EMC and Loop Inductances on Printed Wiring Boards

Wiring loop inductances on printed wiring boards (PWB's) have commonly been suspected of being the primary cause of radiation and susceptibility to interference of equipment with electronic circuitry. It is a fact that, when loop areas are reduced, probleme mostly disappear. Results from calculations however, indicate that loop radiation and loop induction from interference fields are frequently far too low to explain the EMC problems found in practice. It will be shown that radiation or interference pick-up is mainly caused by wires which connect the PWB to the outside world. This (inter)equipment wiring acts as a transmitting antenna that is excited by PWB current loops (radiation) and behaves as a receiving antenna that picks up interference and excites signal loops (susceptibility). In practical situations, this (inter)-equipment wiring may, under unfavourable conditions, increase radiation and susceptibility of equipment by more than 40 dB.