Analysis of the electromagnetic radiation behaviour of motherboard-subboard structures

Due to its antenna nature, subboard-on-motherboard structures act as unintentional radiators of electromagnetic fields. The magnitude of the electromagnetic emissions can exceed even the CISPR 22 limits, when high-speed signals are routed through the interboard connector. A worst-case analysis of the maximum field strength radiated from two mutually-parallel motherboard-subboard PCBs is presented in this paper. A detailed investigation of the radiation behaviour of motherboard-subboard structures leads to the conclusion that with a suitable pin configuration of the interboard connector the field emission can be considerably reduced. It is shown that further field suppression can be achieved, if grounding posts are introduced into the system design.