EMI, EMC, and Shielding Advanced EMC Topics Informational

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

Q: How do I correlate near-field results with the PCB layout?

Overlay the emission map on a photograph or CAD image of the PCB. Most automated scanners provide this overlay automatically with registration markers. Identify the hot spots and trace them to specific components or traces in the layout. Common findings: a particular IC pin (e.g., clock output or switching node), a specific trace (e.g., high-speed data bus), or a ground return path (e.g., a gap in the ground plane that forces current through a slot antenna).

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.

Category: EMI, EMC, and Shielding

Updated: April 2026

Product Tie-In: Shielding, Gaskets, Absorbers, Filters

Near-Field EMI Scanning on PCBs

Near-field scanning is the most effective diagnostic technique for identifying EMI sources on a PCB after a product fails regulatory emissions testing. It pinpoints the exact source of the problem, allowing targeted fixes rather than trial-and-error troubleshooting.

Probe Types

Magnetic field (H-field) probes: Small loop antennas (1-10 mm diameter). Sensitive to current flow (proportional to di/dt). Best for finding: current-carrying traces, switching regulators, clock distribution networks, and IC output drivers. A shielded loop probe provides better spatial resolution and rejects electric field pickup
Electric field (E-field) probes: Small monopole or stub antenna (1-5 mm length). Sensitive to voltage (proportional to dV/dt). Best for finding: high-voltage nodes, crystal oscillators, and radiating slots or apertures
Automated scanners: Commercial near-field scanning systems (Detectus, Amber, Keysight N6141) provide computer-controlled XY positioning with probe heights from 0.5-50 mm and scan areas up to 500x500 mm. They produce calibrated emission maps overlaid on the PCB layout image

Near-Field Scanning Parameters

Magnetic probe sensitivity: V_out = -jωμ₀ × A_loop × H at probe location
Resolution: limited by probe diameter d (approximately d/2 spatial resolution)
For 2 mm probe at 2 mm height: resolution approximately 2-3 mm
Dynamic range: typically 40-60 dB (limited by probe sensitivity and noise floor)
Scan time: (area / step^2) × dwell_time (typically 5-60 minutes per frequency)

Common Questions

Frequently Asked Questions

Can near-field scanning replace far-field EMC testing?

No. Near-field scanning identifies the sources of emissions on the PCB, but it cannot directly predict the far-field emissions that would be measured in a regulatory test (3 m or 10 m distance). The relationship between near-field and far-field is complex (depending on the entire system: cables, enclosure, and grounding). However, near-field scanning is invaluable for: diagnosing failures, guiding design fixes, and comparing before/after modifications. It is a complement to, not a replacement for, far-field testing.

What frequency range is useful for near-field scanning?

The useful range depends on the EMI problem: conducted emissions (150 kHz - 30 MHz): near-field scanning identifies the noise sources on the PCB that are coupling to the power lines. Radiated emissions (30 MHz - 1 GHz): most PCB-level EMI sources operate in this range (clocks, data buses, switching regulators). Above 1 GHz: near-field scanning is useful for identifying on-chip or IC-level emissions, but requires very small probes (< 1 mm) for adequate resolution.

How do I correlate near-field results with the PCB layout?

Overlay the emission map on a photograph or CAD image of the PCB. Most automated scanners provide this overlay automatically with registration markers. Identify the hot spots and trace them to specific components or traces in the layout. Common findings: a particular IC pin (e.g., clock output or switching node), a specific trace (e.g., high-speed data bus), or a ground return path (e.g., a gap in the ground plane that forces current through a slot antenna).

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