Conformity Magazine - November 2007 - (Page 50)

may take into account emission limit levels defined by regulatory bodies (e.g., FCC), or environmental effects such as electromagnetic pulse (EMP) and lightning, or intentional sources such as wireless systems, radar etc. The EMC budget must also make an estimate of emissions from internal components and provide margin of error protection, typically between 10 dB and 30 dB. The shielding effectiveness required is usually plotted against frequency and can be easily compared to the shielding effectiveness of the enclosure as determined in physical testing. In a similar way, the engineers responsible for the thermal side of the design usually begin by determining a thermal budget to guide the fan sizing and bulk airflow necessary to cool the chassis. A relatively rough thermal model of the entire system is created. To cut down on solution time, the model contains minimal detail. For example, two-resistor compact models of the individual components are often used. At this stage, engineers pay special attention to the airflow over the motherboard, which is something that can’t be determined by simple rule of thumb calculations. The type of analysis that is usually performed at this stage is illustrated by the fan-sizing chart shown below (see Table 1). Engineers considered three vent/fan combinations and selected the one with large vent holes and a low capacity fan because it excelled in meeting both thermal and cost considerations. Calculating Shielding Effectiveness The new approach utilizes EMC simulation to evaluate the enclosure shielding effectiveness as a function of frequency. The purpose of this simulation is to identify the areas that are limiting the shielding effectiveness, and to explore design improvements. Figure 3: Shielding effectiveness of the 2U server calculated 3m from the enclosure under test. Blue horizontal line shows the shielding effectiveness of the unit with two conductive gaskets. Black horizontal line shows the emission budget. Table 1: Fan Sizing Comparison—Engineers considered three vent and fan combinations and selected the middle one because it met thermal and cost considerations. The simulation can be arranged to mimic a shielding effectiveness test in a lab, where the enclosure is rotated on a turntable and the peak emissions measured by a receive antenna located 3 or 10 meters away. The advantage of simulation is that it determines not just the level of radiation but the path by which radiation escapes from the enclosure. In this example, the simulation showed that most of the emissions escape through two long seams between the enclosure base and lid. Placing gaskets along these particular seams increased the shielding effectiveness to well above that required by the EMC budget at a much lower cost than would have been required to gasket the entire enclosure. The next stage of thermal management typically involves refining the simulation model in order to take advantage of the fact that the parts list is usually solidifying and to provide the detailed information needed at this stage of the design process. More components are added to the simulation, and compact models used in the model up to now are often replaced by more detailed and accurate models. Figure 4: The color contours show the electric field distribution at 900 MHz. A resonance occurs inside the enclosure in the space between the power supply and fan tray. The seam at the top of the air vent panel is radiating. 50 Conformity noVEmBEr 2007

Table of Contents Feed for the Digital Edition of Conformity Magazine - November 2007

Conformity - November 2007 Contents Editor’s Note Commission Sets Date, Terms for 700 MHz Auction FCC Reaffirms Roaming Obligations Commission Moves to Improve USF Management FCC Proposes Fines for Violations of E911 Handset Requirements Commission Levies $1 Million Plus Fines for Unsolicited Fax Advertisements GR-3150 - Lithium Battery Certification Levels Based on Criteria for General Product, Safety, and Performance, Part 2 ESD Open Forum Machinery Field Evaluation for the United States Validation, Verification, and Immunity Testing Techniques for EMC for Functional Safety Thermal/EMC Simulation Addresses New Mechanical Design Challenges Product Safety Equipment Buyer’s Guide FCC Releases Report on Telephone Numbering Resource Utilization Canada Still Has PCS Spectrum to Assign Updated Standards List for the EU’s Medical Devices Directive Updated Standards List for EU Directive on Active Implantable Medical Devices Call for Papers for the 2008 EOS/ESD Symposium Announced Mobile Phone Use in Hospitals May Be Dangerous CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Product Reviews Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - November 2007

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