How decoupling capacitors may cause radiated EMI

This paper analyzes effects that decoupling capacitor(s) may have on the development of radiated emissions should improper implementation on a printed circuit board (PCB) occur. This applied EMC paper is based on real-world experience and contains an easy solution to help engineers achieve compliance quickly and at low cost, without having to redesign the PCB. Improper implementation of decoupling capacitor(s) includes: routing traces on the outer layers or using boards with or without power and return planes; excessive inductance in the decoupling loop area; lack of charge storage to replenish the power and return planes; and self-resonant frequency of the capacitor outside the harmonic spectrum of periodic signals. We analyze the magnitude of radiated electromagnetic interference (EMI) based on physical placement of decoupling capacitors from digital components and whether distance spacing from a switching element, connected by routed traces on both the top and bottom of a PCB, makes a significant difference in the development and propagation of radiated emissions. The focus is on what occurs on a PCB, regardless of whether the board is single-sided, double-sided or multilayer.