Power Plane Eddy Current Attenuation and Stitching via Parasitics in High Speed Substrates
Stitching via arrays and eddy current losses damp cavity resonances to suppress power plane noise in high-speed substrates.
Plane Wave Impedance
Wave Impedance
Electromagnetic field propagation relationships express the ratio of electric field strength to magnetic field strength in a propagating electromagnetic wave. Compatibility engineers measure plane wave impedance during shielding effectiveness testing of board enclosures and electromagnetic containment structures. This wave ratio excludes characteristic trace impedance within physical printed circuit board conductors.
Field Ratio
Far-field electromagnetic radiation emitted from switching circuits transitions into a plane wave where electric and magnetic field vectors remain orthogonal to each other and to the direction of wave propagation. In free space, plane wave impedance equals approximately three hundred seventy-seven ohms, defined by the ratio of free space permeability to permittivity. When plane waves encounter metallic shielding barriers on a circuit board, impedance boundaries cause partial wave reflection and absorption attenuation.
High conductivity metals like copper provide excellent reflection loss against high impedance electric fields, whereas high permeability materials absorb low impedance magnetic fields. Field probes quantify wave impedance variations near board aperture seams to identify electromagnetic leakage paths. Absorber material placement inside enclosure walls reduces internal standing wave resonances caused by impedance boundaries.
Shielding Limit
Wave impedance determines the reflection and absorption performance of shielding materials. Improper shield enclosure design permits field leakage and electromagnetic compliance failure. Measuring wave ratios validates shielding effectiveness across high frequency operating bands.