Conformity Magazine - March 2008 - (Page 52)

vertical electric field component can couple easily across the aperture. The SE can be found from the ratio of the field strengths without and within the enclosure, as follows: Due to the highly resonant behavior of the box, and in order to speed up the simulation time, a very small value of loss is distributed throughout the solution space by artificially assigning a conductivity (σ=0.002 S/m) to the free space cells of the calculation domain. In [10], it has been already demonstrated how this artifact has practically no influence of the far field calculated results. Figure 4 illustrates a similar approach (as just described), invoking [4] to calculate the near field distribution due to a complex multilayer PCB, and imported as radiation source within [5] to determine the farfield data (left picture). Effect of Slots and Apertures The effect of apertures’ shape and configuration on the value of the radiated electric field and the related shielding effectiveness (SE) is another important aspect to be analyzed. For instance, in Figure 5 it is shown that dividing a specified area into a combination of multiple apertures may reduce the value of the radiated emissions. The critical aspect of this kind of analysis is represented by the high number of details and high aspect ratio due to the possible presence of multiple slots, ventilation holes, honeycomb panels, etc. Some software tools prefer to adopt specific meshing techniques in order to be able to mesh the complex shape of enclosure, while other tools have a build-in library which allow for the easy characterization of slots and apertures without physically meshing them, therefore saving memory requirement. One of the final goals for EMC-EMI analysis is represented by the matching of specific constrains determined by normative regulations; for this reason it is important to be able to perform this check within the same tool used to detect the possible EMI-EMC source problems. Figure 6 illustrates the SE for a metallic enclosure in the frequency range of 0-1GHz and the comparison with FCC regulations (Class A and B) at 3 meters and 10 meters. In this specific case, the design corresponded to Figure 6a would not meet the regulations for the Class B. It is straightforward to point out that the possibility of predicting this behavior a-priori (before any prototyping activity) will save time and money and speed up the design process, thereby also reducing the time-to market of the product. The apertures located in the front panel of a metallic enclosure can be covered by honeycomb panels, as shown in Figure 7. For the left panel, circular holes of 2mm diameter and 1mm distant from each other are employed, while for the right panel circular holes of 4mm diameter and 2mm distant each other are modelled. The calculated results are reported in the same figure (left side) where the comparison with case of regular aperture (Figure 5, top picture, left side) is presented. A sensible improvement (of more than 100dB) can be observed Figure 7: Model of enclosure with honeycomb panel and SE results Figure 8: a): Model of a complex enclosure, b): measurement set-up inside semi-anechoic chamber 52 Conformity marCh 2008

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

Conformity Magazine - March 2008 NewsBreaks Editor's Note Adelstein Renominated to the FCC Industry Canada Publishes FAQs On RF Fields FDA Gives Its Scientific Capabilities Poor Marks FDA Offers Guidance On its Product Review Process FDA Publishes Updates to List of Recognized Standards FDA Warns of Patient Risk From Electric Dental Handpieces Complying with the EMC Directive, Second Edition ESD Open Forum Emerging Issues in Standards - Korean "RoHS": Not Like RoHS at All The iNARTE Informer Challenges in Testing - Field-Induced Charged Device Model: What Really Happens Design Fundamentals - EMI Shielding Technologies for Consumer Electronics Equipment Focus On...Compliance Software Buyer's Guide Additional Standards Published for the EU's ATEX Directive Additional Standards for the EU's Medical Devices Directive Also Published China to Require Bar Codes to Enhance Product Safety CPSC Releases 2007 Self-Assessment CPSC Actions in News 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 - March 2008

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