It is not always necessary for a shield to completely enclose a product in order to be effective. Shields work by reflecting, absorbing or redirecting electric and/or magnetic fields. In many cases, a product in a poorly designed shielded enclosure will radiate more (or be more susceptible) than the same product without the enclosure. an unfiltered cable penetration) can completely eliminate any benefit the enclosure would otherwise provide. Effective enclosures can add significant cost and weight to a product and a single breach of the enclosure (e.g. However, shielded enclosures are a poor substitute for good EMC design at the board level. In other words, even a poorly designed circuit board can meet EMC requirements if it is sealed in a metal box. In fact, a metallic enclosure with no apertures, seams or cable penetrations can typically reduce radiated emissions and improve radiated immunity by 40 dB or more. Shielded enclosures that are properly designed and installed can be a very effective means of attenuating radiated emissions and protecting products from external sources of interference. Computers, cell phones, video games, industrial controls, automotive and avionic systems, etc., all typically come packaged in metal (or metalized) enclosures or have shields located directly over specific components on their printed circuit boards. Virtually all high-speed electronic devices employ shielding in some form. Ask the average engineer on the street about controlling electromagnetic interference and the response will probably involve shielding.
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