Substrate Radiation
The electromagnetic behavior that occurs when an RF signal or noise frequency matches the physical dimensions of a void in a PCB reference plane represents a major source of unwanted emissions. This condition, termed slot resonance, turns a slot in the ground or power plane into an unintentional slot antenna. It can occur when traces are routed across splits or gaps in the copper plane, leading to signal distortion and electromagnetic compliance failures.
Physical Mechanism
When a return current traveling along a trace encounters a slot in the reference plane, it cannot cross the gap directly and must flow around it. This deviation increases the inductance of the trace and creates a localized voltage drop across the slot itself. If the length of the slot corresponds to a half-wavelength of the signal frequency or its harmonics, the slot behaves as a resonant structure that radiates electromagnetic energy into the surrounding environment.
Because of this, slot resonance can couple noise into adjacent traces or create radiation that exceeds regulatory emissions limits.
Design Mitigation
Preventing this issue requires keeping reference planes as continuous as possible beneath high-speed routing. If splits in the planes are necessary to separate different voltages, no high-speed traces should be routed across them. If routing across a slot is unavoidable, design guidelines recommend placing stitching capacitors or stitching vias close to the crossing point to provide a low-impedance path for the return current.
These capacitors allow the high-frequency return current to bypass the gap, preventing the slot from radiating and eliminating slot resonance before it can affect circuit performance, while also keeping the differential trace impedance constant.
