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Questions to Ask Before Purchasing a TEM Cell or GTEM Cell

When it comes to conducting electromagnetic compatibility (EMC) testing, TEM (Transverse ElectroMagnetic) cells and GTEM (GigaHertz Transverse ElectroMagnetic) cells are widely used for their ability to generate controlled and repeatable test environments. These test cells are cost-effective solutions for performing RF emissions or RF Immunity. Other applications include probe calibration, E-Field and magnetic field measurements, sensor calibration, and level comparisons. Both options are nice because the measurement transducer is the device itself. No additional probes/antennas are needed. However, before investing in a TEM cell or GTEM cell, asking the right questions is crucial to ensure you select the most suitable solution for your specific testing requirements. This article presents a comprehensive list of questions to consider before purchasing a TEM or GTEM cell.

First, one must understand the major difference between the two to understand which fits your application best.

TEM cells are essentially coaxial transmission lines that generate a transverse electromagnetic field. They consist of a central conductor and a concentric outer shield, separated by a dielectric material. The inner conductor serves as the signal conductor, while the outer shield acts as the ground reference. By applying appropriate voltage and current to the inner conductor, TEM cells create a uniform and controlled electromagnetic field within the test volume. TEM cells are good for smaller items and in lower frequency ranges. All TEM cells operate from DC up to a top frequency based on the physical size. The larger the TEM, the lower the frequency range it operates up to. There are TEM cells operating to 6GHz or even higher. These are small and good for testing chips/PCB. For more working area, larger TEMs are required, and many only operate up to 200MHz, 500MHz, or 1000MHz.

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Figure 1: Open TEM 500 (500MHz)

 

Figure 1 shows an Open TEM Cell where you can see and have access to the test area. However, it being open allows it to have more possible effects from ambient RF for emissions and has no shielding for immunity testing.

A standard TEM cell is closed and shielded.

Figure 2: TEM 1000 (1000MHz)

 

The equipment under test (EUT) is placed between the septum (center metal plate) and outer shell. It can sit on top or below the septum. One side is normally terminated with a 50Ohm load, and the other is the measurement point for emissions or the injection point for immunity testing.

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GTEM cells, also known as gigahertz transverse electromagnetic cells, are a modern variation of TEM cells. They feature a tapered or wedge-shaped design, with the cross-sectional area gradually increasing from the narrowest point toward the test volume. This unique structure allows for a broader frequency range and improved performance compared to traditional TEM cells. It operates from DC to 20GHz. It can come in many sizes, from tabletop to ones you can stand inside of. The size and frequency range gives the GTEM cell much better versatility. The trade-off is a higher cost. It can also be used for larger EUTs. Because the septum is on an angle giving more working volume and terminated internally to 50 ohms, anechoic cones are used to absorb and stop reflections on the back wall.

Figure 3: GTEM 1000 (septum is 1m)

 

TEM and GTEM cells are incredibly useful for design engineers to test and check their creations throughout the development process. This saves cost and reduces time to market in meeting the final EMC requirements.

What are the specific testing requirements?

You may be trying to perform pre-compliant measurements according to a standard, or you might have an application to test your product’s capabilities and performance for internal standards. Knowing what frequencies or ranges you want to test is essential to selecting the correct device. If performing immunity testing, it is also important to know what field level you will need to reach and, therefore, the required input power.

What is the required test volume?

The EUT size will determine what size cell you may need. The rule of thumb constraint is the EUT should take up ~1/3 or less of the area under the septum. Many times the cells will be specified with a test volume. If your EUT is larger than the test volume, it is still possible to use the device. The results can become less predictable, but the system will still produce repeatable results.

What filters or penetrations are required for your device?

TEM devices can come with a wide variety of filters and shield penetrations to fit your needs. You need to know if your EUT needs AC or DC power and what size filters for the correct voltage and current. It is best to pass all wires through filters to suppress the influences affecting each other from inside to outside the cell. Common filters are USB, Ethernet, DC, and AC power. Additional penetrations for Coax (type N or SMA), Fiberoptic lines, or a general-purpose waveguide pipe are available.

What is the test cell’s termination rating?

A TEM cell is terminated externally with a coaxial termination, which can be purchased to match the application for frequency and power. The GTEM has the termination built into the cell and will have a preset power rating. Selecting a GTEM with a high power rating for future growth and testing is best. Also, remember that power may be limited by the coax cable connector.

 

Selecting the right TEM or GTEM cell for your EMC testing requires careful consideration of various factors. By asking these essential questions before purchasing, you can ensure that the chosen cell aligns with your testing requirements, complies with relevant standards, offers the necessary features, and provides reliable and repeatable results. A well-informed decision will contribute to efficient and accurate EMC testing processes, helping you achieve compliance and product reliability.

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