Open TEM Cells Ease EMC Testing of Large Devices Stephan Gerlach and Juergen Strohal Introduction Designers use transverse electro-magnetic (TEM) cells for radiated emission and susceptibility testing. This assures their integrated circuits and electronic modules are in compliance with EMC (electromagnetic compatibility) rules. This article describes an open TEM cell construction designers can use for their own pre-compliance measurements to achieve EMC performance improvements. Atmel® has developed an open TEM cell to perform these tests on larger DUTs (device under test). The cell can test Automotive Compilation Vol. 10 devices up to 30 x 25cm. It requires low RF power during immunity tests that generate field strengths up to 200V/m. The TEM cell consists of inexpensive dual-layer PCB material (FR4). Each end has N-type RF connectors, and typically one side terminates with a high-power 50Ω load (figure 1). Atmel engineers optimized the TEM cell dimensions to achieve a precise 50Ω design with low reflection coefficients. The internal fields exhibit high homogeneity at a frequency range between DC to 1GHz, and are usable up to 3 GHz. The uniform RF fields provide maximum field strengths with the available RF input power. Atmel has performed validation measurements to ensure proper function of the cell. 48 Open TEM Cell Evaluation and Verification Test Board / DUT Power Supply Test Monitor RF Input 50 Ohm Termination Port A Port B A Rohde & Schwarz ZVM network analyzer evaluated the S parameter and reflection coefficients of the cell (see figures 2, 3, 4). The test used a Narda 520 broadband field meter with an EF0391 E-field probe. This probe is rated at 100 kHz to 3GHz and is used to perform the E-field measurements inside and outside of the TEM cell. Septum CH1 S11 dB MAG 5 dB/ REF -50 dB -7.1054 dB 2.8 GHz Figure 1. Cross Section of the Atmel Open TEM Cell The TEM Cell Benefits of an Open-Cell Design A TEM cell is similar to a coaxial cable, just with a remarkably larger size. There are both open and closed TEM cells. One disadvantage of the closed type is that the E-field orientation changes from vertical in the center to horizontal at the corners of the dividing septum. Although the sum of the vertical and horizontal vectors is always constant, the E-field's influence on the DUT is not constant. The usable area is limited to 1/3 of the septum area. With an open TEM cell the exposure to the E-field orientation is fixed. The usable surface area of an open TEM cell is much bigger. This is very beneficial for larger DUTs such as an LCD display. Another advantage of an open-cell design is its large open area. Designers can insert bigger devices without problems caused by cable connections on various sides of the DUT. A closed TEM cell hampers the insertion of larger DUTs with cables and also creates severe inner-field non-uniformity that affects the homogeneity and repeatability of the measurements. Because the overall dimensions determine the cell's cut-off frequency, the TEM cell size limits the usable frequency range. Designers can still use the cell at a higher cut-off frequency, but this increases the uncertainty regarding the usable inner area. A scale is given in figure 6. The N-type connectors provide the best trade-off between mechanical robustness and stability, while giving good electrical performance in the upper frequency range. CAL OFS 5 dB/ CPL FIL 10k -50 dB START 10 kHz Date: 200 MHz/ 18.JAN.13 STOP 3 GHz 10:16:25 Figure 2. Measurement 1 (S11, 10kHz to 3GHz, Magnitude in dB, Port 2 Terminated with 50Ω) Impedance Matching is Better Than 12dB up to 1GHz CH1 S11 LIN SWR 200 mU/ REF 2 U 3 U 2.8 GHz CAL OFS 200 mU/ 〈 2 U CPL FIL 10k 1 U START Date: 10 kHz 18.JAN.13 200 MHz/ STOP 3 GHz 10:18:22 Figure 3. Measurement 2 (S11 VSWR, 10kHz to 3GHz, Port 2 Terminated with 50Ω) VSWR is Better Than 2:1 up to 3GHz with One Exception at 2.85GHz 49 © 2013 / www.atmel.com CH1 S11 16 ↑1 U 1 14 0.5 12 5 E-Field (V/m) 2 CAL P2 8 P3 0.2 0.5 1 2 5 P4 6 P5 4 OFS 0 P1 10 2 10 0 CPL 1 1.5 2 -5 E-Field Measurements Outside of the TEM Cell -1 10 kHz 18.JAN.13 3.5 -2 -0.5 Date: 3 Figure 7. Measurement 5 1GHz to 3.3GHz, at the Test Points of Figure 5 FIL 10k START 2.5 Frequency (GHz) STOP 3 GHz 10:17:22 Figure 4. Measurement 3 (S11, 10kHz to 3GHz, Port 2 Terminated with 50Ω) The Smith Chart is Well Centered Around 50Ω Safety Distance If you apply 6W input power the cell will create field strengths of up to 200V/m. You should ensure a safety distance of at least 200mm around the TEM cell. E-Field Measurement Inside the TEM Cell Atmel engineers used a Narda 520 broadband field meter for the measurement of the five test points at each side of the dividing septum inside the cell (figure 4). The applied power level is +13dBm. The cell is terminated with a 50Ω load. The results were taken over two frequency ranges (figures 6 and 7). 4 Atmel engineers conducted two measurements with two different safety distances to verify this recommendation (figure 8). Measurement 1 was made at a distance of 40mm to the septum, measurement 2 at 80mm. The input power was 13dBm, applied over a frequency of 100kHz to 3GHz. 5 1 2 4 Distance 1: 40mm Figure 5. E-Field Measurement Test Point Location within TEM Cell Distance 2: 80mm Figure 8. E-Field Measurement Points of Safety Distances 16 7 12 P1 10 P2 P3 8 P4 6 P5 4 2 0 0.0001 6 E-Field (V/m) E-Field (V/m) 14 5 4 Distance 1 3 Distance 2 2 1 0.001 0.01 0.1 Frequency (GHz) Figure 6. Measurement 4 100kHz to 1GHz, at the Test Points of Figure 5 Automotive Compilation Vol. 10 1 0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 Frequency (GHz) Figure 9. Measurement 6 E-Field (V/m) Over Frequency (100kHz to 3GHz) at the Safety Distances of Figure 8 50 The measurements prove that field strengths at a safety distance of 80mm are 5 to 10 times lower than those within the cell (figure 9). Nevertheless, we recommend a safety distance of at least 200mm during measurements when field strengths up to 200V/m. Test engineers should carry out their own evaluation to potentially further increase the safety distance if necessary. Technical Parameters and Dimensions The TEM cell specifications: • Conversion factor: 0dBm (1mW) 2.5V/m P/mW P/dBm E/V/m 0.32 -5 1.375 1.26 1 2.75 5.01 7 5.5 19.95 13 11 79.43 19 22 316.23 25 44 1258.93 31 88 5011.87 37 176 19952.62 43 352 • Example: 38dBm (6.31W) 200V/m • Dimensions: 100cm (L) x 35cm (W) x 22cm (H) Table 1. Power Conversion Table and Resultant Field Strength • Material: FR4 double-side plated • Connectors: N-type References Open TEM Cell—Power Conversion The open TEM cell provides a good conversion of RF input power to the measured E-field strength (see figure 10). The tabulated results expressing power both in mW and dBm come in handy when setting up test protocols (table 1).  Sandee M. Satav, Vivek Agarwal “Do-it-Yourself Fabrication of an Open TEM Cell for EMC Pre-compliance”, IEEE Transactions on Electromagnetic Compatibility, 2008 400 350 E-Field (V/m)  M.L. Crawford, “Generation of standard electromagnetic fields using TEM transmission cells,” IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-16, pp. 189 – 195, Nov. 1974 300 250 200 E/V/M 150 100 50  Shaowei Deng, Todd Hubing, Daryl G. Beetner, “Characterizing the Electric Field Coupling from IC Heatsink Structures to External Cables Using TEM Cell Measurements”, IEEE Transactions on Electromagnetic Compatibility, Vol. 49, No 4, Nov. 2007 0 -20 0 20 40 60 Power (dBm) Figure 10. Electrical Field Strength (V/m) vs. Input Power (dBm) 51 © 2013 / www.atmel.com Atmel Corporation 1600 Technology Drive, San Jose, CA 95110 USA T : (+1)(408) 441. 0311 F : (+1)(408) 436. 4200 | www.atmel.com © 2013 Atmel Corporation. / Rev.: Atmel-9025A-Auto-Compilation_E_A4_102013 Atmel®, Atmel logo and combinations thereof, and others are registered trademarks or trademarks of Atmel Corporation or its subsidiaries. Other terms and product names may be trademarks of others. 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