A Complete Simulation of a Radiated Emission Test according to IEC

34 PIERS Proceedings, August 27-30, Prague, Czech Republic, 2007 A Complete Simulation of a Radiated Emission Test according to IEC 61000-4-20 X. T. I Ngu, A. Nothofer, D. W. P. Thomas, and C. Christopoulos University of Nottingham, United Kingdom Abstract In a radiated emission test according to IEC 61000-4-20 the radiated power of the equipment under test (EUT) is calculated from three measurements. The algorithm used to calculate this power assumes the EUT to be electrically small and have the radiation characteristics of a set of dipoles. Since this assumption is not always valid, the calculated radiated power by this method may be different from the total power radiated by the EUT. This work presents a complete simulation of a radiated emission test according to IEC 61000-4-20 using the TLM method. This is then compared to the total radiated power of the EUT in free space, space above ground plane and in the GTEM cell numerically. The differences in these power values are analyzed to predict a contribution to the uncertainty budget of the radiated emission test. 1. INTRODUCTION IEC 61000-4-20 [1] describes radiated emission measurements using a Transverse Electromagnetic (TEM) or Gigahertz Transverse Electromagnetic (GTEM) waveguide. To perform this test, the equipment under test (EUT) is placed inside the cell in three orientations and the voltages at the cell port are measured. From these three voltages, the power radiated by the EUT is calculated. To enable comparison to the Open Area Test Site (OATS) or a Fully Anechoic Chamber (FAC), the maximum field strength generated by the calculated power is determined above a ground plane and in free space respectively. This work is fully a numerical study where Transmission Line (TLM) [2] simulation method is used. A complete GTEM TLM model is created to study the errors and the construction of the TLM model is explained in Section 2. This paper presents a full 3D TLM simulation of three orientations radiated emissions test according to IEC 61000-4-20. The power radiated by the EUT is then calculated from the three voltages. This is described in Section 3. The total radiated power of the EUT in free space and above a ground plane is determined in separate simulations. In addition, the total radiated power of the EUT inside the GTEM cell is simulated. These simulations are described in Section 4. In Section 5, the results are presented and all these values for the radiated power are compared and the electric field strength that this power would produce is included for each scenario. These preliminary results will help EMC engineers establish an uncertainty budget in their measurements, since the difference in the calculated field strength is an indicator for the potential error caused by the correlation method described in IEC 61000-4-20. 2. GTEM TLM MODEL An important aspect in this work is the GTEM TLM model. A complete detail of the GTEM TLM model is explained in [3]; however some details will be outlined here. The TLM simulation involves the generation of a comprehensive simulation code to represent the geometry of the GTEM model. In this code, the cells in the TLM meshes were arranged as closely as possible to represent the real GTEM cell geometry. Each component in the GTEM model was constructed by joining cell by cell in three-dimensions. Model construction was based on GTEM 5407 manufactured by ETSI Lindgren, USA. An important feature that was included in the modeling of GTEM cell is the inclusive of a realistic sized pyramidal RAM model which has not been reported in other previous numerical models. To increase accuracy, we employ a large mesh in excess of 14 million (224 156 417) cells and one round of simulation took 70 hours to complete. We kept well within the λ/10 limit and also checked that staircasing errors are negligible. This model is fully electrically tested to comply with 50 ohm characteristic impedance. 3. IEC 61000-4-20 GTEM MEASUREMENT Various GTEM to OATS correlation algorithm [4 8] have been produced over the years. This paper concentrates on the correlation algorithm that is used in the IEC 61000-4-20 standard. Any source of radiation can be replaced by an equivalent multipole expansion which should result in the same radiation pattern and same power outside a volume enclosing the source. Once these multiple

Progress In Electromagnetics Research Symposium 2007, Prague, Czech Republic, August 27-30 35 moments are known, the radiation both in free space and over a ground plane can be calculated. The method specified in the standards is the three-measurement method. This three-measurement method requires three major orientations of an EUT inside the GTEM cell shown in Figure 1. In each orientation, the voltage at the cell port is measured. Based on these three voltages, the radiated power of the EUT, and from there the electric field over a ground plane, representing the correlation to OATS, can be calculated. In a similar way, the maximum field at a point in free space can also be calculated, representing the correlation to a fully anechoic chamber. V1 V3 V2 (a) (b) (c) Figure 1: EUT in the (a) first orientation (b) second orientation and (c) third orientation. 4. TOTAL RADIATED POWER Three different environments were considered in this work. These are inside the GTEM cell environment, free space and space above a ground plane. In this work, a realistic model of an EUT is represented by a monopole sitting on the top of a metal box shown Figure 2. The metal box itself is a 29 29 11 cm perfect electric conductor (PEC) with a 19 cm monopole sitting on top of the metal box. The monopole is located 12 cm from the center point of the top of the box. The method used is based on the simulation of the total radiated power [9] where a surface of a rectangular structure can be defined over a certain volume to give the total power passing through the assigned surface with the EUT within the volume shown in Figure 2. The purpose is to examine how this power changes in different environments. The frequency chosen for this work is 200 MHz. The input power for all the simulation setups was equally set to 0.02 V peak. Surface Monopole Metal box volume for total radiated power simulation Figure 2: Total radiated power by the EUT in a volume enclosing the EUT. 4.1. Free Space Radiation The radiation from the EUT is simulated in a free space environment. This result serves as a control to examine the actual total power radiated by the EUT itself without taking reflections into account. The result from this simulation can be used to calculate the maximum electric field E max for later comparison with the other two environments. This method simulates an EMC test performed using full anechoic chamber. 4.2. Space Radiation over the Ground Plane The EUT is then placed over a ground plane to simulate an EMC test performed using OATS. The total power radiated in this method takes the reflection by the image dipole introduced by the ground plane into account. The maximum electric field E max can also be calculated from the simulated result of this setup and later served as a comparison for results obtained from the GTEM to OATS correlation algorithm stated in IEC 61000-4-20.

36 PIERS Proceedings, August 27-30, Prague, Czech Republic, 2007 4.3. Radiation inside the GTEM Cell The radiation inside the GTEM cell is the main observation in this paper. For this, the EUT is placed inside the GTEM cell. In contrast to the other two methods mentioned above, the GTEM cell has unique characteristics in that it ideally only couples to the TEM modes. In addition, at the lower frequencies, the structure of the GTEM is within the near field of the EUT and this may alter the characteristics of the EUT. This is different from the simulation in Section 4 Part A and B because the total power radiated by the EUT inside the GTEM cell is not simulated over an enclosing surface, but it is calculated from the three voltages simulated from three orientations according to IEC 61000-4-20 as described in Section 3. 5. SIMULATION RESULTS AND COMPARISON Table 1 shows the comparison values of the different total radiated power in the different environments at 200 MHz. The EUT in free space has the lowest total radiated power compared to the EUT above a ground plane and also inside the GTEM cell. From Table 1 it can also be seen that the total power calculated from the three voltages from three respective orientations was greater than the total power radiated in the free space by the EUT. The comparison of various values of E max for vertical and horizontal polarizations calculated from the total radiated power in different environments is shown in Table 2 and Table 3. They show the maximum electric field strength at a distance of 3 m between the EUT and the antenna. For Table 2 the field strength was calculated above a ground plane representing OATS, and for Table 3 it was calculated in free space representing FAC. Table 4 shows the value of the total radiated power in the different environments when the above simulations were repeated using higher frequency such as 1 GHz in this case. In such case, the EUT was considered to be electrically large. Interestingly, at higher frequency, the radiated power predicted from the three voltages method seemed to underestimate the total radiated power obtained from other environments. Table 1: Comparison of total radiated in different environments at 200 MHz. Environments Calculated from three simulated voltages In free space In space above a ground plane Total radiated power (W) 3.324 10 7 2.902 10 7 3.178 10 7 Table 2: Comparison of maximum electric field E max in OATS calculated from total radiated power in different environments at 200 MHz. Total radiated power environments E max Horizontal (V/m) E max Vertical (V/m) Three simulated voltages 2.313 10 3 1.589 10 3 Space above a ground plane 2.261 10 3 1.554 10 3 Table 3: Comparison of maximum electric field E max in FAC calculated from total radiated power in different environments at 200 MHz. Total radiated power environments E max Horizontal (V/m) E max Vertical (V/m) Three simulated voltages 1.730 10 3 1.557 10 3 Free space 1.691 10 3 1.522 10 3 Table 4: Comparison of total radiated in different environments at 1 GHz. Environments Calculated from three simulated voltages In free space In space above a ground plane Total radiated power (W) 1.243 10 7 1.784 10 6 1.787 10 6

Progress In Electromagnetics Research Symposium 2007, Prague, Czech Republic, August 27-30 37 6. CONCLUSION Various references in the literature such as [10] and [11] present examples for uncertainty budgets in measurements done using a GTEM cell. The uncertainties may be due to mismatch to instrument errors and poor repeatability. However, most uncertainties reported are easily distributed to instrument errors, cable loss, EUT orientations, impedance matching and so forth. These uncertainties are well defined with a high level of confidence since they are parameters that can be measured and observed easily. However, the GTEM to OATS correlation is one particular uncertainty which is hard to determine during experiments since it is difficult to isolate the other uncertainties. We have shown here a method to define the contribution to the uncertainty budget due to the GTEM to OATS correlation. Simulation work could omit various parameters in real measurements, thus a particular uncertainty could be examined effectively which is the GTEM to OATS correlation in this case. The results gathered in this work would be useful in future examples of uncertainty budget calculations for the GTEM to OATS correlation. In this work, we have shown that the algorithm based on IEC 61000-4-20 for correlating three voltages from the three position method seems to agree reasonably well compared to the free space and above ground plane environment but, it over-predicts the total power radiated from EUT. However, at higher a frequency approaching 1 GHz in our case, the total power radiated obtained from the prediction based on the three voltage method seriously underestimated compared to other environments. The algorithm stated in IEC 61000-4-20 is ideal for the case when the EUT consists of a dipole antenna. The EUT chosen in this work is not a dipole on purpose in order to demonstrate the accuracy of the algorithm when the EUT is not a dipole. It appears that the differences cannot be simply due to normal random errors and more likely are due to systematic factors. The preliminary results shown here justify a further investigation to establish the origin of the differences and if possible eliminate them. One possible reason for the observed discrepancies is the omission of the phase information or the propagation of higher order modes, which at higher frequencies are likely to play a greater part. We hope to be able to report in more detail in the future on work in progress to quantify the impact of phase [12] on these measurements. Another interesting aspect of this work is the importance of using TLM in examining problems in EMC world. The GTEM TLM model in this work was delicately created using 3-D square mesh and has been proven to have high accuracy and useful in the characterization of emission in GTEM cell. REFERENCES 1. IEC 61000-4-20, Electromagnetic Compatibility (EMC) Part 4: Testing and measurement techniques, Section 20: Emission and immunity testing in transverse electromagnetic (TEM) waveguides, International Electrotechnical Commission, Geneva, Switzerland, 2003. 2. Christopoulos, C., The Transmission Line Modeling Method TLM, IEEE Press, Piscataway, NJ, 1995. 3. Ngu, X. T. I., A. Nothofer, D. W. P. Thomas, and C. Christopoulos, A complete model for simulating magnitude and phase of emissions from a DUT placed inside a GTEM cell, to appear in IEEE Transaction on Electromagnetic Compatibility. 4. Lee, A.-K., An algorithm for an advanced GTEM to ground plane correlation of radiated emission tests, IEEE Symposium on Electromagnetic Compatibility, 58 62, Santa Clara, CA, August 1996. 5. Osburn, J. D. M. and E. L. Bronaugh, Advances in GTEM to OATS correlation models, IEEE Symposium on Electromagnetic Compatibility, 95 98, Dallas, TX, August 1993. 6. Thelberg, M. J., GTEM to OATS emission correlation 1 5 GHz, Technical report, Chalmers University of Technology, Gothenburg, Sweden, 1993. 7. Thelberg, M. J., E. L. Bronaugh, and J. D. M. Osburn, GTEM to OATS emission correlation 1 5 GHz, IEEE International Symposium on Electromagnetic Compatibility, 387 392, Chicago, IL, August 1994. 8. Wilson, P., On correlating TEM cell and OATS emission measurements, IEEE Transaction on Electromagnetic Compatibility, Vol. EMC-37, 1 16, February 1995. 9. Paul, J., C. Christopoulos, and D. W. P. Thomas, A 3D time domain TLM electromagnetic field solver regsolve.cc, Regsolve Manual, Electromagnetics Research Group, School of Electrical and Electronic Engineering, University of Nottingham.

38 PIERS Proceedings, August 27-30, Prague, Czech Republic, 2007 10. Nothofer, A., D. Bozec, A. Marvin, M. Alexander, and L. McCormack, The use of GTEM cells for EMC measurements, Measurement Good Practice Guide, No. 65, National Physical Laboratory, York EMC Services Ltd, UK, 2003. 11. Harrington, T. E. and E. L. Bronaugh, EUT directivity and other uncertainty considerations for GHz-range use of TEM waveguides, IEEE International Symposium on Electromagnetic Compatibility, 117 122, Vol. 1, Montreal, QUE, August 2001. 12. Ngu, X. T. I., A. Nothofer, D. W. P. Thomas, and C. Christopoulos, The impact of phase in GTEM and TEM emission measurements, EMC Europe International Symposium on Electromagnetic Compatibility, Barcelona, September 2006.