An investigation of PCB radiated emissions from simultaneous switching noise

Processors are currently operating with fundamental clock frequencies that are at or above the resonant frequencies of typical processor boards and modules. Adequately decoupling printed-circuit boards (PCBs) at high frequencies has become an increasingly urgent task in the light of increasing clock frequencies with decreasing rise times. Providing sufficient charge at frequencies near and above 1 GHz is extremely difficult with lumped-element capacitors. To further complicate the issue, modern PCB power buses may be analogous to microstrip-patch antennas. Exciting a power bus at board harmonics may result in significant radiated EMI from the bus. Much has been done to improve high-frequency decoupling from a signal-integrity perspective. However, the benefit to EMI is somewhat unclear, because the mechanism by which power-bus noise results in radiated EMI is not well understood. Input impedance of a power bus, transfer impedance across a power bus, and radiated emissions from a PCB are presented herein. The results are discussed to provide characterization of radiated EMI directly from a PCB power bus