GTEM

A GTEM cell ( Gigahertz Transverse Electromagnetic Cell ) provides a defined environment for tests and measurements for electromagnetic compatibility. The GTEM cell is used to accommodate the measuring or test objects in the measurement and testing of radiated emission or immunity according to IEC 61000-4-20.

The GTEM cells provide a measurement environment, to detect interference immunity or interference emissions, so that, for example, requirements of the European Electromagnetic Compatibility Directive and the hereby harmonized standards can be tested. The koaxialähnliche form and thus given operating in the TEM mode make sure that upon the application of the test volume approximately adjusts a characteristic impedance of 377 ohms. The GTEM cell can be operated depending on their size in the frequency range from 0 Hz to a few GHz in the TEM mode.

Since the GTEM cell metallic shields her test volume completely closed, the test volume is electromagnetically decoupled from the environment. Therefore, the GTEM cell can be operated in normal rooms, without the need to focus on a special equipment of these laboratories. For manufacturers of products smaller design, this cell has an advantage over the measurement in anechoic chambers, which have a significantly larger amount of space due to their compact size.

Construction of a GTEM cell

The GTEM cell has a pyramid- like shape. Inside is a shifted from the middle plate ( septum ) as the inner conductor, so that there is a coaxial conductor structure. The GTEM cell has at its head a connector for a coaxial cable. From this connection it widens. The inner conductor of the coaxial connection is continuously into the septum of the GTEM cell, the outer conductor is continuous over the connection point in the outer conductor of the GTEM cell. The outer conductor of the GTEM cell has a rectangular cross section. Within the cell, the flat septum is arranged so that a characteristic impedance of 50 ohms and is adjusted is maintained over the length of the GTEM cell. The value is chosen so that standard laboratory measurement or test equipment with cables with a characteristic impedance of 50 ohms also free of reflection can be operated at the terminal of the cell. The expansion of the GTEM cell continues continuously until the inner conductor passes into the flat designed terminating resistor arrangement with the load resistance of 50 ohms also. The connecting opposite metal back panel of the GTEM cell is equipped with absorbers, which prevent the reflection of electromagnetic field wave.

The wave impedance of ~ 377 ohms within the GTEM cell is ensured in this arrangement that a transversalelektromagnetisches field in the dielectric spreads air inside the GTEM cell. The characteristic impedance of 50 ohms is linked via the geometric shape of the inner conductor and the outer structure of the GTEM cell with the field impedance of the air. Waveguide waves, which are higher-order modes, are formed in the GTEM cell depending on their size typically only in the gigahertz range. In Crawford cell is the case with a comparable size already at a few 100 MHz. As such, the GTEM cell is superior to comparable dimensions with respect to the usable frequency range for far-field conditions, the Crawford cell or TEM cell.

Immunity with a GTEM cell

For immunity leads an electrical signal which is applied to the connection of a GTEM cell to currents and voltages in the septum and the outer conductor structure. It arises between the septum and shell of the GTEM cell is a field in which can be tested electronic devices.

EMI measurements with a GTEM cell

The GTEM cell can also be field-conducted interference emissions emanating from devices that measure. The Disturbances ( electric, magnetic or electromagnetic fields ) of a specimen in the GTEM cell produce in septum and shell currents and voltages in between. These currents and voltages can be a suitable instrument (eg a spectrum analyzer or measuring receiver ) Remove the terminal of the cell and provide for further processing.