Testing for Radiated Emission byElectronic Devices

Jim NadolnyAMP Incorporated

ABSTRACTMethods and testing facilities are described. Advantagesand disadvantages are discussed for the gigahertz trans-verse electromagnetic (GTEM) cell, mode stirred chamber(MSC), semi-anechoic chambers (SAC), and open-area testsite (OATS). An example is presented illustrating the use ofthe AMP MSC to assist a customer in reducing radiatedemission from a computer product.

INTRODUCTIONThe main purpose of testing electronic devices for emissionof electromagnetic radiation is to ascertain compliancewith regulations and standards. Manufacturers of elec-tronic related products are required to comply withmandated levels of emissions in order to sell legally theirproducts. The Federal Communications Commission(FCC) regulates emission levels and test techniques in theU.S. The European Community (EC), comprising twelvecountries, has issued a set of European Norms (EN) thatregulate product emission/immunity levels and test tech-niques. Both FCC and EN regulatory documents presumethe use of an open-area test site (OATS) for measure-ments. Alternate testing facilities are acceptable provideddata taken at such can be correlated with data taken at anOATS.

Achieving product compliance with mandated emissionlevels is usually a sequential process. A typical productcompliance cycle involves:

1. A precompliance scan is used to identify frequenciesat which radiated emission is potentially noncompli-ant. This is carried out at a precompliance test facilityto minimize the test time required at the final compli-ance test facility, to minimize error margin, to reduceoverall cost, and gain the benefits of its convenience.Data obtained at a precompliance test facility willhave a margin of error that has been determined ei-ther quantitatively or qualitatively. For instance, a

2.

10-dB error margin would imply that any emissionwithin 10 dB of the limit needs to be scanned at thecompliance facility. If emission outages are encoun-tered, corrective action is undertaken and testing isrepeated. A precompliance test will often includeradiated immunity testing to determine if the upsetthreshold of the device under test (DUT) is abovemandated levels. Immunity testing is required by theEC, but not by the FCC.

In a compliance test, emissions at the marginal fre-quencies of the product will usually be scanned.Appropriate bandwidth and detector functions areselected depending on which agency’s approval isrequired. The focus of the compliance test facility ison radiated emissions. Immunity testing is often doneat other facilities. Testing at the compliance facility isdone to achieve low error margins and to adhere toregulatory procedures. Both the FCC and EC requirethat the maximum radiated emissions be measuredand that the emission be maximized by cable orienta-tion changes. These cable orientation changes arevariables which pose difficulties no matter whichtechnique or facility is used.

TECHNIQUES FOR MEASURING RADIATEDEMISSIONS 1-6

There are four widely accepted techniques for performingradiated emission measurements, each of which will bediscussed. In addition, the ability of the technique to bemodified for immunity testing will be examined.

Open-Area Test Site (OATS)An OATS is situated on flat terrain, free from obstructions(buildings, fences, etc.). It has a ground screen which actsas a standardized reflective surface, a turntable, and ameasurement mast. The OATS is exposed to the localelectromagnetic ambient environment; many are exposedto the weather. Products are placed on the turntable and

© Copyright 2004 by Tyco Electronics Corporation. All rights reserved. t alteration and the Journal reference and copyright notice are included on the

AMP Journal of Technology Vol. 4 June, 1995 J. Nadolny 111

measurements are made at a distance specified by the ap-propriate agency (3 m, 10 m, or 30 m). The height of thereceiving antenna is adjusted by the mast which varies theantenna height within a range of one to four meters. Maxi-mum emissions are recorded as the turntable is rotated, themast height varied and the DUT cable orientation ischanged.

A recognized problem with OATSs is the variability ofresults depending on the facility used. One study compared17 certified OATSs and found wide variation due primarilyto worn or broken connectors and cables, antenna factors,and poor ground planes. Variations of 20 to 30 dB in emis-sion amplitude were reported for identical test setupsmeasured at different OATSs. Other factors that can affectemission readings, which were not varied in the cited study,are radome effects in all-weather facilities, turntable ef-fects, and termination of the ground screen. All theseissues led to the release of ANSI C63.4-1992 which speci-fies site attenuation requirements to be met by the test site.

A key feature of ANSI C63.4-1992 is that an OATS mustmeet a specified performance level referred to as normal-ized site attenuation (NSA). The theoretical NSA is theelectric field insertion loss between a transmitting and areceiving antenna accounting for ground reflection, an-tenna height variation, and free-space loss. If dipoleantennas are used with a 3-meter separation, mutual im-pedance terms are added to account for lower frequencynear-field effects between the antennas. The shape of thetheoretical NSA curve carries the frequency-dependentterm –20 log10 f, which comes from the Friis Transmissionequation:

where

Pr = received power, Pt = transmitted power, = wave-length, R = distance between transmitting and receivingantennas, G0t = gain of the transmitting antenna, andG0r = gain of the receiving antenna.

Since the radiated power is proportional to the square ofthe field strength, the received electric field is proportionalto the wavelength or inversely proportional to thefrequency f according to –20 log10 f.

By specifying the measurement technique and values forerror margin and NSA, regulatory agencies are achievingbetter site to site correlation. Broken or worn cables andconnectors, wrong antenna factors, and ground planeeffects will lead to an NSA which does not meet the re-quirements of ANSI C63.4-1992. For an OATS, themeasured NSA must correlate to within 4 dB of the theo-retical NSA value at a single point, usually the center of theturntable.

As a compliance test facility, the OATS is the standard towhich other facilities are compared. However, an OATS is

not designed for precompliance test activities and conse-quently has major drawbacks if used for this purpose. Oneof them is that for precompliance testing ready access to anengineering laboratory with diagnostic tools is highly desir-able. Such are usually not available at an OATS.Furthermore, iterative testing at an OATS is time consum-ing, a high electromagnetic ambient is present, and thedevice to be tested is exposed to the weather. Radiatedimmunity testing is another aspect of precompliance testingwhich an OATS is not very well suited simply because FCCregulations prohibit the outdoor broadband transmittedpower required for such testing. All these circumstancesmake OATSs not very attractive as precompliance testfacilities.

Semi-Anechoic Chamber (SAC)SACs are indoor radiated emission test facilities. Usingsheet steel bonded to a plywood core, an electromagneti-cally quiet enclosure can be constructed. Ventilation ports,power access, and personnel access are each carefully ad-dressed so that the plane wave shielding effectiveness canbe 100 dB over the frequency range of interest. To dampenthe quality factor, Q, and simulate an OATS, carbon filledpolyurethane cones or ferrite tiles are mounted on theceiling and walls. The exterior dimensions of such a cham-ber can vary from 10 x 10 x 10 ft to 80 x 24 x 12 ft–or evenmore.

The FCC and EC have alternate site provisions for radi-ated emission measurements at other then an OATS.ANSI C63.4-1992 recognizes SACs as an alternate site,provided they meet the volumetric NSA requirements.Radiated emission data taken in SACS which meet thisrequirement can be submitted to the regulatory agency andare recognized as having a direct correlation to radiatedemission data gathered at an OATS. If the SAC does notmeet the volumetric NSA requirement, radiated emissiondata can vary more than acceptable to the regulatoryagency.

Unlike an OATS, which must meet the NSA requirementsat a single point, the SAC must meet the NSA over thevolume occupied by the (DUT). The better the ability ofthe anechoic material to dampen the Q-factor of the cham-ber or to absorb RF radiation, the closer actual NSAmatches the theoretical NSA.

Carbon-filled polyurethane cones perform well when thelength of each cone is at least one quarter wavelength. At30 MHz this requires approximately 8-ft-long cones. Coneshape and doping levels can optimize performance. Thedielectric loss mechanism in the carbon filler is due to thefrequency-dependent, complex permittivity.

A complimenting technology for RF absorption is the useof ferrite tiles. They are approximately 1/4 inch thick and 4inches square. The loss mechanism of ferrite tiles is due totheir frequency-dependant complex permeability. The tilesare usually applied in combination with carbon-filled poly-urethane cones; above 100 MHz even small cones of that

112 J. Nadolny AMP Journal of Technology Vol. 4 June, 1995

l

l

material are effective RF absorbers. Typically, the ferrite The other end is a wide bandwidth termination made oftiles are first attached to the chamber wall and the carbon resistors and RF absorbing cones. The wide bandwidth offilled polyurethane cones are then attached to the tiles. operation is denoted by “gigahertz.” As with SACs, the sizeThe combination of tiles and cones gives wide bandwidth varies with the application. For desktop equipment testingperformance without the need for very long cones. Thus, a GTEM cell would be 25 ft long with a cross section of 11the size of the chamber can be reduced. ft by 13 ft at the flared end.

Using a geometric ray-tracing algorithm the NSA can bepredicted. Required input data are the chamber size andthe RF absorber material return loss as a function of fre-quency and incident angle. A manufacturer of SACS hasgenerated NSA graphs for two different chambers. Thesegraphs compare ideal and predicted NSA for vertical andhorizontal polarizations. Only the frequency range of 30MHz to 200 MHz has been computed since this range is theSAC design challenge. Predictions were made for both alarge (80 x 48 x 25 ft) 10-m compliance facility and asmaller (24ft x 14ft x 12ft) precompliance facility. Resultsfor the 10-m compliance facility show a maximum depar-ture of only 2 dB from the theoretical NSA. Theprecompliance chamber achieves a ±4-dB variation atfrequencies above 80 MHz. Below this frequency departurefrom the theoretical value can be as much as 16 dB. Tocorrelate precompliance radiated emission data from aDUT with data taken at an OATS requires statistical aver-aging. This averaging of many products is required only atfrequencies where the measured NSA deviates from thetheoretical NSA by more that 4 dB. A first-order approxi-mation is that the deviation between theoretical andmeasured NSA is the margin of error.

For immunity testing acceptable to regulatory agencies,field uniformity may have a maximum amplitude variationof ±3 dB over the product volume. As with the NSA, thisrequirement is most difficult to meet at lower frequencies(30 MHz to 100 MHz) and may require ferrite tiles or 8-ftcones. A common technique to achieve better field unifor-mity is to add cones or tiles on the floor when this testing isperformed since a ground plane is not required for immu-nity testing. For military product testing, the only techniqueallowed by MIL-STD-462D is with a SAC. The SAC musthave absorber material above, behind, and alongside theDUT, as well as behind the measurement antenna. Thespecified absorption characteristics are easily met with 2-ftcarbon-filled polyurethane cones.

A major advantage of a SAC is the ability to vary the DUTcable configuration to find the maximum emission. Thedominant radiation source of many products are the cables,which requires the ability to orient them for maximumemission. The major disadvantages of a SAC are its sizeand the initial cost.

Gigahertz Transverse Electromagnetic (GTEM) CellThe GTEM cell has emerged as the most recent emission/immunity test facility. It is a hybrid between an anechoicchamber and a TEM cell; it has many advantages overOATSs and SACs. The GTEM cell is a rectangular-flaredcoaxial structure. One of its ends, the cell apex, is equippedwith a connector for interfacing to standard coaxial cables.

The fundamental theory behind GTEM measurements isthat any radiating structure, for instance, a DUT, can beaccurately modeled as a sum of electric and magnetic di-pole moments. This theory is actually applied twice: first,when the total radiated power from the DUT is measuredand, second, when predicting the radiated emission of theDUT that will be measured at an OATS. The GTEM isdescribed analytically as a parallel plate waveguide wherethe dominant (lowest) frequency of propagation mode isTEM.

Two key assumptions are made in the theoretical develop-ment:

1. All dipole moments are assumed to be in phase. Un-intentional radiators are not designed to act as highlydirective antennae or behave with complicated phasedifferentiation in their radiation patterns.

2. OATS measurements are made in the far field thatallows the DUT to be modeled as a resonant dipole.Near-field predictions are possible but would requiretwelve measurements.

The DUT can be thought of as a system of electric andmagnetic dipoles with its integral behavior being the resultof a superposition of the effects of the individual dipoles.Using this model, the DUT is characterized by a set oftwelve independent parameters, because each of the tworesulting dipoles will have in general three spatial compo-nents with each component comprising a real and animaginary part. Thus to determine all values for the result-ing electric and magnetic dipoles, in principle twelveindependent measurements are required to find the twelveunknown parameters. Introducing two reasonable assump-tions simplifies the problem considerably and reduces thenumber of parameters to three. First, assuming that alldipole moments are in phase reduces the number of un-knowns and consequently the number of equations to six.The problem can be reduced further to three unknownsand three equations by assuming TEM power only atthe cell apex and by taking advantage of symmetryconsiderations.

It has been shown that if the three apex voltages corre-spond to three orthogonal rotations of the DUT, the vectorsum of these voltages measured in three independent di-rections is equal to the sum of the dipole moments for thisgeometry under the assumptions stated above. Three or-thogonal rotations are required to couple the dipoles to thecavity and excite the TEM mode. The total radiated poweremitted by the DUT can be determined from the propaga-tion constant and the sum of the dipole moments.

As stated above, the analysis assumes that the GTEM is a

AMP Journal of Technology Vol. 4 June, 1995 J. Nadolny 113

parallel plate waveguide and that the dominant propaga-tion mode is TEM. For this structure the higher ordermodes TE

n and TM

n will propagate, but are weakly

coupled and the broadband match provided by the termi-nation suppresses the formation of higher order modes.Thus, essentially the total power radiated from the DUT inthe TEM mode is measured.

To predict OATS performance of the DUT, it is assumedthat the total radiated power, as calculated from the apexvoltages, is present at the terminals of a fictitious resonantdipole. The predicted radiation pattern from the resonantdipole accounts for ground plane effects and the effect ofscanning in height with the receiving antenna. The calcu-lated peak amplitude is recorded and compared to theradiated emission limit line.

The GTEM radiated emission test procedure is as follows:

1.

2.

3.

4.

Strap the product to a platform.

With the product operating, measure the terminalvoltage from the GTEM cell.

Repeat the above test with the product orthogonallyrotated.

Repeat the above test in the third orthogonalrotation.

The GTEM correlation algorithm performs the followingcomputations:

1.

2.

3.

4.

5.

6.

7.

Calculates the vector sum of the three measured volt-ages. The three voltages being the result of the DUTorthogonal rotations.

Computes the total radiated power emitted by theDUT as determined from the summation of the threevoltages and the TEM mode equations for theGTEM.

Computes the current excitation of an equivalentHertzian dipole when excited with the powercalculated in step 2.

Places the hypothetical Hertzian dipole at a specifiedheight over a perfect ground plane.

Computes the horizontally and vertically polarizedfield strength at appropriate height intervals over thetotal operator selected correlation algorithm heightwithin the range of one to four meters.

Selects the maximum values of the horizontally orvertically polarized field strengths over the heightselected.

Presents the maximum value for comparison to thechosen EMC limit.

The test data generated from this technique can achievevery good correlation to data taken at an OATS. As ofDecember 2, 1993, the FCC will accept data taken in aGTEM as equivalent to OATS provided that

1. Data equivalence achieved by comparison of resultsbetween OATS and GTEM is established. Emissions

2.

are to be measured using both GTEM and OATSmethods for the specific product type as a test case.If correlation is established, the GTEM results willsuffice in the future.

If the DUT has any cabling, the cabling must be setup identically to the test case for which equivalencewas established.

The GTEM cell can be an excellent tool for radiated im-munity testing. Its advantage is the generation of a uniformTEM wave over an appreciable volume and high fieldstrength per unit input power. This feature and theGTEM’s ability to measure radiated emissions make itattractive as a precompliance facility.

The major drawback of the GTEM is in measuring cableemissions. Orthogonal rotations of a cable are not verypractical and this is required for the theory to predict thetotal radiated power from the DUT. In practice, the cablesare orientated to generate the highest apex voltage. Prod-ucts which have test setups or cabling dimensions largerthan about 1.5 meters cannot be accurately tested in aGTEM.

Mode Stirred Chamber (MSC)The MSC, like the SAC, can be constructed of sheet steellaminated to a plywood core. The laminated panels areassembled to form a chamber. Attention to doors, air flow,and wiring is similar to those for a SAC. The MSC does nothave any RF absorbing material on the walls, instead, theinterior surface is highly reflective. A motorized paddlewheel or vane is located inside the chamber and providesmode stirring action.

Unlike the GTEM, which primarily supports TEM propa-gation, the MSC is designed as a resonant cavity. At fivetimes the first rectangular cavity resonant frequency, themode spacing in frequency space is very dense. This resultsin the loss of antenna directivity in the MSC. Stated an-other way, the field density within the chamber is nearlyconstant over the interior volume. This feature is usuallystated as the gain of any antenna in a MSC being unity.The antenna factor or ratio of electric field to terminalvoltage of a receiving antenna does not become unity in aMSC.

The MSC as a precompliance tool can be used effectivelyas a device for measuring radiated immunity. It is possibleto generate a calibration curve relating free space powerdensity to average power density in the MSC. System re-sponse to MSC average power density is similar to freespace power density within about 5 dB at any given fre-quency. The MSC can also be used to measure differencesin radiated emission by a DUT when changes to the DUTare made. The property of unity gain results in repeatablemeasurements since the effects of cable movement areminimized.

Figure 1 shows an example of how radiated emission

114 J. Nadolny AMP Journal of Technology Vol. 4 June, 1995

measurements made in a MSC have aided in productimprovement. A computing product, after having failedradiated emission tests at an OATS, was characterized inthe AMP MSC. The baseline radiated emission data, rep-resenting the original product, are shown in Figure l(a).The product was modified and retested. Figure l(b) showsmeasurement for the modified product. Subsequently, themodified product was retested at an OATS. It was foundthat the radiated emissions were reduced by approximatelythe same amount as had been measured in the MSC.

Figure 1. Illustration of radiated emission measurements ina MSC. (a) original emission of a computing product; (b)emission after product modification.

Total radiated power from a DUT can be measured withthe MSC. The MSC differs from the GTEM in that energycarried by non-TEM modes is also measured and includedin the total MSC-radiated power measurement. However,the high Q-factor caused by the reflective walls of the MSCartificially enhances the field strength at resonant frequen-cies. The result is a radiated power measurement that isdifferent from the total radiated power that would begenerated by the same DUT at an OATS.

SUMMARYThe gigahertz transverse electromagnetic (GTEM) cell isideal for radiated immunity and emissions testing providedthe DUT has no or minimal cabling. Emission measure-ments made in a GTEM cell are accepted by regulatoryagencies. The mode stirred chamber (MSC) is good forstatistical averaging of data for improvement/degradationcomparisons and for radiation immunity testing. Emissionmeasurements made in a MSC are not accepted by regula-tory agencies. Semi-anechoic chambers (SAC) are usuallylarge and expensive but provide the greatest degree offlexibility for immunity and emission testing. If militaryproducts are of interest, the SAC is the only facility recog-nized by MIL-STD-462D for radiated emissioncompliance. Emission measurements made in a SAC areaccepted by regulatory agencies. The open-area test site(OATS), if properly constructed and periodically cali-brated, provides precise measurements but suffers fromexposure to local electromagnetic ambients. Emission mea-surements made at an OATS are accepted by regulatoryagencies.

REFERENCES1. W.B. Halaberda and J.H. Rivers, “Measurement Com-

parisons of Radiated Test Facilities,” IBM, Boca Raton,FL, IEEE Internatl. Symposium on EMC, 1992.

2. P. Willson, D. Hansen and D. Koenigstein, “SimulatingOpen Area Test Site Emission Measurements Based onData Obtained in a Novel Broadband TEM Cell,” ABBEMI Control Center, IEEE National Symposium onEMC, 1989.

3. E.M. Millen, “FCC Accepts GTEM Data!”, EMC Testand Design, January 1994.

4. J.L. Bean and R.A. Hall, “Electromagnetic Susceptibil-ity Measurements Using a Mode Stirred Chamber,”U.S. Naval Surface Weapons Center, IEEE Symposiumon EMC, 1978.

5. C.A. Balanis, Antenna Theory Analyis and Design(Wiley, New York, 1982).

6. D.M. Pozar, Microwave Engineering (Addison Wesley,Reading, MA, 1990).

James Nadolny is a Member of Technical Staff in the Tech-nology Division of AMP Incorporated, Harrisburg,Pennsylvania.

Mr. Nadolny holds a B.S. in electrical engineering from theUniversity of Connecticut and a M.S. in electrical engineer-ing from the University of New Mexico. He joined AMP in1993 after having worked as an EMI design specialist forseven years at Honeywell in Albuquerque, New Mexico.Current research interests focus on electromagnetic model-ling for EMC and electromagnetic characterization ofconnector and cable assemblies.

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