Conformity Magazine - July 2008 - (Page 20)

Clearly, as a result of using an isotropic antenna, the SVSWR method exposes more of the chamber to the antenna beam and therefore requires taking greater care of the anechoic layout. In both the SAC and FAR configuration, this leads us to increasing the coverage of the foam part of the hybrid absorber in chambers where only a partial lining had been sufficient until now, and many chambers will need upgrading as a result. The method itself is a volumetric test (Figures 5 and 6), involving the positioning of the transmit antenna along the line of sight of the receive antenna (which remains fixed) to 6 specified test points, starting at a 3m distance referenced to the front of the test volume, and then moving further away to 5 other positions as specified by the standard. This is repeated at the centre, right and left positions. The SVSWR for each position is calculated, so that this becomes a relative measurement that compares a set of data points at one location to itself, and not to data points at another location. The acceptance criterion is SVSWR < 6dB. The implications so far are that compliant FARs will pass easily, while SACs may require additional floor absorbers between the antenna and the turntable, and possibly on the turntable itself. It should be noted that wall and/or ceiling absorbers may be required in those SACs that were constructed with partial absorber lining, sufficient to meet the NSA criterion below 1GHz. CISPR Product Measurements While CISPR 16-1-4 defines chamber validation methods below and above 1GHz, CISPR 16-2-3 [9] defines measurement methods for products. Methods are well described for both FAR and SAC below 1GHz, but agreement has yet to be established on the methods above 1GHz. Radiated emission standards now cover the range 30 MHz - 5 GHz. In most cases, the approach above 1GHz is simply a continuation of the methods used below 1GHz. Since typically the same antenna and set up is used, this is convenient and does not require a major reconfiguration of the chamber (although some would also say that any change in the floor absorber set up is a major reconfiguration). Even though most would agree that there cannot be a huge change in the wavelength between say 999 MHz and 1.01 GHz, it has often been the natural break point for defining new measurement methods. There are of course clear RF reasons for this, including the different type of antennas typically used in this frequency range, the increasing impact of the reflection properties of the test environment and positioning equipment, and the increasing complexity of the EUT radiation pattern. First, below 1GHz we use antennas that can normally illuminate the test equipment within their 3dB beam width. However, semi-anechoic chambers cannot take advantage of this as a result of the ground plane, and have to height scan the antenna in emission tests, while FARs and all radiated immunity tests take full advantage of this and this therefore simplifies the tests. This however assumes that, below 1GHz, a typical radiation pattern of the EUT (a so called unintentional radiator) will also be relatively simple and not require a high degree of spatial sampling and height scanning. Above 1GHz, we start to see an issue in that the radiation pattern of both antennas and EUTs start to become more directive and complex, thus making it necessary to measure a greater number of positions Figure 5: Top view of SVSWR test positions Figure 6: Side view of SVSWR test positions 20 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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