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Anechoic Chambers_ Past And Present

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					Anechoic Chambers     4/11/05    11:08 AM     Page 1




                                                                                                Feature Article
                                                                                                         Brian F. Lawrence




                                                                                    Anechoic Chambers,
                                                                                      Past And Present


           his article will review the historical   communications outside of the test          shielded chamber could solve some of
                                                    area.                                       these problems, its imperfections will
      T    development of absorber materials
           and anechoic chambers, which play
       an important role in the work of today’s     Different industries and regulatory
                                                                                                result in internal surface reflections,
                                                                                                cavity resonances, yielding poor site
       EMC test engineer. We will discuss           authorities place different priorities on   attenuation (in the case of emissions
       early attempts to achieve correlation        emissions in comparison with                tests) and non-uniform field conditions
       between Anechoic Chambers and Open           susceptibility. If a home computer          (in the case of susceptibility testing).
       Area Test Sites and trace the                malfunctions it can be inconvenient,
       improvements made through the years          but if an automobile, or worse, an          Lining the internal surfaces of RF
       that resulted in current industry            aircraft were to malfunction it could be    Shielded chambers with an ideal
       practice.                                    disastrous. On the other hand, if a         absorbing material would have the
                                                    mass-produced item must be removed          effect of simulating OATS conditions
       As international regulatory agencies         from store shelves as a result of           within a convenient, indoor, weather
       introduced RF emission and                   regulatory spot checks, this could be a     protected test chamber. An RF anechoic
       susceptibility requirements and              different kind of disaster — an             chamber could become an ideal EMC
       standards in the 1970’s and 1980’s, the      economic one — for the manufacturer         test site, useful for both emissions and
       need to make accurate EMC tests              and retailer. Responsible companies         susceptibility tests, if the absorber
       gained increasing importance.                and independent test laboratories,          materials can adequately eliminate
       Regulatory standards define not only         therefore, responded to the emerging        internal surface reflections over the test
       the permitted characteristics of the         EMC Regulations by developing their         frequency range. Producing such
       equipment under test (EUT), but also         own EMC testing capabilities,               absorber material was the challenge
       the test procedures and the calibration      including the construction of OATS          presented to the anechoic chamber
       of the test equipment and test facility.     facilities and RF Shielded chambers.        industry during the 1970’s.
       Only by addressing all of these points
       can the standards foster correlation         The ideal OATS, as defined in the           Absorber materials were not new. They
       between measurements made at                 standards, is practically impossible to     had been used in anechoic chambers
       different locations and times by             create, although with the right location    for many years to create test facilities
       different engineers using different          and careful design, there are now a         for radar and microwave antenna
       instrumentation. In general, standards       number of near perfect OATS facilities      evaluation. Absorbers were typically
       on the measurement of radiated               in operation. Typical problems              manufactured by impregnating
       electromagnetic emissions prescribe the      associated with the use of an OATS          conductive carbon into a foamed plastic
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       use of open area test site (OATS),           could include; ambient RF interference,     medium, such as polyurethane or
       while those concerned with RF                poor grounding conditions, inclement        polystyrene. These carbon-impregnated
       susceptibility define an RF Shielded         weather, remote locations and testing       materials were fashioned into tapering
       environment in which a uniform field         time limited to the daylight hours. If      wedge and pyramid shapes to provide a
       can be established. Surrounding the          weather protection is provided,             suitable impedance match between free
       susceptibility test area with an RF          dielectric reflection from wood or          space and the resistive absorber
       shield is necessary to prevent the test,     plastic walls, as well as reflection from   medium. Balancing the carbon content
       which deliberately creates strong            wiring and lighting, are also of            with the shape of the tapering material
       radiated signals, from interfering with      concern. While theoretically an RF          provided efficient and predictable


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                        Feature Article

                        absorption of RF energy from                   3-meter range, EMC Anechoic                distribution of pyramidal shapes as a
                        Microwave frequencies to below 500             Chamber was built for IBM in Boca          series of layers having different
                        MHz, where the tapered length of the           Raton, Florida.                            impedances. This absorber model gave
                        absorber would be greater than one                                                        industry the capability to design and
                        wavelength. The lower frequency of             This chamber’s site attenuation was        build Absorber Materials with much
                        good absorption performance was                tested according to the site attenuation   improved performance in the VHF
                        strongly related to the length of the          methods developed for ANSI C63.4           band.
                        absorber (and still is, for conductive         and accepted by the FCC as modeling
                        foam pyramidal units).                         open area test site performance and        A chamber simulation program was
                                                                       suitable for testing to the FCC Part 15    also developed that imported the
                        Extrapolating this established                 Rules. The chamber was designed and        material performance files from the
                        technology down to 30 MHz and below            built by Ray Proof at a cost of almost     absorber models to predict the field
                        was the basis of many early EMC                $2M (two million dollars) and required     conditions that would exist in the final
                        Anechoic Chambers of the 1980’s.               Ray Proof’s Absorber Division to           chamber construction. This new tool
                        Pyramidal absorbers six feet, eight feet,      install a 50 foot long, walk-in            allowed design engineers to optimize
                        and even up to twelve feet long were           waveguide to test the 8 foot long          chamber shaping and absorber layout
                        produced and installed in large RF             pyramids of foam that lined the walls      to provide the desired OATS
                        shielded chambers with mixed success.          and ceiling of the IBM Chamber.            equivalence.
                        Not only were there physical problems
                        in manufacturing, handling and                 More 3m range anechoic chamber             In parallel, the chamber industry had to
                        installing these materials, but new            installations followed the success at      design and install more sophisticated
                        methods of factory quality testing also        IBM, but this chamber performance          and accurate test equipment able to
                        had to be developed in order to make           and Absorber technology did not            verify the actual performance of the
                        meaningful and reliable production             conveniently scale up to a 10m range       optimized absorber designs. A huge 6 ft
                        tests on such large pyramidal shapes           length, desirable for testing Class A      square coaxial line, having a 2 ft square
                        and at frequencies down to 30MHz.              computing devices. Something               center conductor was installed at Ray
                                                                       different was needed.                      Proof, able to measurement the low
                        Multi-national corporations such as                                                       frequency reflectivity of absorbers in
                        IBM and Hewlett Packard were                   The next step in chamber development       groups of 8 units. This original Ray
                        particularly interested in taking              came as the result of a partnership        Proof test system, together with an
                        advantage of anechoic chambers for             between customers, industry, and           array of more modern systems
                        their EMC Test Programs. Available             academia. Funding provided to the          instrumented with network analyzers is
                        OATS facilities, often remote from             University of Colorado at Boulder by       installed at the ETS-Lindgren absorber
                        their manufacturing plants, could not          IBM and Ray Proof resulted in the          plant in Durant, Oklahoma.
                        keep up with their demanding test              development of a numerical model for
                        schedules. The RF chamber industry             absorber materials. The model used a       Anechoic Chambers that could meet
                        responded and in1982 the first full size,      homogenization principle to simulate a     both the 3m and the 10m range OATS
                                                                                                                  characteristics of site attenuation
                                                                                                                  according to such standards as ANCI-
                                                                                                                  C63.4 and CISPR16 became available
                                                                                                                  by 1990. However, they were monsters,
                                                                                                                  large and expensive, and outside the
                                                                                                                  economic range of the majority of
                                                                                                                  potential customers.

                                                                                                                  In 1969 the University of Tokyo
                                                                                                                  patented the use of ferrite tiles in EMC
                                                                                                                  Anechoic Chambers. Sintered ferrite
                                                                                                                  tiles, only a few millimeters thick, can
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                                                                                                                  exhibit excellent absorption properties
                                                                                                                  at frequencies below 100 MHz. By the
                                                                                                                  late 1980’s many ferrite tile lined
                                                                                                                  chambers were being used in Japan as
                                                                                                                  EMC Test Sites. The great advantage of
                                                                                                                  the ferrite tile technology was that
                                                                                                                  chambers could be dramatically
                                                                                                                  reduced in size. The surrounding shield
                                                                                                                  did not have to be sized to

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                                                                                                               Feature Article

       accommodate a large volume of thick            and design of the Antennas that will be   fully qualified, non-anechoic
       absorber lining in addition to the active      used in the chambers.                     reverberation chambers. Today there is
       test volume. However, ferrite was still a                                                a test chamber for almost every EMC
       very expensive material to produce,            As the EMC practice has evolved, so       test requirement, making the emulation
       and was even more expensive when the           too has chamber test site design. For     of a wide variety of test conditions
       tiles were packed and shipped to sites         less than a $100K investment,             possible.
       outside Japan.                                 companies can now own and operate a
                                                      compact 7m x 3m x 3m pre-                 EMC test engineers can now take
       When the original ferrite tile patent          compliance EMC Chamber. Facilities        chamber anechoic performance for
       expired in the mid 1980’s, competitive         like this demonstrate +/- 6dB             granted and concentrate on selecting
       pressures reduced the cost of a ferrite        correlation to NSA at low frequencies     other chamber features and accessories.
       lined chamber. Again, it was IBM who           and within +/- 4dB at frequencies from    The optimal choice of antenna
       in 1986 became the first company in            100 MHz to millimeter wave                frequency ranges, antenna patterns,
       the U.S.A. to install a high                   frequencies. Slightly larger chambers     equipment handling ramps, hoists,
       performance 10m range chamber using            can be designed to demonstrate +/-4dB     towers, turntables, automated sliding
       ferrite tile technology at their facility in   correlation to NSA for smaller EUTs       doors, and other accessories will
       Austin, Texas.                                 and with reduced scanning height of       improve the ease of use, lower the cost
                                                      the antenna. For Engineers who are        of ownership, and allow the chamber
       By the 1990’s ferrite tile suitable for        evaluating product susceptibility and     user to take full advantage of the
       EMC test chambers were being                   who self certify product emissions,       performance that the modern chamber
       produced by several companies in Asia,         such pre-compliance chambers provide      can deliver.
       the U.S.A. and Europe. The original            ideal indoor test site convenience.
       absorber numerical modeling programs                                                     About The Author
       and their later derivatives had been           The next step up from the pre-            Brian Lawrence is the Director of
       modified to include ferrite parameters         compliance chamber is the full-           Sales & Marketing for ETS-Lindgren,
       together with dielectric matching              compliance, 3m-range facility. Such a     Europe. Prior to the sale of Lindgren
       layers. As a result, a new generation of       chamber which cost IBM $2M in 1982        RF Enclosures, Inc. to ESCO
       optimized, hybrid absorbers combining          is available today in a much reduced      Technologies Corporation in March of
       the best features of ferrite and               9m x 6m x 6m size, or even smaller,       2000, Brian Lawrence was responsible
       conductive foam could be designed and          offering exceptional performance, for     for Lindgren’s EMC Test Chamber
       applied to EMC chambers.                       around $300K. At the higher end, base     business worldwide. Brian Lawrence
                                                      models of 10m range anechoic              has over 40 years experience in
       Chamber simulation programs,                   chambers start below $1M.                 Anechoic Chamber and Absorber
       incorporating hybrid absorber models                                                     Material development and has worked
       have been responsible for the modern           Emerging test requirements have lead      for Ray Proof USA and Ray Proof UK
       generation of EMC Anechoic                     to chamber designs that offer             during his career.
       chambers. These chambers cannot only           specialized or combination test
       meet OATS standards but will beat              capabilities.
       almost every actual OATS site in terms         These include,
       of correlation to the theoretical ideal        for example,
       site model across the entire test              EMC and
       frequency range. Typical regulatory            wireless testing
       standards require an acceptable OATS           in a 3m to 5m
       test site to demonstrate site attenuation      range length
       correlation to the ideal model within          according to
       +/- 4dB. Modern 10m and 3m range               ETSI standards
       chambers available from ETS-Lindgren           and special
       are guaranteed to correlate to within +/-      chambers for
       3 dB of the ideal standard, using              automotive test
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       optimized hybrid absorber technology.          applications
                                                      according to
       Having reached this point of                   CISPR-25. For
       development with the EMC Test                  the ultimate
       Chamber, the focus on improving site           susceptibility
       attenuation correlation to the                 testing of
       Normalized Site Attenuation, NSA, of           complex and
       an ideal OATS has moved on from the            large EUTs
       absorber and chamber to the calibration        there are also

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