3.13 Radiation from Board Edges

The energy stored between conducting planes is in parallel with the energy stored in decoupling capacitors. When a demand for energy is made, waves propagate into both the decoupling capacitors and into the space between the planes. Reflections return energy to the point of demand. This wave action was discussed in Section 2.4. Consider waves that travel between ground and power planes. When a wave reaches the edge of the board, the open circuit reflects the wave and doubles the voltage. Wave fronts reach the board edge at different times. Waves double in amplitude as they reflect. The wave amplitude is inversely proportional to the distance from the point of demand. If the wave amplitude is 1 mV at the board edge and the planes have a 2 mm spacing, then the E field strength is 2 mV/2 mm or 1 V/m. This E field level would be measurable.

In multilayer boards, each ground/power plane stores field energy. Energy is also stored in parallel decoupling capacitors. Every logic transition requires field energy. This energy comes from the nearest sources, which includes decoupling capacitors, ground/power plane capacitances, and connected logic. When logic crosses layers, it carries field energy to a new region. This energy must eventually be radiated or dissipated. Field energy cannot travel through conductors. It can only travel through holes in conductors (vias) or it can leave at the board edge. When logic lines are terminated, most of the energy used for driving the transmission lines is dissipated in the terminations. For unterminated lines, the field energy must be lost in trace and switching resistances and in radiation. The path that supplies the transmission line energy is the same path that is used to dissipate the energy (see the discussion in Section 4.16).