Conformity Magazine - July 2008 - (Page 16)

the COMTS is validated over the range 30-1000 MHz, with the transmit antenna positioned at various locations in the test volume (Figure 1) of the chamber and the receive antenna scanned in height from 1-4m. The measurement distance required will be 3m and/or 10m, depending on the size of the test volume, with any volume larger than 2m normally having to be tested at 10m. Most CISPR standards require a 10m distance, except for CISPR 22 which allows 3m measurements only for Class B (domestic) equipment. In reality, many chambers will test at 3m anyway if the equipment under test (EUT) is not larger than 2m in diameter. We will see later that a new standard will specifically define the NSA test distance as a function of the size of the EUT in order to harmonize this issue. The design of SACs today is based on sufficient experience with ferrite tile technology that has been gained over the last 15 years. Nowadays, 3m and 10m chambers are built that vary significantly in regard to test volume size and NSA specifications (±4dB NSA is not always appropriate for some implementations). The SAC is one of two chamber types described in CISPR 16-1-4, with the other being the fully anechoic room (FAR). The FAR eliminates the ground plane effect with the introduction of an absorber covered floor, and this will be described in the next section. For SACs, the challenge will come if they must also be used as a FAR and also meet the requirements above 1GHz, as we will see later in the article. In terms of the future of the method, we need to first look back. The NSA method validates the site in a similar way that the EMC test is performed, using the same measurement distance, height scanning and the same types of antennas. There are, however, two main problems with the NSA method, which has led CISPR/A to look at what is called the reference site method (RSM). First, there was no regulation of the site on which the antennas were calibrated, and therefore the antenna factors were only as good as the site on which they were calibrated, so that the validation of another site by the NSA method would include the errors of the original calibration site. At one point it was actually claimed by some that poor OATS were intentionally being built to make it easier for chambers to pass the NSA test. The introduction of the CALTS method in CISPR 16-1-5 solved this. A second problem with the NSA method is that the published three-antenna method assumed that the three antenna factors could be obtained from three site attenuation (SA) measurements, whereas in fact one of the antennas is height scanned in one measurement and at a fixed height in a second measurement. There are four unknowns because this antenna is in two states, due to effects of mutual coupling with its ground plane image and the attenuation via the radiation pattern at different heights. The introduction of dual antenna factors, in which the antenna factors of individual antennas were not determined during the calibration process, solved this problem. Having accepted that the NSA method is a comparison of the validation site to the calibration site, we can demonstrate by working through the NSA equations that the antenna factors are redundant. Figure 1: NSA measurement in a semi-anechoic chamber Therefore, as an amendment to CISPR 16-1-4 and CISPR 161-5, CISPR/A is currently considering the use of the RSM. This is an alternative method for the validation of compliance test sites. As with NSA, the determination of Vdirect and Vsite is required. These are obtained with the exact same geometry and polarization that is defined for the NSA method. The difference between the two methods is the calculation of the site attenuation deviation: ∆SA = Vdirect - Vsite – SA APR Instead of the two antenna factors and the NSA, the antenna pair reference site attenuation (SAAPR) is required, where SAAPR includes the antenna factors as well as the coupling of each antenna to the ground plane and the coupling between antennas. In addition, the radiation patterns of the antennas are included, unlike the NSA method in which the radiation patterns are only approximated to those of Hertzian dipoles. Even though RSM retains the same antenna geometry Figure 2: FSNSA measurement in a FAR with walkway absorbers covering the whole floor 1 Conformity JULy 2008

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Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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