Investigation of the Calibration Procedure from CISPR 25, Annex B, for use with Vehicle Component Testing.

Transcription

1 Investigation of the Calibration Procedure from CISPR 25, Annex B, for use with Vehicle Component Testing. Dennis D Swanson, EMC Engineer TijV Product Service 1775 Old Highway 8 New Brighton MN USA Abstract: CISPR 25 [ I testing is intended to protect receivers installed in vehicles from disturbances produced by conducted and radiated emissions. The radiated emissions testing is performed in an absorber-lined shielded enclosure (ALSE). Annex B of CISPR 25 describes a calibration procedure for an ALSE, by comparing measurements in the ALSE to measurements at an open area test site (OATS). The maximum error caused by reflected energy in the ALSE is limited to db. The research for this paper began as an attempt to qualify ALSE sites for compliance testing. Suggestions for further research and for changes to the procedure were generated. INTRODUCTION Throughout the international automotive industry, there are several recognized methods for measuring radio frequency (RF) emissions from components and sub-assemblies. In addition to a myriad of proprietary requirements by vehicle manufacturers, several attempts have been made to standardize the procedure. CISPR 25 was published in 1995, and the Society of Automotive Engineers (SAE) has closely followed it in their publication J 1113/41I*l. The European Union has defined a procedure in Council Directive 95/54/EC 13], which deals with passenger and transport vehicles, and in Council Directive 97/24/EC 14], which deals with motorcycles. The International Standards Organization (ISO) also specifies a method for RF emissions measurements in at least two proposed standards, IS0 137 t51 for construction machines and IS t1 for agricultural and forestry machines. These automotive standards identified above all have one important feature that is in contrast to RF emission measurement standards used for testing typical commercial products (i.e., CISPR 11, CISPR 22, FCC Part 15, etc.). These automotive standards all specify that the device under test (DUT) be placed on an elevated ground plane while measurements are made. Each of the listed standards also mandates or permits RF emissions measurements to be made inside an ALSE, with the stipulation #hat a correlation between the ALSE and an OATS must be performed. However, 95/54/EC, 97/24/EC, IS0 137 and IS do not specify any procedure for performing the correlation, nor do they mandate any tolerance on the correlation. CISPR 25 includes a correlation procedure in Annex B and a tolerance requirement in clause (Technically equivalent requirements appear in J1113/41.) Features of the CISPR 25, Annex B correlation procedure include: Requirementhat the ALSE is at least 7.0 m x.5 m x 4.0 m (23.3 ft x 21.7 ft x 13.3 ft). (But also note clause 4.3). A standard noise source, attached to a 1.5 meter wire harness, used to generate frequency signals that are measured both at the OATS and in the ALSE. The distance from the antenna to the wire harness is 1 meter. The referenced setup is found in of the CISPR 25 (refer to Figure 1 and Figure 2 of this paper). Although comparison plots of the field strength versus frequency are to be generated, and the delta between the OATS measurement and the ALSE measurement must be less than db, the procedure prohibits the application of a correction factor to measurements taken inside an ALSE. SITESELECTIONANDTESTINGPRECAUTIONS Measurements were made at three OATS sites and three ALSE chambers owned by TUV Product Service (TUV PS) in Minnesota. Table 1 is a description of the sites. Table 1 - Description of Sites Tested Symbol 1 Location Description LTS 1 Wild River Lab, IA&weather (enclosed) OATS ITaylors Falls, MN ) STS 1 Wild River Lab, 1 Fair-weather (non-enclosed) OATS Taylors Falls. MN OW Oakwood Lab, All-weather (enclosed) OATS Millville, MN TA#3 New Brighton Lab, ALSE with 24 inch absorbing foam New Brighton, MN on walls d ceiling and ferrite on floor. Size: 24 s 1 s feet TA#I New Brighton Lab, ALSE with ferrite on walls, ceiling New Brighton. MN & floor. Size: 24 s 15 s feet TA#7 New Brighton Lab. ALSE with 24 inch absorbing foam New Brighton. MN on walls Sr. ceiling. Size: 20 x 20 x ~/98/$ % IEEE is

2 Figure 1 - Calibration Procedure Setup - Plan View Figure 2 - Calibration Procedure cm-+,,;r\ *cm + t 1OOcm 4 L c * -b cm t 90 cm 100 cm I 1. Spectrum Analyzer 2. ALSE (if applicable) 3. Preamplifier & Bulkhead connector 4. Cable 5. Antenna. 250 cm long elevated ground plane cm Test harness (elevated 5 cm) * An error in placement of the wire harness during early testing was continued for the sake of consistency. Refer to figure 11 of CISPR 25 for other details * An error in placement of the wire harness continued for the sake of consistency. Refer to figure of CISPR 25 for other

3 The CISPR 25, Annex B correlation procedure is significantly different from ANSI C3.41, Clause 5.4. normalized site attenuation (NSA) measurements, which typically are used as the basis for comparing several OATS. Variation between OATS attenuation measurements would be expected, and such a comparison of the OATS owned by mv PS was previously publishedt81. Because of these expected variations, measurements at multiple OATS, using the CISPR 25, Annex B correlation procedure, was deemed necessary to assure the quality of the reference measurements. The three ALSE sites were chosen because TUV PS would like to qualify each of these rooms for testing components used on automotive machines and vehicles. While the rooms are too small to strictly meet the CISPR 25 requirements, the Annex B correlation procedure still seemed to be a logical method of meeting the less stringent requirements of the EU directives and IS0 draft standards. Precautions were taken to introduce as little error as possible into the data gathering process. The same antennas, cables, field site source, and wire harness were used for each set of measurements. The same operator was responsible for setting up all tests and taking all data. Only a few miscellaneous cable adapters, the spectrum analyzer, and the preamplifier were different for some tests. Correction factors for the preamplifiers were used. The field site source generated spikes 30 at 10 MHz intervals. A mid-course refinement in the analyzer settings made it possible for reliable measurements to be made from 30 to 1000 MHz at OW and the three ALSE sites. Early data was only reliable up to 700 MHz at the LTS and STS sites, and there was no opportunity to retake the data. However, the early data was still valuable when determining if the measurements met the tolerances specified in CISPR 25. Two of the ALSE rooms, TA#3 and TA#7, each has a permanently installed copper bench inside. At the other sites, it was necessary to fabricate an elevated ground plane. This typically was a copper or aluminum sheet, which was bonded to the metal floor or ground plane using copper tape and aluminum foil. At least four -inch wide strips of aluminum foil were used to bond the elevated metal plane to ground. 54 :: % 42 PRESENTATION OF THE TEST DATA 4 Data was gathered for both the horizontal and vertical antenna orientations, but only the horizontal data is discussed in this paper. In the horizontal orientation, the antenna was parallel to the driven wire harness fixture. Stronger signals were detected more easily at higher frequencies in this orientation. The data presented has also been edited to show only the frequency points at which readings were measured at all sites. At the higher frequencies, many readings were in or below the noise floor of the spectrum analyzer at some sites, and were thus unavailable for consideration. The CISPR 25, Annex B procedure mandates that field strength data versus frequency plots be presented. See Figure Figure 3a - Field Strength Levels Comparison of Measured Field Strength - All Sites I I I I I I I FA_. -RM31M --r.-rm41m -A-RM7IM ---k.otv-1m -cat-ltslm -*-STSIM - Figure 3b - Field Strength Levels Comparison of Measured Field Strength - All Sites I--cRM31M..m..RM4IM -A-RM7lM -.+.OW 1M -.x-ltslbl -..-STSIM] Figure 3c - Field Strength Levels Comparison of Measured Field Strength - All Sites I

4 3a, 3b, and 3c for this information. While the field strength data is shown, of primary interest is the delta between the field strength measurements from the various sites. Figures 4a and 4b show the comparison among the three OATS, referenced to the OW site. (This data is only available up to 700 MHz.) Figures a, b and c show a comparison of the three ALSE rooms, referenced to the OW site. Figures 5a, 5b and 5c show a comparison of the three ALSE rooms, referenced to TA#3. Since the Oakwood site was the only OATS with reliable measurements above 700 MHz, it was chosen as the reference for the ALSE sites. For consistency, it was also chosen as reference for the other OATS. TA#3 was chosen as the reference ALSE because it has some similarities in construction with each of the other two rooms. In general, the selection of a reference site for delta measurements would be considered arbitrary. - Figure 5a - ALSE Sites Referenced to TA#3 TA#3 as Reference Site - All ALSE sites w-.rmtlm -RM4lM x RM3lM 1 Figure 4a - OATS Sites Referenced to OW OW OATS as Reference Site - All OATS Y---\ I I I I 1 I I Figure 5b - ALSE Sites Referenced to TA#3 TA#3 as Reference Site - AII ALSE sites 0 50, c..m..STSlM -LTSlM x OWlM I oo IRMTIM -RM4lM x RMJIM 1 Figure 4b - OATS Sites Referenced to OW Figure 5c - ALSE Sites Referenced to TA#3 TA#3 as Reference Site - AU ALSE sites F@ , II.~.!..m..STSlM -LTSlM x ow IM L..m..RM7lM -A-RM4lM _----- x RMJiti-1 371

5 Figure a - ALSE Sites Referenced to OW OW OATS as Reference Site - All ALSE sites , wrh43lm -.w..rm41m -._ -.._ -h-rm7lm x OWIM ; i - - Figure b - ALSE Sites Referenced to OW OW OATS as Reference Site - AM ALSE sites e--rm3lm --*..RM41M -.A- RM7lM x OW-IM 1 --_ Figure c - ALSE Sites Referenced to TA#3 OW OATS as Reference Site - All ALSE sites IOOC crm31m..m..rm41m -A- RM71M._.._...-.-_.- x e_j 1 ANALYSIS AND DISCUSSION One initial goal for this project of performing the CISPR 25, Annex B correlation procedure was to qualify TijV s ALSE sites for performing radiated RF emissions tests on automotive components and subassemblies to other standards. As demonstrated in Figures a, b, and c, all of the ALSE sites have deviations from the reference site which exceed the db tolerance specification. Several observations can be made about the non-compliance of the ALSE sites to CISPR 25. Excessive deviations in the similar sites Measurements which compared the OATS to each other were included as a check on the integrity of the calibration procedure and the setup. It is disconcerting that the measurement deviations exceeded db between the LTS and the OW sites, since the construction of the ground plane is similar at these sites. Even the difference between OW and STS exceed db at some frequencies. Since the LTS data and the STS data were the earliest samples included in the study, it is more likely that they would contain any errors due to inconsistencies in the test setup and in the measurement procedures used by the equipment operator. The same comments can be made about the measurements comparing the ALSE to each other. Since TA#3 and TA#7 have similarly constructed copper benches and have anechoic foam installed, would it be possible to achieve close agreement in the correlation? Since TA#3 and TA#4 both have ferrite absorber on the floor, would that improve the correlation? The obvious answer from this data was no to both questions. Of course a more satisfactory answer would come from a more comprehensive statistical approach, which should be a future project. Because the ALSE data sets were collected most recently, they are less likely to contain errors due to inconsistencies in the setup and in the operator s technique than the earlier measurements. I Measurement Uncertainty Issues In an era when the quantification of measurement uncertainty is a widely discussed topic, it is trendy to include a few appropriate comments. There seems to be room for some, improvement in the procedure. Compounding of errors. For ANSI C3.4 NSA measurements, the permitted tolerance between OATS is +4 db from an ideal sire. One problem with the CISPR 25 calibration is that there is no ideal site against which to compare. There were many frequencies shown in Figure 3 where a deviation between one ALSE and one OATS is under db, but the deviation between a second OATS and that ALSE is greater than 10 db. For example, at 30 MHz, the delta between in TA#4 and OW is about 1 db, but the delta from OW to STS is about 9.5 db, and the delta from OW to LTS is about 13 db. This situation could result in a DUT passing at one location and failing at another. Correction factor prohibition. It seems uncharacteristic to permit a tolerance of + db, and yet to prohibit that the final 372

6 measurement be corrected, as is mandated by clause B.4. The EMC community would never think of using an antenna or a preamplifier without adding a correction factor. Even cable loss factors are commonly considered. Of course, a correction factor based on deviation between an ALSE and a particular OATS would not be desirable. The correction factor should be against a universal or ideal standard. Yet the use of factors for an ANSI C3.4 NSA would not be appropriate because of the elevated ground plane Elevated Ground Plane: Good or Bad? As noted earlier, there are fundamental differences between CISPR 25 and other standards which specify procedures for measuring RF emission, One foremost difference is that CISPR 25 mandates that the DUT and harness be positioned on or just above a ground plane, which elevated 90 cm above the floor of the chamber. Commonly stated rationale for this is that most components in a vehicle are anchored to some metal part in the body or engine compartment, and that the cabling is routed next to the metal framework. Also cited is a history of using this method in the industry and also a comparison to MIL-STD-42. However, there are vehicle manufacturers whose internal standards do not require the ground plane for radiated emissions test. Also, the latest release of MIL-STD- 42D, clause 4.5 and figure 3 specify that some radiated emissions is to be performed on a non-conductive table. tgl It would be appropriate in the future to perform some computational analysis to evaluate the effect of this simple geometry radiator. An analytical comparison could be made of the predicted radiation pattern from the driven wire harness, positioned 5 cm above an elevated, finite ground plane versus the predicted pattern from the wire harness above the floorlevel ground plane. In CISPR 25, figures 11 and, the tolerances are precisely defined for positioning of the wire harness. In practice, some harnesses are thicker than the prescribed tolerances. Some have branches which break out in such a manner that they preclude meeting the tolerances. Some have been designed for precise placement in the vehicle, with intentional bends that are hard to straighten. Computational analysis could help determine more quickly which tolerances from the procedure are most critical, and even predict whether they can be met in a practical setup. Dimensional requirements. When using a ferrite lined room, it is possible to meet the size constraints of an ALSE specified in CISPR 25, clause 4.3, without meeting the minimum size specified clause B.l. This is demonstrated in Figure 7 for TA#4. The size requirement in clause B.l does not seem to consider the use of ferrite chambers. Of course, any proposal to adjust the ALSE dimensions in clause B.l would be stronger if data from a ferrite-lined chamber can be shown to comply with the CISPR 25 tolerance requirements, Figure 7 - Demonstration that TA#4 Dimensions meet the requirements of Clause 4.3 t 0.8 m Elevated Ground Plane 0.19m. pdq.. -*.. *-.. 7-t **..*. *... 2-m \10.8m c CONCLUSION During an effort to correlate three ALSE sites to an OATS per CISPR 25, the attempt generated several questions about the Annex B procedure. The data from the ALSE sites did show non-compliance with the standard. However, verification data which compared several OATS sites also indicated noncompliant results, as did verification data which compared the ALSE sites. An analysis of the situation was made to identify possible source of errors and deficiencies. The likely sources seemed to be human error in performing the procedure, site deficiencies, and deficiencies in procedures. Efforts were made to identify and minimize errors and inconsistencies in the process of performing the procedure. The OATS sites were not considered deficient, since all comply with the NSA requirements of ANSI C3.4, clause The author did express some concerns about the CISPR 25, Annex B calibration procedure. l l Some concern dealt with issues related to measurement uncertainty. It was noted that the use of the correlation curve as a correction factors is prohibited. Also, large variances in measured levels were seen at different OATS. Since the NSA procedure can not be used, because an elevated ground plane is used in the setup, it is difficult to compare similar sites. Several suggestions were proposed as projects that could be used to determine whether the elevated ground plane should be used. If the interaction between the elevated ground plane and an actual wire harness is such that it grossly affects the directivity of the actual emission, then changes to the CISPR 15, Annex B procedure should be considered. n 373

7 l Finally, a suggestion was made that CISPR 25 consider ferrite-lined chambers when setting the requirements for the minimum size of an ALSE As CISPR 25 continues to evolve, it is hoped that some of these suggestions can be used to improve it. REFERENCES AND BIBLIOGRAPGHY 1 CISPR 25 Limits and Methods of Measurement of Radio Disturbance Characteristics for the Protection of Receivers used on Board Vehicles, First edition, [21 SAE Jl Limits and Methods of Measurement of Radio Disturbance Characteristics of Components and Modules for the Protection of Receivers used on Board Vehicles, July ] Commission Directive 95/54/EC, Official Journal of the European Communities, 31 October Council Directive 97/24/EC Is1 ISO/DIS 137, Earth-moving Machinery - Electromagnetic Compatibility, Draft International Standard, July 199. R ISOKD 14892, Agricultural and Forestry Machines - Electromagnetic Compatibility - Test Methods and Acceptance Criteria, Final Committee Draft ISO/TC 23/SC 2, March 199. [ I ANSI C3.4, Methods of measurement of radio-noise emissions from low-voltage electrical and electronic fs uipment in the range of 9 khz-40 GHz, O Shea, Timothy P, Site Attenuation Comparison as a Function of Turntable Size and Location, EMC ESD International Conference Proceedings, Denver, CO, April MIL-STD-42D, Measurement of Electromagnetic Interference Characteristics, January 1993.