Eliminating EMI Caused by Circuit Board/Chassis Resonances

Printed Circuit Boards (PCBs) are often mounted in close proximity to a metal chassis using metal or plastic posts as illustrated on the right. At high frequencies, the cavity formed between the PCB and the chassis can resonate resulting in elevated levels of radiated emissions. Whether the posts are conductors or insulators, cavity resonances occur. At frequencies near these resonances, small amounts of energy coupled from the PCB to the cavity can result in significant levels of unintended emissions resulting in electromagnetic interference.

These cavity resonances can be a real problem in systems where little attention is paid to the method of mounting circuit boards to the chassis. The resonances can be readily damped however, using simple inexpensive components mounted on the board or the posts. The least expensive solution involves placing a few SMT resistors on the board that electrically connect the board ground to the chassis. The goal of this project was to develop a simple method for determining the optimum value of these resistors in different situations and experimentally validate the effectiveness of this approach.

Publication

X. He, T. Hubing, H. Ke, N. Kobayashi, K. Morishita and T. Harada, "Calculation of optimal ground post resistance for reducing emissions from chassis-mounted printed circuit boards," IEEE Transactions on Electromagnetic Compatibility, vol. 53, no. 2, May 2011, pp. 475-481.