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.
Standing Wave Modes
Resonance Pattern
Resonant electromagnetic distributions form when high frequency wave reflections from the boundaries of conductive structures interfere constructively. These standing wave modes occur within the parallel-plate waveguide created by the power and ground plane layers of a printed circuit board. They generate localized voltage peaks and nulls across the board area at specific frequencies that depend on the board dimensions.
The resonance is constrained by the physical edges of the planes and the dielectric constant of the substrate.
Power Distribution
The presence of standing wave modes degrades the effectiveness of the power distribution network by creating high impedance regions at resonant frequencies. If a switching integrated circuit is located at a voltage peak, the local supply voltage can fluctuate wildly and cause logic errors or timing failures. The placement of decoupling capacitors must be optimized to suppress these resonances by loading the plane at the voltage peaks.
This optimization prevents the generation of high frequency noise that would otherwise couple into signal traces or radiate from the board edges.
Simulation Method
Identifying these resonant regions requires three-dimensional electromagnetic simulation of the board plane geometry. This analysis identifies the critical frequencies and guides the placement of decoupling components before fabrication.