Conformity 2008 Engineers' Reference Guide

EMC Testing Using Field Probes as EMI Diagnostic Tools by Scott Roleson inding solutions to nettlesome electromagnetic interference (EMI) problems in electronic devices is sometimes like buying a jigsaw puzzle at a garage or lawn sale, that is, there are probably some pieces missing, and maybe some irrelevant pieces from another puzzle mixed in. Nevertheless, the goal is still to assemble enough pieces to “get the picture.” EMI work is like that because the goal is usually to build and sell some electronic product, where EMI is a side show. There may simply not be the time or money available to develop a detailed or complete understanding of all EMI problems, so long as your product passes its final regulatory tests. In these situations, speed is more important than precision, and a qualitative understanding of the EMI phenomena may have to suffice. This is not to say that one should have a single-minded pursuit of the quickest “fix” that seems to solve an EMI problem. Quick “fixes,” where the reason why it works is not clearly understood, have the nasty tendency to sometimes work only once. Consequently, it’s still important to gain insight into the physics underlying an EMI problem. Magnetic and electric field probes are effective diagnostic tools for providing that insight. Final EMI testing to demonstrate compliance with regulatory requirements is necessary, but usually is not an efficient way to solve EMI problems. EMI tests at an open area test site 88 Conformity 2008 AnnuAl guide F © Photographer: Infocus | Agency: Dreamstime.com (OATS) or in an absorber lined chamber (ALC) are usually the most accurate, but also are very time consuming. Trying to solve EMI problems in such an environment is usually a matter of trying some change and seeing if the EMI drops or not. Such an approach is usually hit-and-miss, and success is as much a case of luck as perceptiveness. Electrical circuits produce magnetic and electric fields which will radiate radio-frequency (RF) energy that may exceed EMI limits. This is particularly true in digital circuits or switching power supplies, where high-order harmonics can produce strong RF fields. In cases where RF currents are forced to flow in other than straight lines, magnetic fields will be strongest. In other cases, such as those where RF currents are low but potential differences are high, near electric fields may dominate. Trading off speed for accuracy, magnetic or electric field probes can be effective tools to speed EMI diagnosis by helping you to find the sources of EMI at the PC board or assembly level. That doesn’t mean that one form of testing should be done instead of the other, but rather that they complement each other. For example, start by identifying the frequencies where emissions are troublesome with a preliminary test of a product prototype in an ALC or OATS. Then, use field probes to

Table of Contents Feed for the Digital Edition of Conformity 2008 Engineers' Reference Guide

Conformity Engineers' Reference Guide 2008 Contents Letter from the Editor 2007: The Year in Review Company Profiles EMC Requirements for ITE A List of EMC Directive Standards Automotive EMC Standards Standards for Medical Devices EMI Issues in Manufacturing Using Field Probes as EMI Diagnostic Tools Meeting DFS Testing Requirements Selecting an Independent Testing Laboratory Filters PCBs Enclosures Antennas Anechoic Chambers Gaskets Spectrum Analyzers Compliance Software Product Safety SAR Standards Expansion Continues Update on China's CCC Compliance Program Machinery Field Evaluation ESD An Annual Progress Report on ESD Standards An Update on IEC 61000-4-2 Automotive ESD Testing Handling Class O ESDS Devices Telecom Part 68 DSL Interoperability Testing RoHS An Update on RoHS Regulations EU Directives The EU's Physical Agents Directive Buyer's Guide Consultants and Services Product Spotlights Catalog Spotlights Product Directory Vendor Directory Advertisers Index

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