GTEM cell
A GTEM cell ( G igahertz T ransverse E lectro m agnetic Cell) provides for tests and measurements for electromagnetic compatibility provides a defined environment. The GTEM cell is used to hold the measurement or test objects when measuring and testing radiated emitted interference or interference immunity according to IEC 61000-4-20.
The GTEM cells provide a measurement environment to record interference immunity or interference emissions, so that, for example, the requirements of the European directive on electromagnetic compatibility and the standards harmonized with it can be checked. The coaxial shape and the resulting operation in TEM mode ensure that a field wave resistance of approximately 377 ohms occurs when the test volume is applied . Depending on its size, the GTEM cell can be operated in the frequency range from 0 Hz to a few GHz in TEM mode .
Since the GTEM cell shields its test volume completely closed with metal, the test volume is electromagnetically decoupled from the environment. The GTEM cell can therefore be operated in normal rooms without having to pay attention to the special equipment of these laboratory rooms. For manufacturers of smaller products, this cell has an advantage due to its compact size compared to measurements in absorber rooms , which have a considerably larger space requirement.
Structure of a GTEM cell
The GTEM cell has a pyramid-like shape. Inside there is a plate (the septum) displaced from the center as an inner conductor, so that a coaxial conductor structure results. The GTEM cell has a connection for a coaxial lead at its tip . It expands from this connection. The inner conductor of the coaxial connection goes continuously into the septum of the GTEM cell, the outer conductor goes continuously from the connection point into the outer conductor of the GTEM cell.
The outer conductor of the GTEM cell has a rectangular cross section. Inside the cell, the flat septum is arranged so that a line impedance of 50 ohms is established and is maintained over the length of the GTEM cell. The value has been selected so that common laboratory measuring or testing devices with lines with a characteristic impedance of 50 ohms can also be operated reflection-free at the connection of the cell. The expansion of the GTEM cell continues until the inner conductor merges into the flat terminating resistor arrangement with the load resistance of also 50 ohms. The metal back wall of the GTEM cell opposite the connection is equipped with absorbers that prevent the reflection of an electromagnetic field wave.
The field wave resistance of ~ 377 ohms inside the GTEM cell is guaranteed in this arrangement by the fact that a transverse electromagnetic field spreads in the dielectric air inside the GTEM cell. The line impedance of 50 ohms is linked to the field impedance of the air via the geometric shape of the inner conductor and outer structure of the GTEM cell. Waveguide waves , which are higher-order modes , typically only develop in the GTEM cell in the gigahertz range, depending on their size. In the case of the Crawford cell , this is already the case at a few 100 MHz with a comparable size. In this respect, the GTEM cell is superior to the Crawford cell or TEM cell with comparable dimensions in terms of the usable frequency range for far field conditions.
Immunity tests with a GTEM cell
For immunity tests, an electrical signal that is applied to the connection of a GTEM cell leads to currents and voltages in the septum and the outer conductor structure. This creates a field between the septum and the shell of the GTEM cell in which electronic devices can be tested.
Interference emission measurements with a GTEM cell
The GTEM cell can also be used to measure field-related emissions from devices. The interference emissions (electrical, magnetic or electromagnetic fields) of a test object in the GTEM cell generate currents in the septum and envelope and voltages in between. These currents and voltages can be picked up by a suitable measuring device (for example a spectrum analyzer or a measuring receiver ) at the connection of the cell and made available for further processing.
Web links
- TU Dresden, Professorship TET and EMV
- PTB ( Memento of February 11, 2007 in the Internet Archive )