What is a TEM cell?

A TEM cell is an apparatus or a device that allows an EMC test engineer to measure the radiated field strength of a product (Radiation Emission test) as well as to conduct the Radiated immunity tests (i.e., subject a product to given electric field strength). It is a widely used cost-effective method for conducting EMI/EMS tests. It was originally developed by the U.S NBS lab in the 1970s. The TEM cell is suitable for conducting both the Radiation Emission test and Radiated susceptibility test of a smaller test object (the typical size for test objects is 15 × 5 × 10 cm). It is not a complete replacement for an anechoic chamber, but the size and cost of the TEM cell make it a convenient alternative to an anechoic chamber for conducting EMC tests of small products.

Geometry and construction of a TEM cell

TEM stands for ‘Transverse-Electromagnetic (Mode)”. The transverse mode of electromagnetic radiation represents a specific field pattern of radiation in the plane perpendicular (i.e., transverse) to the radiation’s propagation direction. A TEM cell is a rectangular-coaxial transmission line tapered at each end to form a geometry that is specifically designed to create the TEM mode over the given frequency region (typically from DC to a few hundred MHz). The TEM cell is constructed by gradually expanding the size of a coaxial transmission line to the dimensions that are large enough for a small EDT to be placed between the inner and outer conductors without significantly altering the properties of the line. 

Fig. 1 TEM cell

The geometrical view of the TEM cell is shown in figure 1. It is basically a tri-plate structure with one center conductor (septum) and two ground plates (outer shields) surrounding the septum. It also consists of two ports, one RF connector port, and the other port is terminated with a 50-ohm load. The RF connector is used to connect the TEM cell to either a spectrum analyzer or signal generator/power amplifier through a coax cable, depending on whether it’s being used for emissions or immunity testing. The DC block is used to protect the sensitive input to an analyzer just in case a DC voltage is accidentally applied to the septum.

The geometry determines the useful or operating frequency range of the TEM. Generally, if the geometry size is larger, then the upper usable frequency of the TEM cell will be lower. 

Uses of TEM cell:

1. Used for Radiated Emissions Pre-Compliance Testing

Fig. 2 Radiated emission test setup

The radiation emission test setup is shown in Figure 2. A DUT is placed between the septum and the outer conductor. When a TEM cell is used to measure the radiation emission of a DUT inside the cell, there will be abandoned electric power generated on both the terminals of the TEM cell by the radiation field. Now, the user can measure it by tuning the spectrum analyzer to the frequency range of interest.

2. Used for Radiated immunity testing:

Fig. 3 Radiated immunity test setup

The test setup for radiated immunity tests when using a TEM cell is shown in Figure 3. A signal generator or RF power generator and a power amplifier are connected with one port of the TEM cell, and a 50-ohm termination resistor is connected to the other port. A DUT is placed between the septum and the lower conductive plane. Whenever the RF power is applied through the input port, there will be electric field strength (E=V/d) developed across the septum and lower (or upper) plane. Here, V is the applied voltage of the signal, and d is the distance between the septum and lower (or upper) plane. The relation between the applied RF power and the field strength between the cell height is given below.

In radiated immunity tests, the TEM cell measures the value of path loss up to the DUT and analyzes the performance of the DUT when it is subjected to the electric field of specified amplitude (V/m) across the range of frequencies.  

• 3. Used for EMI Troubleshooting

• If a product of a company failed at an EMC test lab and the company decides to make modifications to pass the test standard, the TEM cell helps to test the effect of the modifications. The TEM will allow the company to do the EMC test in their office rather than use an expensive test lab. For example, it can be used for debugging PCB-level EMI issues.
  
  4. Used for component evaluation:

The TEM cell can help catch unscrupulous suppliers by analyzing the EMI performance of identical components from different vendors.

Specification of TEM cells:

Frequency range: Represents the operating frequency range of the TEM cell. Usually, it is in the range of MHz to GHz.

Septum height: Represents the distance between the septum (center conductor) and lower (or upper) plane. It is represented in mm.

Cell height: Represents the height of the TEM cell in mm.

Field Strength at Center of Cell: Represents the electric field strength at the center of the cell. It is represented in dB mV/meter at 1 mV input. 

TEM cell connector type: Represents the TEM cell connector type.

VSWR: Represents the voltage standing wave ratio of the TEM cell.

Maximum RF input power: Represents the maximum RF input power that can be handled by a TEM cell when conducting the immunity test. It is represented in Watts.

Return loss: It is the difference between forwarding power (from source to load) and reflected power (from load to source) in dB.

Transmission loss: Represents the power loss in the TEM cells. It is represented in dB

Third-order intermodulation: Represents the signal strength of third-order intermodulation products in dBc. 

Overall dimension: Represents the overall dimension of the TEM cell. It is represented in (W) × (D) × (H) mm.

EMC Directory has listed TEM / GTEM Cells from the leading manufacturers. Use the parametric search tools to narrow down on products based on your requirement. Once you find parts that meet your requirement, view product details, download the datasheet, compare products and request quotations. Inquiries sent via EMC Directory are directed to the manufacturers who get back to you with a quote or information.