Conformity Magazine - July 2008 - (Page 47)

For immunity: • Analog (Op-amps and voltage regulators) for RFI • Digital for ESD and power transients Where is the shield needed? Once you have identified those circuits that need attention, you need to select the shield boundaries. This is not a clear decision, but a little thought will narrow the choices. Each shield needs to accomplish the following: 1. Enclose the selected circuit up to the highest vulnerable frequency, whether emissions or RF immunity. Modules might be a power supply, a microprocessor board, or even a chip on the circuit board. Where noisy and vulnerable circuits exist in close proximity, they may need to be isolated from each other. All lines entering the shield need to be either filtered or shielded. Power and audio frequency data lines can usually be filtered, but high speed data lines will probably need to be shielded. 2. Once you have decided which elements need to be shielded, your shield boundaries would consider which would involve the fewest cables and which are physically the easiest to implement. Designing an Enclosure If you follow the above rules for shielding and filtering, you will have little problem meeting EMC requirements. Realistically, you will probably take some short cuts (not that we advise such an approach!). If we continue with the approach that we have identified and addressed the high risk modules and lines, what do we do with the low risk elements? We can’t cover all possibilities, even if we wrote a book on it. But we can adopt an alternate approach that will minimize the demands placed on the shield Ground In principle, shields don’t need to be grounded. In practice, when using multiple shields in an enclosure, things go much easier if you have a good ground system. This doesn’t involve earth ground, but is basically a local reference we call ground, usually (but not necessarily) connected to the enclosure which, in turn, is often connected to earth ground. But a shielded enclosure doesn’t need to be connected to earth ground to work. Suffice it to say that, for an enclosure, ground is a substantial amount of metal, preferably planar, that circuit modules can be bolted to. The textbook case of a good high frequency ground is a ground plane, which provides a low impedance path between two points. It is hard to overemphasize the merit of a ground plane; at higher frequencies (say, 100 MHz), the impedance of even a short wire length (a couple of centimeters) is easily 1000 times as high as that of a plane. But we don’t have to have a flat plane to achieve a low impedance ground system. You can bend it like an L, splice it like a T, shape it to meet form factor requirements. You can cut holes to allow wire harnesses or mechanical members to pass. If you have to splice several metal members, make sure the mating surfaces are conductive, and use lots http://www.nec-tokin.com/english/ http://www.nec-tokin.com/english/

Table of Contents Feed for the Digital Edition of Conformity Magazine - July 2008

Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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