This paper presents a new decoupling circuit for suppressing radiated emissions due to power plane resonance in multilayer printed circuit boards (PCBs). This circuit is based on transmission line theory, and consists of two decoupling capacitors and one power trace. The two capacitors, one mounted on the power pin of an IC and the other mounted on the common power distribution bus in a board, are connected through the power trace. The characteristic impedance of the trace is much higher than the impedance of the capacitors. In addition, the length of the trace between the capacitors is less than 1/4 the effective wavelength for high frequency (e.g., 1 GHz). Tests we performed on simple PCBs confirm that our decoupling circuit suppresses radiated emissions due to power plane resonance.

This paper analyzes mechanisms of radiated emissions from multilayer printed circuit boards (PCBs) and presents a model to describe the characteristics of such radiation. The radiation mechanism from a four-layer PCB, including the internal power and ground planes, is investigated using a time-domain magnetic field measurement near the PCB. Measurement of the waveform indicates that the main source of radiation is in the power distribution planes. To investigate the characteristics of the radiation from the power distribution, the S21s of the board are measured; the board impedance and the transmission characteristics of the power distribution planes are found to be directly related to the S21 between the two points in the board. The results indicate that the power distribution acts as a transmission line at frequencies higher than 100 MHz. A model that can explain well the radiation properties of these planes treats them as a parallel-plate transmission line interconnected by decoupling circuit comprising a decoupling capacitor and interconnect inductance. From the transmission line theory it is deduced that the line resonance gives rise to strong radiated emissions. The interconnect inductance is an important factor in determining the radiation characteristics.

This paper describes the mechanisms of power-distribution-plane resonance in multilayer printed circuit boards and the techniques to control the resonance. The power-distribution-plane resonance is responsible for high-level emissions and circuit malfunctions. Controlling the resonance is an effective technique, so adequate characterization of the resonance is necessary to achieve control. The resonance characteristics of four-layer printed circuit boards are investigated experimentally and theoretically by treating the power-distribution planes as a parallel-plate transmission line with decoupling circuits. Analysis of the forward traveling wave shows that the resonance frequency is determined by the phase delay due to wave propagation and by the phase progress of interconnect inductance in the decoupling circuit. Techniques to control the resonance characteristics are investigated. The resonance can be shifted to a higher frequency by adding several decoupling circuits adjacent to the existing decoupling capacitor or by increasing the number of via holes connecting the capacitor mounting pads to the power-distribution planes.