Conformity Magazine - December 2008 - (Page 30)

diathermy apparatus]) that industry should use measurement distances of 30 meters, 100 meters, and 300 meters. Measurement Uncertainty and Measurement Bias The measurement uncertainty (MU) of EMC test equipment at a 10-meter horizontal antenna distance is lower than at a 3-meter horizontal antenna distance. The two most wellknown MU EMC documents, CISPR 16-4-2 and the United Kingdom Accreditation Service (UKAS) Lab 34 document, both reflect this MU variance. The CISPR 16-4-2 standard has two measurement uncertainty tables in the body of the standard, one for three-meter measurements and one for 10-meters. The 3-meter MU table (table of standard uncertainties) shows a MU of 6 dB for the vertical log periodic antenna, while the 10-meter distance shows a MU of 4.9 dB for the vertical position. So, there is a MU of 1.1 dB for the vertical measurement just due to equipment MU for the 3 to 10 meter adjustment. LAB 34 shows a similar difference. It shows a MU difference total of 3.5 dB that is primarily due to antenna directivity (2 dB), antenna height (0.4 dB), phase center (0.8 dB), and measurement distance (0.3 dB). For example, the antenna directivity is 3.0 dB for log periodic antennas in the vertical position at a 3-meter horizontal distance, while it is only 1.0 dB at a 10-meter position. Also, antenna height contributes 0.5 dB to the MU table at 3-meters, while it is only 0.1 dB at 10-meters. The measurement bias from 3 meters to 10 meters may be greater than the measurement uncertainty. This measurement bias can range widely as shown by actual measurements with real products. The measurement bias is most likely due to gain compression from near-field phenomena, and from complex interactions between the size of the equipment under test and the physical size of the measurement antennas. This measurement bias needs to be researched further by the EMC technical community. Comment EMC labs (both independent testing labs and those captive to corporations) that measure product emissions at a 10-meter antenna distance must compare the levels directly to the allowed standard/regulatory limits shown in CISPR 11, CISPR 22 and FCC Part 15. EMC labs that measure product emissions at a 3-meter antenna distance are allowed to compare the measured levels to a limit 10 dB higher (more relaxed) than the regulatory limit. This leads to the paradox that a lab that builds a 3-meter chamber can invest a smaller amount capital, and be rewarded by being permitted to meet a relaxed design-suppression limit. On the other hand, the organization that makes a larger capital investment and develops a 10-meter measurement capability must meet the required regulatory limit set at 10 meters. 30 Conformity DeCember 2008 Other Considerations Measurement Distances There are four other common horizontal measurement distances (other than 10-meters) in the measurement world of EMC. They are 1-meter, 3-meters, 5-meters, and 30 meters. A 1-meter antenna distance is commonly used for testing military products (MIL-STD 461), and for commercial aircraft products (RTCA-DO-160) testing. This measurement distance is based on historical precedent, and on the fact that electronic equipment is closely combined in airplanes and other military vehicles (tanks, armored carriers, etc.) A onemeter antenna distance has never been popular for commercial electronic products (non-avionic) because it does not represent actual usage of commercial electronic devices relative to the sensitive receivers of licensed broadcast signals. A 3-meter antenna horizontal distance has been used for commercial electronic products such as information technology (IT) equipment and industrial, scientific, and medical (ISM) equipment. However, measurements made at 3-meters have near-field problems below 1000 MHz that generate complications with meeting ten-meter regulatory limits. The near-field problems also contribute to system failures at 10-meters when products are tested individually at 3-meters, and then unsuccessfully combined into systems at 10-meters. (Note - Strong arguments may be made for making measurements above 1000 MHz at 3-meters, due to smaller receiving antennas and the antenna being in the far-field of the product under test. For example, the wavelength of electromagnetic waves is 0.3 meter at 1000 MHz, so the receiving antenna at 3-meters is 10 wavelengths from the product under test at that frequency.) A 5-meter antenna distance has been used as a compromise between 3 and 10 meters. In many cases, it allowed larger products to be tested in the far-field at a lower frequency. Onewavelength at 5-meters correlates to 60 MHz electromagnetic energy. So, a measurement at a 5-meter antenna distance allows the emission above 180 MHz to be measured in a field greater than three-wavelengths from the product under test. In reality, a five-meter SAC (a chamber designed to permit an antenna to be placed five-meters from the product under test) was often used for testing larger products on a turntable, with the antenna being placed at 3-meters from the product under test. A 30-meter antenna distance was originally specified in FCC rules to allow measurements 3 wavelengths from the product at 30 MHz. It was found to be impractical because of the ambient levels at many outdoor sites exceeded the regulatory limits at 30 meters. However, arguments can be made for a 30-meter measurement distance with a loop antenna below 30 MHz, due to the long-wavelengths of the electromagnetic energy below 30 MHz.

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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