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Printed Circuit Board Design and Layout Questions

1. How do I model the effectiveness of shielded enclosures?

There are several different ways to define shielding effectiveness and there are different kinds of shields. Shielded enclosures that attempt to reduce radiated emissions (or susceptibility) by completely encasing a source in a conductive enclosure are generally limited by the size of the seams and apertures in the enclosure as well as the quality of the filtering applied to wires that penetrate the enclosure. Since shielded enclosures tend to enhance the fields inside the enclosure at frequencies corresponding to enclosure resonances, it is not generally sufficient to define shielding effectiveness as the ratio of the field inside the enclosure to the field outside. It is better to compare the radiation from a structure with no enclosure to the radiation from the enclosed structure.

A simple formula for estimating the radiation from PCB/heatsink structures in a shielded enclosure is provided in [1]. Note that electric and magnetic field shields do not necessarily encase the structure being shielded and this analysis does not apply.