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The Clemson Vehicular Electronics Laboratory has a great little tutorial on electromagnetic shielding. The intro reads:
Shielded enclosures generally surround a product's circuitry on all sides. Care is taken to ensure that all apertures are small and every seam is sealed. On the other hand, it is not uncommon to find shields that don't surround the entire product. Often a shield partially covers only a few circuits. These shields may be penetrated by unfiltered wires and sometimes consist of a single plate of metal that may or may not be connected to ground.
Why are apertures and seams so important in some applications and completely irrelevant in others? The answer relates to the fact that there are different kinds of shielding for different applications. In the crosstalk notes, we saw that whether a cable shield needed to be grounded on one or both ends depended on the type of crosstalk we were trying to prevent. Similarly, circuit and system shielding requirements depend on the application.
It is convenient to divide shielding requirements into 3 categories: electric-field shielding, magnetic field shielding and shielded enclosures. Shielding strategies depend on the type of coupling we are trying to prevent.
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