How to locate the I/O module in EMC near-field testing?

Near-field testing is key to efficiently addressing EMC issues in I/O modules. A near-field probe kit, including magnetic and electric field probes, is used. First, the out-of-range frequency is identified through conducted or radiated emission testing. Then, with the module powered on, the near-field probes are used to scan the PCB surface, chips, cables, and connectors at close range. The intensity changes of the signal at that frequency are observed on a spectrum analyzer; the location of the strongest signal is the primary radiation source or coupling path. Magnetic field probes are sensitive to current loops and are suitable for locating switching power supply loops or clock traces; electric field probes are sensitive to voltage changes and are suitable for locating chip pins or unshielded conductors. For immunity issues, noise coupling at internal sensitive nodes can be monitored while interference is applied to identify weak points. Combined with an oscilloscope, time-domain waveforms can be observed. Once the problem area is located, targeted measures can be taken, such as adding shielding to chips, adding filtering to traces, or modifying grounding methods. Near-field testing does not require an anechoic chamber and can be performed quickly in a research and development laboratory, making it a powerful tool for EMC design and remediation.