Minimizing EMI caused by radially propagating waves inside high speed digital logic PCBs

One of the primary modes of radiation from printed circuit boards (PCBs) are due to emissions along the edges of printed circuit boards. A variety of edge treatment techniques are used to minimize these two effects including adding a string of vias around the periphery of the PCB to short out ground planes (fencing), totally shielding the printed circuit board between two outer ground-planes and edge plating, and pulling back power planes from the edges by a factor of up to 20 times the plane-to-plane separation distance (20 H rule) to minimize PCB resonance effects. This paper describes the underlying mechanism of how radiation from PCB edges actually takes place, and then shows how these techniques impact the EMC aspects of printed circuit board edge structures.