Conformity Magazine - August 2008 - (Page 44)

frequency in megahertz, B = bandwidth factor (in band: B = 1; out of band: B = circuit attenuation), and S = shielding (ratio) protecting circuit. This equation indicates that susceptibility is directly proportional to loop area (A), frequency (F), and the bandwidth factor (B). Frequency (F) is dictated by the specification and the operating environment, as is the field strength (E). Of course, the engineer has no control over 2π, or 300, which is the speed of light divided by 1,000,000 for this equation. From these equations, it is possible to determine some key information. Emission levels are: • Directly related to loop area • Directly related to signal current • A function of frequency squared • Inversely related to shielding effectiveness Susceptibility levels are: • Directly related to loop area • Directly related to bandwidth • Directly related to the transmitted frequency and field strength • Inversely related to shielding effectiveness From the point of view of an EMC engineer, loop area is defined as the length times the separation distance of the conductors on which a signal travels from the time it leaves its source until it returns. In an automotive electronic system, this can be very hard to define, especially if the signal return path is the frame or metal shell of the vehicle. There are two common ways to minimize loop area for circuits in which the signal travels along wires or along other conductors within the vehicle. One method is to place the signal conductor as close as possible to the metal ground of the vehicle. The second is to provide a signalreturn conductor that runs along the signal conductor. Often, good design techniques and the use of filtering devices such as ferrite chip beads and ferrite cable cores can reduce emissions and increase immunity to acceptable levels. With today’s complex automobile designs and systems, it takes a complete EMI control plan to meet the mandatory requirements and obtain the desired operation without EMI issues. Shielding in Automotive Applications Operating at the lowest possible frequency and minimizing the loop area, circuit current, and bandwidth do not always reduce emissions sufficiently. With all of the intentional transmitters now included in automobiles, many of which operate at high frequencies, the potential EMI problems become profound. If the design still cannot meet its requirements, the only parameter left in the equation is shielding. Shielding, which is noninvasive and does not affect high-speed operation, works for both emissions and susceptibility. It can be a standalone solution, but it is more cost-effective when combined with other suppression techniques, such as filtering, grounding, and proper design to minimize the loop area. Shielding can usually be installed after the design is complete. However, it is much more cost-effective and generally more efficient to RF capabilities enable Bluetooth within a vehicle, which is low power, short range communications. Conformity AUGUSt 2008

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

Conformity Magazine - August 2008 Contents Editor’s Note FCC Seeks Comment on Reauction of 700 MHz D Block Commission Cracks Down (Again!) on Junk Fax Marketer Canada Levies Fees for Public Safety Radio Spectrum FDA Issues Guidance on Medical Device Tracking Greenpeace Challenges Video Game Manufacturers The IEEE EMC Society: A Proud History of Accomplishments ESD Open Forum An Update on Changes to the Automotive ESD Standard, ISO 10605 Modeling of Radiated Electromagnetic Disturbances in Automotive Applications Design Issues in Automotive Radio Frequency Systems IEEE EMC Symposium IEEE 2008 Show Preview Exhibitors Buyer’s Guide Product News Most Eco-Friendly Companies Updated Standards List For the EU’s Pressure Equipment Directive Commission Issues Updated Standards List For Directive on Gas-Fired Appliances HHS Releases Fact Sheet on U.S. Imports from China Customs Agents Seize Holiday Lights IEEE PSES Symposium Set for October CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Product Reviews Advertisers

Conformity Magazine - August 2008

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