Conformity Magazine - December 2008 - (Page 32)

Some of the earliest results were reported in the CBEMA study, which stated in paragraph 10.8.1.2 that “Results of measurements in such practical test sites at varying distances between the equipment being tested and the measurement antenna, have been found to be within + or – 6 db of those predicted using a 20dB/decade fall-off relationship between the equipment and the antenna.” Other published results include the paper by H.J. Garn, E.Zink, and R. Kremser on 3 and 10 meter testing (1993 International IEEE EMC Symposium), and recent results from an independent testing lab in the U.S. The 1993 paper by Garn, Zink and Kremser was titled “Problems with Radiated Emission Testing at 3 meter distance according to CISPR 11 and CISPR 22.” (3) This paper showed extrapolation results varying from -18 dB to -1 dB from 3 to 10 meters. The 1993 paper measured a generator inside of a computer case and the generator itself. The paper concluded “based on the present specifications, compliance tests at 3-meters should not be allowed.” Northwest EMC (an independent EMC testing laboratory) tested four products in 2008, first at a three-meter antenna distance and then at a ten-meter antenna distance, including two ITE Systems (CISPR 22), and two medical Systems (CISPR 11). The first ITE System had a fall-off minimum of 3.3 dB and a maximum of 14.1 dB over a frequency range of 30 – 960 MHz. The second ITE System had a fall-off maximum of 7.8 dB and a minimum of 2.4 dB. EMC Standards CISPR 11 – Industrial, Scientific, and Medical (ISM) RadioFrequency Equipment – Electromagnetic Disturbance Characteristics - Limits and Methods of Measurement. CISPR 22 – Information Technology Equipment – Radio Disturbance Characteristics - Limits and Methods of Measurement. CISPR 16-4-2 – Specification for Radio Disturbance and Immunity Measuring Apparatus and Methods – Part 4-2: Uncertainties, Statistics, and Limit Modeling – Uncertainty in EMC Measurements. LAB 34 – The Expression of Uncertainty in EMC Testing – United Kingdom Accreditation Service, Edition 1, August 2002. ANSI C63.4 – Standard for Methods of Measurement of RadioNoise Emissions from Low-voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz. MIL-STD 461 – Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, MIL-STD-41E, 20 August 1999. RTCA DO-160 - Environmental Conditions and Test Procedures for Airborne Equipment, RTCA DO-160E, December 9, 2004. 32 Conformity DeCember 2008 The first medical System showed a fall-off from 3 to 10 meters of a minimum of 0.5 dB and a maximum of 13.6 dB over a frequency range of 30 to 400 MHz.. The second medical System had a fall-off minimum of 3.2 dB from three to 10 meters and a maximum of 11.6 dB. Considering the range and average of the fall-offs, an inverse distance fall-off prediction is clearly not appropriate for the spatial change from 3-meters to 10-meters. Also, the fall-off appeared to be source dependent and not frequency dependent. Conclusions The preferred measurement venue for EMC labs for radiated emission testing today is a 10-meter semi-anechoic chamber (SAC). A 10-meter SAC consists of a shielded room approximately 20 meters (long) by 17 meters (wide) and 6 meters (high), which is then lined with ferrite tiles and pyramidal absorbers. The ferrite tiles are effective radio frequency (RF) absorbers below 1000 MHz, while the carbon-loaded foam-pyramidal absorbers are very effective above 1000 MHz. The use of RF absorbers on the walls of the shielded room permits the SAC to perform as an open area test site (OATS). An OATS has been the historically correct site to make accepted measurements. The advantages of the 10-meter SAC over an OATS are keyed to no ambient signals and no weather-related issues. Since there is no ambient, measurements should be made at 10-meters because the argument for shorter measurement distances due to high ambient noise levels goes away with the elimination of the ambient signals. The ambient issue becomes more important with each generation of new RF transmitters and receivers. These RF transmitters and receivers include cellular phones, hand-held portable electronic entertainment and business products, laptop computers, desktop computers, entertainment boxes, and similar electronic units. The increasing proliferation of these electronic units, coupled with the greater use of digital television and radio transmissions, assure a densely packed spectrum of signals that result in an ambient of high-, medium-, and low-level signals in the environment. A standard OATS is referred to as a reference test site by ANSI C63.4. (An OATS with no weather protection housing around the EUT is called a reference test site.) It is called a reference test site because all other measurement distances are “referenced” to this ideal test site of a metallic ground plane on a flat surface that has no reflecting objects in the near vicinity of the ground plane. However, most standard OATS are soon protected by some building material (plastic, vinyl, wood, etc.) in order to allow testing of products during inclement weather. This weather protection may be just around the turntable or it might encompass the entire area between the measuring antenna and the product under test.

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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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