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.
Plane Impedance
Electrical Characteristic
Total electrical opposition encountered by currents flowing through the power and ground layers of a printed circuit board determines the stability of the voltage supply. Effective distribution of power requires maintaining low values of plane impedance across a wide range of frequencies. The physical dimensions of the planes and the dielectric constant of the material between them form a parallel plate capacitor.
This structure provides a path for high frequency noise to return to the source.
Frequency Dependence
At low frequencies, the resistance of the copper and the inductance of the planes dominate the electrical behavior. As frequency increases, the capacitance of the plane pair provides the primary path for current. High frequency peaks emerge when the physical dimensions of the board lead to electromagnetic standing waves within the dielectric space.
Resonance effects can significantly alter the performance of the power distribution network.
Design Control
Reducing the dielectric thickness between power and ground layers increases the plane capacitance and lowers the overall plane impedance. Strategic placement of vias and the avoidance of large cutouts in the copper ensure that current paths remain direct. Minimizing this value across the operating spectrum of the digital circuits prevents voltage drops and logic failures.