Conformity Magazine - July 2008 - (Page 45)

The shielding approach needs to be planned from the beginning. All too often, the problem is such that major structural changes are required in order to resolve the problem, changes that could have been avoided with a little forethought. The Basics of Shielding We need to start with the basics of shielding, no matter how you expect to contain the interference. Shielding is needed to block RF fields, whether to keep internal emissions from escaping the enclosure, or to keep external RF sources from penetrating the enclosure. Figure 1 shows a shielded enclosure with openings and penetrations. For effective shielding, you need to keep the openings small enough to prevent RF from passing through. Shielding effectiveness (SE) is given by: SE = 20*log(λ/2L) Where λ is wavelength and L is the longest dimension of the opening. (Wavelength, λ = 300/f, where λ is in meter and f is in MHz.) As can be seen, SE is zero at L = ½ wavelength or longer. We consider 1/20 wavelength to be a practical maximum opening, giving you a 20 dB shield. From Table 1, we see that at lower frequencies, the 1/20 wavelength criteria is easy to meet, but above 100 MHz, openings become increasingly critical. As a crude rule of thumb, we figure frequencies above ten times the clock frequency to be the approximate upper limit for the 1/20 wavelength criteria. Above that, the emissions and immunity needs are diminishing rapidly. Note that the concern is the longest dimension of the opening; a round 10 cm hole will leak about the same as a 10 cm slot. If you can’t get the slot length small enough, you need to consider using EMI gaskets to close the seams. If you have a ventilator opening, you may need to consider using a conductive screen or perforated metal to reduce the maximum dimension to an acceptable level. The number of holes is not a key parameter; it’s the biggest hole that sets the pace. The second aspect of shielding is the penetrations, namely the data and power lines that penetrate the enclosure. Any interference currents on the wire will freely pass through the enclosure boundary, from either direction. Your solution is to shield the cable or filter the wires in the bundle. Where isolation is required, as would be the case with a patient connected cable, there are severe limitations as to how the cable can be shielded or filtered. Note that this concept is completely general. You are free to select the boundary of the shield, at the entire box, at the circuit board, or at the chip. If the icebox approach isn’t feasible, you will be forced to go inside the box and selectively shield certain functional modules, say, the power Custom Shields & Precision Metal Parts Shield Design Options: Board-level Removable lid Multi-cavity Complex geometry Shielding Materials: Brass Steel Aluminum Titanium Nickel Silver Fotofab works with hundreds of alloys RF Shields & More… Whether you need a multi-cavity shield or a simple five-sided box, Fotofab has a solution for you. Visit our Shielding Design Guide at www.fotofab.com/RF. Have a shield or metal part ready for quote? Submit your specifications and drawing online. Of course, you can always call one of our Technical Sales Staff at (773) 463-6211 to discuss shielding material, plating, design options and cost saving ideas. Precision Metal Parts We work with hundreds of alloys ranging from 0.0002” (0.05mm) to 0.090” (2.29mm) thick. Fotofab offers chemically etched parts, laser cutting, plating, electro polishing, tape & reel packaging and more. Fast Delivery Fotofab understands the importance of a deadline. Our standard lead-time starts at 2 weeks with multiple expedite options available. You can even go from print-to-part as fast as same day! Ask about our 24-hour expedite service! For over 40 years, Fotofab has been manufacturing quality shields and precision metal parts. Would you like a copy of our “Design Guide to Photochemical Machining”? Visit us at www.fotofab.com/RF and we will send you a FREE copy. Prototype or production, think of Fotofab for your shielding needs. 3758 Belmont Avenue phone 773-463-6211 www.fotofab.com/RF Chicago, IL 60618 USA fax 773-463-3387 sales@fotofab.com Innovative solutions - exceptional capabilities JULy 2008 Conformity 5 http://www.fotofab.com/RF http://www.fotofab.com/RF http://www.fotofab.com/RF http://www.fotofab.com/RF

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.