Conformity Magazine - December 2008 - (Page 29)

Second, the majority of emission measurements are being made in environments that are semi-free-space. That is, they have air around the product under test (as in an open area test site, or OATS), or they are located in a semi-anechoic chamber, or SAC (a shielded room with anechoic material on the walls and ceiling). In both the OATS case and the SAC case, the floor is a reflecting plane (a solid metal surface or metal-screen surface). So, the measurements are not being made in a “free-space” environment. Third, most measurements are not made in the far-field environment. A far-field environment can be thought to start at 3 times the wavelength of the source of electromagnetic waves, as found in Paragraph 5.1.1 of Clayton Paul’s “Introduction to Electromagnetic Compatibility.” (1) So, at 30 MHz, the criteria would be three times a 10-meter wavelength, or a 30 meter measurement distance. At 90.9 MHz and above, the measurements made at 10-meters are at least three-wavelengths away from the source. However, at a 3-meter measurement distance, one has to make measurements above 300 MHz to satisfy the “three-wavelength” rule. It should be noted that the available frequency spectrum from 90.9 MHz to 300 MHz is full of licensed broadcast services, including FM radio, VHF TV, aviation services, and other susceptible public RF services. In the 1970s, an extensive study was undertaken by the Computer and Business Equipment Manufacturer’s Association (CBEMA), now known as the Information Technology Industry Council (ITIC) (2). This study was done in anticipation of rule settings by the U.S. Federal Communications Commission (FCC). Results of the study indicated that a 30-meter horizontal measurement distance was appropriate for commercial computers (called Class A computers in the FCC Rules.) At 30 MHz, the wavelength of the electromagnetic waves is 10-meters, so the receiving TV or other licensed broadcastservice antenna computer was in the ‘far-field’ of the Class A computer when measured at 30 meters (three-wavelengths away). However, the receiving antennas of licensed broadcast services were always in the “near-field” of a home computer (Class B) at a 3-meter antenna distance from 30 MHz to 300 MHz. Another supporting position for a 10-meter measurement distance is that it more closely typifies the source-receptor separation distance in the practical world. For example, a computer to a rooftop antenna within the same household is clearly represented by a 10-meter separation. Also, an apartment-to-apartment distance is fairly represented by a 10-meter separation distance. The original 1977 CBEMA study on EMI from computers utilized a 30-meter separation for Class A equipment, The FCC soon modified their stance on Class A to 10 meters to reflect the realities of the ambient at 30 meters (with a corresponding limit 10 dB higher than at 30 meters), and because it more closely represented the real-life situation. This inverse distance fall-off is reasonable, since both the 10-meter and 30-meter distances are in the far-field of the source from 90.9 MHz and higher frequencies. In 1977, the typical Class B computer possessed a 4.77 MHz or slower clock speed (oscillator circuitry) and could perform only a minimum of operations. Today’s laptop and desktop computers are equivalent to the 1970 supercomputers, and a strong argument can be made that they should be tested at 10-meters since they are equivalent to Class A computers. Measurement distances shorter than 30 meters are a modern development. For example, CISPR Publication 7 (Second Edition – 1969) - Recommendations of the CISPR states in its Recommendation No. 39 (Limits of Interference from ISM RF Equipment [Excluding RF excited arc welders and surgical DeCember 2008 Conformity 29 http://www.spira-emi.com http://www.spira-emi.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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