Figure 1: TEM cell geometry (Open type TEM cell shown)

A TEM cell (Transverse electromagnetic cell) is a kind of test chamber used to perform EMC (electromagnetic compatibility) or EMI (electromagnetic interference) testing of small-size electronic products such as wireless pagers, receivers, portable phones, ICs, etc. It is also used for the calibration of RF probes and for biomedical experiments. The cell allows an engineer to perform radiated emission measurement testing as well as immunity testing on electronic products.  

The geometrical view of the TEM cell is shown in the above Figure. It is basically a tri-plate structure that consists of a septum (conductive plate in the middle) and two outer shield plates connected to the ground. The Cell features two ports, one RF connector port, and the other port is terminated with a 50-ohm load.  Depending on whether the cell is being used for emissions or immunity testing, the RF connector helps to connect the TEM cell to either a spectrum analyzer or signal generator/power amplifier through a coax cable.   Electrically, the cell is equivalent to a rectangular-coaxial transmission line tapered at each end to adapt to standard coaxial connectors

The device under test (DUT) is placed between the bottom of the septum and the outer plate. The TEM can generate accurate standard electromagnetic waves over a wide frequency range: DC (0 Hz) to a few GHz while EMC immunity testing on DUT. The cell design establishes fairly uniform electric field lines between the septum and the outer plates. The radiation emitted from the DUT can also be measured by using an EMI receiver/spectrum analyzer connected to the RF port of the cell.   We can consider that conducting an EMC test inside the TEM cell is the conducting test inside a coax cable (i.e., DUT is placed between the inner conductor and outer shield).

The geometry of the TEM cell is specifically designed to create the TEM mode (Transverse electromagnetic mode). TEM mode means that the electromagnetic wave traveling through the cell is similar to that one produced far away from an antenna (i.e., far-field conditions) and has essentially free-space impedance (377 ohms), and has the same characteristics as a plane wave. Therefore measurements made using a TEM cell have a close approximation to measurements made to a far-field plane propagating in free space.

The TEM cells can be used to test the product according to standards such as  IEC 61000-4-20, IEC 61967-2, IEC 61967-8, IEC 62132-8, ISO 11452-3, IEEE 1309,..etc. They are used in compliance testing, pre-compliance testing, EMC/EMI troubleshooting, and component evaluation.

TEM cell types:

There are two types of TEM cells: open-type TEM cells and closed-TEM cells.

An open TEM cell design does not have any side walls (Figure 1&2). The key advantages of open-type TEM cells include handling of DUT is easy, lightweight, and low-cost, and no filters are required for cabling entering/exiting the TEM. The key drawback of this type of cell is open to ambient noise.

 A closed TEM cell design has metal side walls (figure 3). It reduces/eliminates ambient noise.  The cell has a shielded door at the side that provides access to insert the DUT and the door may be equipped with a shielded glass window to observe the DUT. The electrical connection to the DUT is provided via feed-through filters. The drawbacks of this type of cell are DUT size is restricted to door size, and getting cables out of the TEM is a challenge and costlier than open-TEM cells.  

Figure 4: Shield tent

Note: It’s also possible to use an open TEM within a shielded tent or shielded bag (figure 4) instead of a closed TEM. The combined cost of a shielded tent + open TEM cell is lower than the cost of a closed TEM cell.

TEM cell uses:

Used for Radiated Emissions Testing

Figure 5: Radiated emission test setup

The radiation emission test setup is shown in the Figure 5. A DUT is placed between the region of the septum and the lower plate and the DUT is switched ON. Electric power will be generated on both terminals of the TEM Cell by the radiation field of the DUT. Now, the testing engineer can measure it by tuning the spectrum analyzer/EMI receiver to the frequency range of interest.

Used for Radiated immunity testing:

Figure 6: Radiated immunity test setup

The test setup for the Radiated immunity test is shown in the Figure 6. A signal generator and a power amplifier are connected to the RF port of the TEM cell, and the other port is terminated with a 50- resistor.  A DUT is placed between the region of the septum and the lower plate and the DUT is switched ON. When the RF power is applied via the RF port, there will be uniform electric field strength (E=V/d) developed across the septum and lower (or upper) plate. Here, V - applied signal voltage, and d - distance between the septum and the lower (or upper) plate.

In the radiated immunity test, the performance of the DUT when it is subjected to the electric field (E) of specified amplitude (V/m) across the range of frequencies is analyzed.   

TEM cell used for EMI Troubleshooting

If a product of a manufacturer fails at an EMC test lab then the manufacturer will decide to make modifications to the product design in order to pass the EMC test standard. The TEM is a cost-effective tool to test the effect of the modifications (related EMC/EMI performance of the product) at their office rather than use an expensive test lab. For example, the TEM cell can be used for debugging the PCB level EMI issues.

Figure 7: Debugging PCB level issues

TEM cell used for component evaluation:

The TEM cell helps to identify unscrupulous/unethical suppliers by analyzing the EMI performance of identical components from different vendors.

The other uses of TEM cells include monitoring ongoing compliance after component changes, isotropic field probe calibration, and shielding effectiveness testing.

Advantages of TEM cells: 

• Small size
• Cost-effective tool for EMC pre-compliance testing
• Since the TEM cell itself functions as a receiving structure while the emission measurement test and also itself creates the EM field for the immunity test, no antenna setup is required.
• Easy to setup & use

TEM cell limitations: 

Traditional TEM cell has a limit on useable operating frequency range due to its geometry. In the TEM cell, beyond certain operating (called cut-off frequency), higher order modes can be excited, which disturbs the TEM wave propagation in the TEM cell. It will affect the measurement results. The higher order modes in the TEM cell are caused by the transitions between the rectangular and the tapered part of the TEM cell.

Figure 8: Red circle shows transitions between the rectangular and the tapered part of the TEM cell

Also, the useful operating frequency range of the TEM is determined by its dimension. The dimension of the TEM cell is inversely proportional to the operating frequency. For higher frequency ranges (e.g., several GHz range), the TEM cell will have a very small dimension (e.g., less than 15 cm), which is not suitable for placing a DUT.

To overcome these drawbacks of TEM cells, the GTEM cell has been developed. The GTEM cell is the frequency-extended version of the TEM cell. The TEM cell operating frequency range is typically DC to 6 GHz, and a GTEM cell operating frequency range is typically DC to 20 GHz.

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