A TEM or transverse electromagnetic cell is a type of
test chamber
An environmental chamber, also called a climatic chamber or climate chamber, is an enclosure used to test the effects of specified environmental conditions on biological items, industrial products, materials, and electronic devices and components ...
used to perform
electromagnetic compatibility (EMC) or
electromagnetic interference (EMI) testing. It allows for the creation of
far field electromagnetic fields in a small enclosed setting, or the detection of electromagnetic fields radiated within the chamber.
Description
A TEM cell is an enclosure acting as an electromagnetic
transducer
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and cont ...
that is shielded to provide isolation from external electromagnetic fields. Within the enclosure lies
conductive
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...
material, forming a section of transmission
stripline that can be connected to standard coaxial cables. The interior of the cell acts as a
waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
and converts electric signals into homogeneous electromagnetic fields with approximately
transverse mode
A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and microwav ...
distribution, similar to free space. The electric and magnetic field inside the cell can be accurately predicted using numerical methods. The original design of the cell is rectangular in shape, although many variations and improvements have been made since its creation in the early 1970's.
Principles of operation
The cell acts to either receive internal emissions or transmit emissions within the chamber.
* When measuring radiated emissions, one end of the stripline is connected to a
spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...
. The other end is terminated with an RF load.
* When performing radiated immunity, one end of the stripline is connected to a source of radiation (e.g. a
signal generator
A signal generator is one of a class of electronic devices that generates electrical signals with set properties of amplitude, frequency, and wave shape. These generated signals are used as a stimulus for electronic measurements, typically used i ...
). The other end is terminated with an RF load.
* In operation, emitted
radiation (whether from an antenna or from the
DUT) travels along the length of the chamber and is absorbed by the
absorbent load at the end.
* For immunity tests, the field uniformity and the cross-polar coupling of the cell have to be within certain limits set by
IEC
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
61000-4-20.
Variations
GTEM cell
A GTEM cell is a design variation of the TEM Cell that allows the cell to operate in the
Gigahertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
frequency range. The external enclosure forms a long rectangular base pyramid. The GTEM is terminated on a lined surface made of
radiation-absorbent material
In materials science, radiation-absorbent material, usually known as RAM, is a material which has been specially designed and shaped to absorb incident RF radiation (also known as non-ionising radiation), as effectively as possible, from as m ...
such as carbon-loaded foam, and absorbers line the side walls. A slightly spherical wave propagates from the source to the tapered waveguide, and since the solid opening angle is small, the undistorted spherical wave can be considered a plane wave. The termination load section uses absorbing material for an electromagnetic wave and a distributed resistive load for current termination. At low frequencies, it operates as a circuit 50 Ohm load; at high frequencies, the absorbers attenuate the incident waves as in an anechoic chamber, In this way, a termination from DC to several Gigahertz is achieved. Some restrictions and compromises limit the applications such as:
* The DUT dimension is limited to 1/3 – 1/2 of the internal cell height to maintain a safe distance that doesn't interfere with measurements of immunity and emissions ensuring uniformity of the EM-Field volume within +/-3dB or +/-5dB respectively.
* Surface resistivity of the conductive metal adopted in the GTEM case (
stainless steel it is not suitable in the GHz range) due to its surface resistivity being too high.
* Choice and quality of the type of the absorber material (carbon foam is inefficient under 80MHz, while a combination of hybrid absorbers is able to cover the frequency range from DC to over 20GHz)
* Increasing the operating frequency of the cell over GHz increases the percentage of non-transverse mode components (Not TEM Mode) that decrease the quality of the measure introducing complex and cross-polarization mode.
References
Bibliography
*Nothofer, A, Alexander, M J, Bozec, D, Marvin, A and McCormack, L
Measurement Good Practice Guide No. 65: The use of GTEM cells for EMC measurements National Physics laboratory, 2003. ISSN 1368-6550.
Using GTEM cells for immunity testing{dead link, date=December 2022 - Evaluation Engineering
Electromagnetic compatibility