip probe (2 mm) targets magnetic fields emitting vertically from flat PCB surfaces and can reach obstructed areas; and a specialized probe (5 mm) detects surface and circular magnetic fields on conductive paths, connectors, and cables. When combined with EMI pre-compliance software and a signal analyzer, the probe set delivers sensitivity, resolution, and spatial diversity that make it a cost-effective tool for isolating interference sources early in the design cycle.