Free-Space Radiated Emissions Certification Testing for IT Equipment

The standard for radiated immunity testing in accordance with IEC 61000-4-3 is free space. Free space is also accepted for radiated emissions certification testing of wireless equipment in accordance with standards published by the European Telecommunications Standards Institute. With free space an accepted reference environment for many other product certifications, it may seem a bit strange that self-certification for information technology equipment (ITE) based on free-space emissions measurements is not already widely accepted.

Will free space soon be recognized as an acceptable environment for international radiated emissions certifications of ITE? The answer to this question depends largely on whether the concepts proposed by a CENELEC working group in European standard prEN 50147 Part 3¹will gain acceptance within IEC/CISPR as amendments to either CISPR 16 or 22.

One thing is certain: changing the certification system for ITE affects the largest number of products certified internationally, probably more than all the other product types combined. ITE manufacturers have a legitimate concern that any new test method must not be a tightening of specification limits in disguise. A test method that loosens specification limits could equally concern regulators. For now, it seems that both regulators and industry are content with the existing open area test site (OATS) standards. This article examines whether any benefits would be gained by adopting an alternative radiated emissions approval for ITE based on measurements in free space.

Understanding the potential advantages of free space requires an appraisal of the successes and failures of the current OATS standards. The best argument in favor of maintaining OATS standards is that they have been successful in controlling interference for more than 30 years. Therefore, it is widely agreed that any alternative test methods must preserve a link in principle to this historically successful model. In fact, some have argued that new methods based on free space cannot be developed legitimately, simply because the specification limits enforced by regulators such as FCC are so deeply rooted in OATS methods.

But the arguments in favor of identifying alternatives to OATS methods are strong too. Currently, an OATS is the theoretical reference environment upon which manufacturers must rely to make critical product decisions. However, such OATS are subject to a wide range of uncontrollable weather as well as local RF ambient conditions that vary from site to site and from day to day.

OATS testing is so fundamentally complex that some operators believe the OATS-based test standards are not being applied uniformly within reasonable bounds of measurement uncertainty. Consider that proper OATS testing requires an operator to investigate and maximize signal amplitude as a function of at least four simultaneous parameters: frequency, turntable angle, antenna scan height, and equipment under test (EUT) placement, including placement of interconnect cables. Also consider that the received signal amplitude varies greatly on an OATS as a function of some parameters even while other parameters remain fixed. For outdoor OATS facilities, testing must take place in the presence of a local RF ambient that can jam the frequency spectrum with continuous transmissions from sources such as television and radio broadcasts, as well as intermittent interference from mobile telecommunications, airports, and military installations.

Because of these complications, manufacturers have found that obtaining consistent problem frequencies from two different accredited OATS laboratories is often nearly impossible. Deciding whether a product should be modified or just retested at another laboratory based on a report of noncompliance is a potentially costly decision for manufacturers. Measurement uncertainty is simply too often the most significant determinant of compliance or noncompliance for products that are close to the CISPR 22 or FCC specification limits.

Viewed against these conditions, free-space chambers have a lot to offer. The technical advantage of testing indoors is straightforward: maximized emissions can be obtained in an environment that eliminates external ambient influence. In addition, once indoors, a typical 3-m free-space chamber can be constructed for approximately 25% of the cost of a 10-m, full antenna scan height semianechoic chamber (the indoor equivalent of an OATS). Free-space chambers minimize variations caused by EUT and cabling placement and eliminate the fundamental need for a receive antenna height scan. This effectively reduces the required operator considerations in a free-space test to just two major components: frequency and turntable angle.

The improved field uniformity yielded by the free-space environment was a major factor in the adoption of free-space concepts for IEC 61000-4-3 radiated immunity testing.^2,3 An important consideration is whether field uniformity is an issue that is relevant for emissions testing. To obtain the maximized levels on an OATS, an operator (or test software) must properly vary frequency, receive antenna height, turntable angle, and equipment and cabling placement. Achieving this on an OATS would require incrementally adjusting the turntable angle and performing a full antenna height scan over the entire frequency range for each incremental turntable angle. Such a procedure would require a significant amount of test time. Consequently, current industry practice is to use a fixed antenna height for initial measurements, obtain a frequency list, and then vary the antenna height at the frequencies obtained from the fixed-height measurements. This method is specified in ANSI C63.4.⁴Unfortunately, this abbreviated procedure uses a fixed antenna height on an OATS, which results in large variations in the received signal as a function of frequency and the equipment's emissions source location within the test volume. Figures 1 and 2 illustrate this problem for a fixed-height 3-m OATS or semianechoic chamber by analyzing the relative signal level received from three locations (front, center, side) at three source heights (0.4, 1, and 1.5 m), as shown in Figure 3.

Figure 1. Variation in received signal, fixed 1.5-m-height receive antenna in a semianechoic chamber or OATS (3-m distance; vertical polarity).

Figure 2. Variation in received signal, fixed 1.5-m-height receive antenna in a semianechoic chamber or OATS (3-m distance; horizontal polarity).

In many cases, varied field uniformity on an OATS invalidates the assumption that the scan height can be fixed to obtain a preliminary list of frequencies. This is because once an operator varies the turntable angle or attempts to maximize the signal by moving the EUT and cabling, the list of frequencies may change. Then, once the antenna height is varied again, the angle at which the maximum readings occur may change, requiring a new cable maximization and height scan, and so on. Therefore, the theoretical quality test that might be achieved by a full scan height OATS or semianechoic chamber is, in fact, rarely achieved because much of the critical information for the test--the preliminary frequency list, maximized equipment placement and cables, and maximized turntable angle--is usually performed with a fixed antenna height even in full scan heightcapable facilities. This is just one plausible explanation for the difficulty of obtaining the same frequency list from even two accredited, high-quality OATS or semianechoic chambers.

In free space, much less variation occurs in signal amplitude using a fixed antenna height, in either vertical or horizontal polarity. The free-space results shown in Figure 4 are obtainable using a fixed antenna height, yet the field uniformity is closer to that of a full scan height OATS, represented by Figures 5 and 6. This comparison between facility types was developed using the model for normalized site attenuation (NSA) as contained in CISPR 22 and ANSI C63.4. Figures 16 were experimentally verified using a sample source radiator and were published at the 1999 IEEE International Symposium on EMC held in Seattle in August 1999.⁵

Figure 4. Variation in received signal, fixed 1.5-m receive antenna height, free-space chamber (3-m test distance; horizontal or vertical polarity).

Improved field uniformity in free space effectively eliminates the need to scan antenna height and reduces the dependence on EUT placement and cable maximization. Signal amplitude at each frequency in a free-space chamber, therefore, is primarily affected by only one variable: the turntable angle.

Setting aside the ambient and operator issues, it is possible to compare basic measurement uncertainty from OATS to free-space chambers by two methods: calculation of measurement uncertainty by a combination of Type A and Type B factors as defined by the International Standards Organization (ISO),⁶ or calculation of measurement uncertainty by sample EUT measurements. In principle, as long as the free-space chambertoOATS uncertainty is within the typical OATS-to-OATS uncertainty, including free-space chambers in the list of acceptable facility types based on measurement uncertainty should pose no additional risk.

Uncertainty Comparison Using Instrumentation Considerations. Tables I and II compare measurement uncertainty factors contained in an annex to prEN 50147 Part 3. The tables illustrate that, in theory, a free-space chamber and an OATS should provide approximately the same measurement uncertainty based on the factors that can be quantified for the measurement instrumentation, including the facility and the antenna.

Component	Probability Distribution	Uncertainty dB
.		Bicon	LPDA
Antenna factor calibration	Normal (k = 2)	±1.0	±1.0
Cable loss calibration	Normal (k = 2)	±0.5	±0.5
Receiver specification	Rectangular	±1.5	±1.5
Antenna directivity	Rectangular	+0.5,0	+2,0
Antenna factor variation with height	Rectangular	±2	±0.5
Antenna phase center variation	Rectangular	+0	±2
Antenna factor frequency interpolation	Rectangular	±0.25	±0.25
Antenna balun imbalance	Rectangular	±1.0	±0
Measurement distance error ±2 cm	Rectangular	±0.1	±0.3
Height of antenna above ground plane Height error ±2 cm	Rectangular	±0.1	+1.0,0
Height of EUT above ground plane Height error ±2 cm	Rectangular	±0.05	±0.05
Site imperfections	Rectangular	±1.0	±1.0
Mismatch	U-shaped	±1.1	±0.5
System repeatability	Normal	±0.5	±0.5
Ambient interference	--	Large	Large
Reproducibility of EUT/cable layout	--	Poor	Poor
Combined standard uncertainty uc(y)	Normal	+1.92,1.90	+2.1,1.8
Expanded uncertainty U	Normal (k = 2)	±3.8	+4.2,3.6

Table I. Uncertainty budget for emission measurements on a 3-m open area test site.

Table II. Uncertainty budget for emission measurements in a 3-m free-space chamber.

Experimental Verification of Free-Space Chamber Measurement Uncertainty by a Simultaneous Method of Emission/Immunity Facility Calibration. Using a NISTtraceable isotropic field probe, it is possible to use data typically collected during an IEC 61000-4-3 uniform plane calibration in a free-space chamber to obtain both traceable error and uncertainty terms in accordance with ISO and NIST guidelines.^6,7 This method of simultaneous, traceable calibration of free-space facilities can be summarized as follows: ⁸

With an isotropic field probe, directional coupler, and power meter, data are collected at multiple locations in the volume to be occupied by the EUT (the uniform plane in the case of the IEC 61000-4-3 geometry). At each calibration frequency, the E-field is recorded as measured by the isotropic field probe. The forward power required to generate the field is also recorded. After the E-field and forward power data are collected, the results are normalized to represent the required forward power to generate 1 V/m at each sample location. The average power (in watts) required to generate 1 V/m is computed as follows:

where Pf_i is the normalized forward power required to generate 1 V/m at each of the individual sample points, and n is the number of sample points. The standard deviation of the required normalized forward power is then also computed:

The system gain over isotropic is then computed from the average value of P_F and the following equation:

Note that for Equation 3, a distance must be assumed. It is suggested that by using the specification protection-zone distance (i.e., 3 or 10 m), the resulting average gain over isotropic is computed directly referenced to the ideal, free-space isotropic result at that protection zone distance, regardless of the actual geometry of the sample-collection points. The average system transducer factor (C_dB) is calculated based on the gain by the following equation:

This is the average transducer factor for the free-space system (including the antenna, cabling and chamber) to be used for future emissions measurements and is traceable to a national reference such as NIST through the instrumentation used.

It is recommended that gain, transducer factor, and standard deviation be computed separately for horizontal and vertical polarity data populations if a linearly polarized antenna is used. Although a free-space measurement system should theoretically have no difference in characteristics depending on polarity, real (non-ideal) measurement systems will exhibit a difference. Furthermore, during the measurement of an actual EUT the transmit or receive antenna may be polarized with respect to structures on the EUT, suggesting that both polarities should be investigated during a certification test and, therefore, should be calibrated.

The Type A (standard, k = 1) uncertainty in the average gain and system transducer factor is calculated from the standard deviation. This value is computed as follows:

where s is the standard deviation of the sample-population data, and n is the number of data points included in the computation of s. Note that for a standard 16-point IEC 1000-4-3 plane, n = 16 for each polarity.

The resulting Type A uncertainty of Equation 5 is then combined with Type B factors for the instrumentation used during the calibration. This includes the isotropic field probe and equipment used to monitor forward power. The Type A and Type B factors are combined by the root-sum-squared (rss) technique and then expanded by an expansion factor of k = 2 to represent a 95% confidence level. This represents the 95% confidence uncertainty in the gain and transducer values determined by the calibration.

The measurement uncertainty for a sample free-space facility, obtained using the rss method, is shown in Figures 7 and 8. Note that this uncertainty is slightly higher for emissions than for immunity because an emissions preamplifier was included in the uncertainty calculations. The results shown in Figures 7 and 8 have been obtained for multiple free-space chambers and are also consistent with the analysis shown in Table II.

Figure 7. Measured chamber uncertainty (k = 2, Gaussian) using simultaneous emissions/immunity calibration based on IEC 61000-4-3 uniform plane method (horizontal polarity).

Figure 8. Measured chamber uncertainty (k = 2, Gaussian) using simultaneous emissions/immunity calibration based on IEC 61000-4-3 uniform plane method (vertical polarity).

Other methods for evaluating free-space chamber performance based on NSA measurements are contained in prEN 50147 Part 3 and have also been discussed in previous works.⁹ Although NSA is a more familiar and perfectly acceptable method of evaluating chamber performance, simultaneous calibration presents several advantages over the NSA method. Besides the obvious economic benefit that only a single calibration is required for both emissions and immunity, the simultaneous method produces traceable error and uncertainty terms, in accordance with the ISO guidelines, for the same equipment, in the same geometry used for subsequent EUT measurements. By contrast, NSA is only a verification test, which does not directly produce traceable error and uncertainty terms that apply to future testing. This is in part because the NSA verification is often performed with different antennas and geometries than will be used for subsequent EUT tests.

Because the simultaneous method calibrates the free-space chamber, antenna, and feed cabling together as a system, there is no need to calibrate these components individually. This eliminates the need to ship equipment (primarily the antenna) to a different facility for calibration. The uncertainty of the isotropic field probe used for the simultaneous method is substantially larger than the theoretical uncertainty obtainable by an insertion loss measurement such as NSA. However, this does not affect the resulting overall, combined expanded uncertainty that must be stated for the equipment and facility during EUT testing. In fact, the uncertainty is comparable by either method because even though the NSA uncertainty may itself be quite low, that uncertainty must then be combined and expanded for the antenna and cables actually used during subsequent testing. The similarity of the bottom-line uncertainty obtained by either method can be observed by comparing the results in Table II, developed assuming NSA verification, to the results in Figures 7 and 8, which were obtained by the simultaneous calibration method using an isotropic field probe. The agreement in results obtained by these substantially different methods provides a significant validation for both the proposed simultaneous calibration and the traditional approach currently described in prEN 50147 Part 3.

Although it has been known for some time that OATS-to-OATS deviations can be substantial, very few studies have been conducted until recently to evaluate OATS uncertainty based on a controlled comparison of measurement data from several laboratories. Furthermore, laboratory accreditation according to ISO Guide 25 was not required in the United States for FCC measurements until 1997, so previous studies could not compare data based on quality ensured by similar accreditations.

A recent study, conducted by Ghery Pettit of the Intel Corp., gathered data for a sample EUT from seven laboratories operated by five different companies. A representative traveled with the sample to ensure that the test setup and device operation were consistent between laboratories. However, to better ensure the independence of the results, the representative provided no additional assistance (such as frequency information) to the participants.

Figure 9 shows the deviation from mean value for the EUT sample in the frequency range of 30 MHz through 1 GHz. The results of this study illustrate that the measurement uncertainty determined by interlaboratory comparison measurements for an OATS is comparable to the result predicted by considering just the instrumentation factors (Table I). The slightly higher uncertainty in the experimental study could be partially attributed to transportation of the equipment samples or some variation between laboratory procedures in obtaining maximized emissions, although the representative observed no specific differences in test methods.

Figure 9. OATS-to-OATS correlation study using sample equipment under test (EUT) (results of seven OATS combined).

It is important to note, however, that the results of Figure 9 showing decent (± 5.3 dB, 95% confidence) overall OATS-to-OATS correlation is based only on those frequencies identified by two or more of the participating laboratories. The difficulty in obtaining identical frequencies was observed in the OATS-to-OATS study: a majority of the laboratories (4 of 7) agreed on only 17 of the 35 frequencies reported in the frequency range of 30 MHz through 1 GHz; less than 50% of the time. All seven laboratories agreed on just three frequencies, which was less than 9% of the frequencies identified by two or more laboratories.

A similar study to evaluate free-spacetoOATS correlation was performed by CKC Laboratories Inc. using data compiled from three sample systems tested in two free-space chambers and on three different OATS. The combined results from this study are shown in Figure 10. To achieve the results of Figure 10, a theoretical correction factor based on a 1-m source height above ground was used to correct the 3-m free-space results for comparison to the 10-m OATS readings. The Figure 10 results were obtained using a fixed antenna height in the chamber, and a full 14-m receive antenna scan height on the OATS. All samples included in Figure 10 were tabletop equipment.

Figure 10. Free-spacetoOATS correlation study using sample equipment under test (EUT) data (results combined for three EUTs and two free-space chambers).

The results of Figure 10 for the free-spacetoOATS correlation demonstrate excellent agreement with the theoretical uncertainty presented in Table II and the uncertainty obtained by the method of simultaneous calibration illustrated in Figures 7 and 8. Furthermore, the free-spacetoOATS experimental results of Figure 10 are within the typical OATS-to-OATS uncertainty quantified analytically in Table I and experimentally in the OATS-to-OATS study presented in Figure 9.

Free-space chambers for radiated emissions certification testing should be viewed as an opportunity for ITE manufacturers to gain uniform worldwide access to the same low-cost, high-quality test facilities that are already used for radiated emissions certifications of other equipment. The adoption of free-space alternatives for ITE would allow manufacturers to perform all of the required radiated EMC measurements in a single facility. Because free-space chambers eliminate interfering local RF ambient, tests are less complex, providing an obvious advantage over OATS. Compared to 10-m semianechoic facilities, these chambers can be built for about 25% of the cost and yet still provide improvements in field uniformity and test repeatability. Compared to both OATS and semianechoic chambers, free-space chambers significantly reduce test complexity. The use of the free-space environment substantially reduces variations due to equipment and cable placement and effectively eliminates the need to scan the receive antenna height to obtain maximized emissions in the frequency range of 30 MHz through 1 GHz. This claim is supported theoretically by the comparisons of Figures 16 and experimentally by the free-spacetoOATS results of Figure 10 for tabletop equipment. The results of Figure 10 were obtained using a fixed antenna height in the 3-m free-space chambers and compared favorably to the results obtained using a full 14-m receive antenna scan height on the OATS at a 10-m test distance.

Because OATS limits have been successful in controlling interference, manufacturers should be allowed to continue to certify products based on the existing OATS standards. This is particularly applicable to large, floor-standing equipment designed to be installed on a ground plane or computer floor. For tabletop equipment, free-space correlation to an OATS is well within the typical OATS-to-OATS uncertainty as demonstrated through both theoretical and experimental considerations. The cost, convenience, and technical advantages that free-space testing offers can provide economic benefits without sacrificing the success that OATS have provided in controlling interference.

"prEN 50147 Part 3: Electromagnetic Compatibility Basic Emission Standard Part 3: Emission Measurements in Fully Anechoic Rooms," CENELEC, TC 210, WG 4-9905, January 1999.
Heinrich Garn, Wolfgang Mullner, and R Kresmer, "A Theoretical and Experimental Investigation of Field Homogeneity in Absorber-Lined Chambers for Radiated Susceptibility Tests According to IEC 801-3 second edition," IEEE 1994 International Symposium on Electromagnetic Compatibility (Chicago: IEEE EMC Society, 1994).
"Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques, Section 3: Radiated, Radio-Frequency, Electromagnetic Field Immunity Test," IEC 61000-4-3, International Electrotechnical Commission, Geneva, 1995. This standard has also been published with some modifications as a European Norm entitled EN 61000-4-3 (1996).
"American National Standard for Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz," ANSI C63.4 (1992).
Clark Vitek and Ed Blankenship, "An Analytical and Experimental Comparison of Radiated Emissions Test Facilities," IEEE 1999 International Symposium on Electromagnetic Compatibility (Seattle: IEEE EMC Society, 1999).
Guide to the Expression of Uncertainty in Measurement, 1st ed. (Geneva: International Organization for Standardization, 1993).
"Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results," NIST Technical Note 1297, U.S. Department of Commerce, National Institute of Standards and Technology, 1994.
Clark Vitek, "A Method of Simultaneous, Traceable Calibration of Free-Space Measurement Systems for Radiated Emissions and Radiated Immunity Testing," IEEE EMC Society Newsletter no. 181 (1999): 21.
Roger A McConnell and Clark Vitek, "Calibration of Fully Anechoic Rooms and Correlation with OATS Measurements," IEEE 1996 International Symposium on EMC (Santa Clara, CA: IEEE EMC Society, 1996).

Free-Space Radiated Emissions Certification Testing for IT Equipment

Free-space chambers may be a viable, cost-effective alternative to open area test sites for testing tabletop information technology equipment.