TABLE OF CONTENTS This chapter looks at that classic of EMC design, the shielded enclosure. It is only too easy to say the product s in a metal box, it must be shielded, that ll do . In fact there is much more to designing a shielded product than simply putting it in a metal box. This section explores the theory of ideal shields and the limits that this theory encounters when you look at the performance of real shields, and then the practical design measures that can be taken to deal with these limitations.
Shielding and filtering are complementary practices. There is little point in applying good filtering and circuit design practice to guard against conducted coupling if there is no return path for the filtered currents to take. The shield provides such a return, and also guards against direct field coupling with the internal circuits and conductors. Shielding involves placing a conductive surface around the critical parts of the circuit so that the electromagnetic field which couples to it is attenuated by a combination of reflection and absorption. The shield can be an all-metal enclosure if protection down to low frequencies is needed, but if only high frequency (> 30MHz) protection will be enough then a thin conductive coating deposited on plastic is adequate.
Will a shield be necessary?
Shielding is often an expensive and difficult-to-implement design decision, because many other factors aesthetic, tooling, accessibility work against it. A decision on whether or not to shield should be taken...
