How to optimize the EMI test using the TEM cell

Article by: Edward Pan, Rigol Technology Co. Ltd

Amid the latest trends and developments in the electronics industry, how does the test and measurement industry keep pace? This month’s In Focus looks at the factors driving innovations and developments in T&M, new challenges and opportunities.

Generally speaking, traditional EMI radiation emission tests are usually performed in large shielding rooms, using diverse antennas to obtain radiated signals. Due to bandwidth limitations, multiple antennas are required to cover the entire frequency range. In addition, it requires a lot of space, and the cost of equipment for standard consistency parameters is higher than usual.

Engineers in small and medium-sized businesses typically need to draw on their experience and best practices to design EMC-compliant products. However, it is estimated that over 50% of products fail the test of the first test. Anytime an engineer sends a new product to a compliance testing lab. The cost of the failure rate is very high. Not only is the cost of new tests high, but the cost of adjustments in certified labs is also very expensive, typically charging up to $ 150 per hour, and project timing and marketing are delayed. It also causes a delay in announcements of new products.

TEM cell

What engineers need is a reasonably priced lab that can measure radiation emissions in their own lab before compliance testing. The TEM cell is the right device for desktop radiated emissions testing. Our R&D team recently developed an open TEM cell to cover the entire frequency range up to 2 GHz, and even at higher frequencies.

Figure 1: The radiation test structure using the TEM cell.

This TEM cell can be used with a spectrum analyzer to test products before and after EMC related design changes. Devices equipped with TEM batteries may not provide exactly the same quantitative results as measurements made in a certified test room, but this will be a good indication of whether the design has experienced too much radiated noise. The engineer will clearly see if his changes improve or degrade EMC performance, or if they stay the same. Using the TEM cell eliminates most of the unnecessary background interference.

The TEM cell is an in-line device used for radiated emission and immunity testing of electronic devices. It is not a substitution, but due to its size and cost, it is a practical alternative to measurements in a shielding room. The TEM cell consists of a diaphragm, a conductive strip in the central part and a grounded wall. The geometry is designed to present a 50Ω strip line. The device under test (DUT) is placed between the bottom wall and the septum. It has no side walls to facilitate the placement of the DUT. It can pick up RF background noise, but it can be taken into account by measuring the unit’s output signal before powering up the DUT.

Compared to standard TEM cells of similar size, general open TEM cells have a better frequency response. TEM cells suffer from the limitation of higher order wave modes, which limit the available bandwidth. The unique design function of the specially designed TEM unit realizes the resistance perpendicular to the desired wave propagation direction. Therefore, the higher order mode and resonance are suppressed. All TEM cells are supplied with 50Ω / 25W RF terminations to protect the spectrum analyzer DC module or RF receiver input and N-Male to N-Male coaxial cable.

Figure 2: Simulated schematic of the TEM cell device.

RIGOL EMI test solution with TEM cell device

Take the example of a medical pacemaker. Because this is a medical grade product, the requirements for EMI testing certification are particularly stringent. The operating frequency of most medical grade electronic instruments and equipment is very low, so it is necessary to strictly filter the background noise to observe the true EMI characteristics and performance. A TEM cell is therefore a good auxiliary tool. We placed the pacemaker motherboard in the TEM cell and connected one end of the TEM cell to the SMA receiving terminal of the RIGOL RSA5065 Real-Time Spectrum Analyzer, and the other end was connected to a terminator of 50 ohms, so that would not happen. The reflected noise in turn affects EMI testing. Then set the RSA5065 spectrum analyzer to the EMI option operating mode to start the measurement. We first scan the EMI traces without placing the TEM cell. The yellow Trace 1 result can be obtained.

And set the detection mode of peak detection, quasi-peak detection and average detection respectively, turn on the counter function, and then set the limit value of the limit line standard. After completing the first step of scanning, place the motherboard in the TEM cell, then open Trace 2 and rescan again to get the green Trace 2. According to the results of the digitization solution, it can be seen that after adding the TEM cell, the background noise part is obviously much reduced, which is the only contribution of the TEM cell.

Figure 3: RSA5065 spectrum analyzer with TEM cell for pacemaker EMI test.

Figure 4: EMI test result without TEM cell.

Figure 5: Comparison of EMI test results between with and without TEM cell.

About the Author

Edouard Pan is currently Senior Technical Marketing Director for the International Area of ​​Rigol Technology Co. Ltd. He has 20 years of experience in the field of electronic measurement technology. It specializes in digital communication, measurement of radio frequency microwave communications, certification of electromagnetic interference tests and logical verification of signal integrity. So far he has conducted over 100 technical seminars.

Edward holds an MA in Optoelectronic Engineering from National Central University and an MA in Business Administration from National Chengchi University. He is currently working on a doctorate at the Institute of Telecommunications Engineering, National Taiwan University.

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