What is the near field scanning technique for locating EMI sources on a PCB?

The near-field scanning technique for locating EMI sources on a PCB uses a small magnetic or electric field probe scanned across the surface of the board to create a spatial map of the electromagnetic emissions, identifying the specific traces, components, or areas that are generating the most interference. The technique involves: using a near-field probe (a small loop probe, typically 1-10 mm diameter, measures the magnetic field H and identifies current-carrying traces and components; a small monopole or dipole probe measures the electric field E and identifies voltage sources and radiating structures), scanning the probe across the PCB surface at a fixed height (typically 1-5 mm above the components) while recording the received signal level at each position using a spectrum analyzer or EMI receiver, building a 2D map of emission intensity at the frequency of interest (the map shows hot spots that correspond to the EMI sources), and correlating the hot spots with the PCB layout (the identified sources may be: switching regulator inductors, high-speed clock oscillators, high-di/dt traces, connector interfaces, or poorly bypassed power pins). The measurement can be performed manually (moving the probe by hand while recording) or with an automated scanning system (an XY positioner moves the probe in a raster pattern while a computer records and processes the data). Resolution is limited by: the probe size (smaller probe = higher spatial resolution but lower sensitivity), the scan height (closer = better resolution), and the scan step size.