Power Distribution Plane Cavity Resonance and Radiated Emissions
Power distribution plane cavity resonances create high impedance peaks that drive edge fringing fields, forcing radiated emissions beyond CISPR 32 limits.
Near Field Magnetic Probe
Emission Sensing
A diagnostic sensor detects localized alternating magnetic fields radiating from printed circuit board traces, components and plane edges. This tool, known as a near field magnetic probe, consists of a small shielded loop of wire that produces a voltage proportional to the rate of change of the magnetic flux passing through it. It operates in the immediate vicinity of the circuit board surface to identify the specific traces or components responsible for electromagnetic interference.
This detection capability is limited to the reactive near field region of the radiating source.
Spatial Resolution
The physical dimension of the loop determines the balance between sensitivity to weak fields and the ability to isolate closely spaced emission sources on the board. A smaller loop of a near field magnetic probe provides high spatial resolution to pinpoint individual pins or narrow traces but outputs a lower signal level. Conversely, a larger loop integrates the field over a wider area to detect overall board emission trends but cannot isolate the specific source.
Engineers select the loop size based on the frequency of interest and the trace spacing of the layout under evaluation.
Scan Procedure
Manual scans are performed by sweeping the probe across the circuit board surface while monitoring the output on a spectrum analyzer. This quick test reveals the hot spots of electromagnetic activity.