Conformity Magazine - December 2008 - (Page 28)

Challenges in Testing by Dan Hoolihan, Independent EMC Consultant Revisiting 10-Meter and 3-Meter Radiated Emission Measurements: A Technical Perspective 10-meter horizontal distance from a product under test to a receiving antenna for measuring radiated emissions has been the preferred measurement distance for regulators and EMC testing labs for a number of years. Other horizontal-measurement distances of 1, 3, 5, and 30 meters have been used for various reasons, but the predominant measuring distance is 10 meters. Important international emission standards (CISPR 11, CISPR 22) specify a 10-meter antenna measurement distance. For example, paragraph 5.2.2 in CISPR 11 states “class B, group 1 or group 2 equipment (as well as class A, group 1 equipment) shall be measured at a distance of 10 meters.” In paragraph 6.1 of CISPR 22, it is stated that “The EUT shall meet the limits of Table 5 or Table 6.” Table 5 is titled “Limits for radiated disturbances of class A ITE at a measuring distance of 10 meters” and Table 6 is titled “Limits for radiated disturbance of class B ITE at a measuring distance of 10 meters.” However, both of these standards allow a measurement distance of less than 10 meters (CISPR 22 – Par. 10.3.1 – “because of high ambient noise levels, or for other reasons” and CISPR 11 – Par. 7.2.3 – “If the field strength measurements at the specified distance cannot be made because of high ambient noise levels or for other reasons 28 Conformity DeCember 2008 A [see 6.1], measurements may be made at a closer distance.”), provided the test lab assumes an inverse-distance fall-off of the radiated field with distance. This article looks at this historically based relationship, and examines the technical basis for continuing or discontinuing the inverse-distance fall-off assumption, especially when measurements are made in a semi-anechoic chamber with its inherent low ambient noise levels. Background For many years, the EMC engineering community and some regulatory agencies have assumed an inverse-distance fall-off for electromagnetic fields from products under test when extrapolating measured values at 3-meters to predicted values at 10-meters. This inverse-distance fall-off is based theoretically on Maxwell’s equations, but the “fall-off” assumes three elements: 1) a point source (a very small source of electromagnetic energy similar to an electric dipole); 2) a free-space environment; and 3) a far-field environment. The great majority of electronic products tested in EMC laboratories are distributed sources, that is, they have a width, a height, and a length of some dimensions. So, the first assumption that the labs are testing a “point-source” is not valid for electronic products. © Greenstockcreative | Dreamstime.com

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

Conformity Magazine - December 2008 Contents Editor's Note FCC Warns Regulatory Fee Payers About Potential "Phishing" Commission Prowls the Web to Enforce Its Regulations First DTV Rollout Initiated in Wilmington, NC New Standards List Released for the EU's Directive on the Safety of Toys ESD Association Call for Papers ESDA. JEDEC Announce Standards Development Cooperation Commission Releases Data on Local Telephone Competition The Future of ESD Testing ESD Open Forum Emerging Issues in Standards: China RoHS: One Year and Counting Challenges in Testing: Revisiting 10-Meter and 3-Meter Radiated Emission Measurements Design Fundamentals: Meeting the Shielding Challenges of Injection-Molded Plastics Focus On...Microwave Absorbers Buyer's Guide FDA Recalls Automated External Defibrillators FDA Revises Guidance on Medical Device Tracking Guidance Document Safety of Passive Implants Released FDA Offers Guidance on Marketing Diagnostic Ultrasound Systems EU Commission Issues Correction on Machinery Directive Standards Other CPSC Actions in the News Product Reviews IEC Standards Update UL Standards Update Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - December 2008

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