Engineering:GTEM cell

A GTEM or gigahertz transverse electromagnetic cell is a type of electromagnetic compatibility (EMC) test chamber used for radiated EMC testing.

Description

The GTEM is an alternative EMC site (than SAC = Semi-anechoic chamber, FAC = fully anechoic chamber or OATS = Open Area Test Site) to perform emission and immunity tests correlated at "Far-field" conditions according to the requirements of the standard EN 61000-4-20. A GTEM cell provides homogeneous electromagnetic fields with TEM (Transverse electromagnetic wave mode) distribution, similar to the free space. The electric and magnetic field inside the cell can be accurately predicted using numerical methods. A GTEM cell external enclosure is made of conductive material such as metal, in the shape of a long, rectangular base pyramid. The pyramid is normally laid flat on the major side (bottom), although occasionally it may be stood on its base. Basically, the GTEM cell is a tapered rectangular 50 Ohm transmission stripline closed on a 50 Ohm load where the inner is called SEPTUM. The GTEM cell begins with a precision CNC craft transition: APEX, where a standard 50 Ohm coaxial connector (7/16", N, or SMA type) to the asymmetric rectangular waveguide is done. The GTEM is terminated on a lined surface made of radiation-absorbent material (RAM) such as carbon foam loaded; metal oxides multilayers hybrid absorbers with ferrite tiles with the side walls able to act as a waveguide. A slightly spherical wave propagates from the APEX source to the loaded tapered waveguide, since the solid opening angle is small, the undistorted spherical wave can be considered as a plane wave. To minimize phase delays that may occur in big GTEM, the termination surface of the absorbers is shaped as a spherical sector. 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. Theoretically, the GTEM is a broadband device able to operate from DC to Microwaves with some restrictions and compromises that conditioning the applications such as:

• The EUT (Equipment Under Test) dimension depends on the internal height H of the cell under the Septum that it is reduced at 1/3-1/2 H to maintain a safe distance that doesn't interfere with the measurements
• Surface resistivity of the conductive metal adopted in the Gtem case (i.e. the inox steel, considered a bad conductor: it is not suitable over the GHz);
• Choice and quality of the type of the absorbers RAM (Carbon foam: is inefficient under 80 Mhz), a combination of Hybrid absorbers: Ferrite matched with carbon-foam absorbers or ferrite with multi-layers metal oxides pyramidal absorbers are able to cover all the frequency range from DC to over 20GHz!),
• Increasing the operating frequencies appears a reduction of the % TEM dominance mode with the introduction of NOT TEM mode components.

The main goals of the GTEM against Semi- and full anechoic chambers are:

• A compact solution at a low cost
• Tests performed are always in Far-field conditions with a plane wave propagation starting from DC frequency (impossible with Anechoic chambers!)
• The GTEM acts as a unique real wideband antenna. you will forget the tedious changement of antennas for each frequency band.
• Small amplifiers are necessary to produce the same field levels with a significant economic gain.
• Test process Speedier and, simplified with a significant gain of time.

Principles of operation

The GTEM cell forms an enclosed TEM (transverse electromagnetic mode) stripline, which acts as receiving emissions or transmitting emissions antenna:

• When measuring radiated emissions, one end of the stripline is connected to a spectrum analyzer. The other end is terminated with an RF load (e.g., 50Ω).
• When performing radiated immunity, one end of the stripline is connected to a source of radiation (signal generator). The other end is terminated with an RF load.
• Under test operation, the D.U.T. (Device Under Test) is placed in the test volume in between the septum and the cell floor. According to the standard EN61000-4-20, the test volume zone is considered as one cube of 1/3 up to 1/2 of the septum height than the floor of the GTEM. Within the test volume, the TEM (Transverse Electromagnetic) field is considered uniform with an uncertainty of +/- 3dB or +/-5dB respectively.
• In operation, emitted radiation (whether from an antenna or from the equipment under test) 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 61000-4-20.

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 - Evaluation Engineering