IEC 61726:1999 pdf download – Cable assemblies, cables, connectors andpassive microwave components – screening attenuation measurement by the reverberation chamber method

IEC 61726:1999 pdf download – Cable assemblies, cables, connectors andpassive microwave components – screening attenuation measurement by the reverberation chamber method.
IEC 61726 describes the measurement of screening attenuation by the reverberation chamber test method. sometimes named mode stirred chamber, suitable for virtually any type of microwave component and having no theoretical upper frequency limit. It is only limited toward low frequencies due to the size of the test equipment, which Is frequency dependent and Is only one ot several methods of measuring screening attenuation.
For the purpose of this standard. examples of microwave components are waveguides, phase shifters. diplexers/multiplexers, power dividerscombiners etc.
2 BasIc description of the reverberation chamber method
The reverberation chamber method for measurement of the screening attenuation of microwave components Consists 01 exposing the device under test (DUT) to an almost homogeneous and isotro9ic electromagnetic field and then measuring the signal level induced into the device.
These conditions are achieved by the use of a shielded enclosure, which acts as an oversized cavity (In terms of wavelength), with a high quality factor. Its boundary conditions are continuously agitated by a rotating reflective surface (mode stirrer), mounted within the chamber, which enables the field to approach homogeneous and isotropic conditions during one revolution.
Electromagnetic power is fed to the chamber by means of an input or transmitting antenna.
The strength of the field inside the chamber is measured through a reference antenna. The ratio of the Injected power (Input antenna) to the received power (reference antenna) Is the insertion loss of the cavity. The insertion loss is strongly frequency dependent and is also dependent on the quality factor of the cavity.
It has been shown that, due to the isotropic field, any antenna placed inside the cavity behaves as if its gain was unity (IJI), therefore no directional effect is to be expected. If the device under test is electrically short, its screening attenuation will be directly related to usual transfer parameters (Z1 and Zt). II the device under test is not electrically short the screening atlenuation may still be related to 2 and Z1 in some simple cases (evenly distributed leakage. periodically distributed leakage) using summing functions derived from antenna network theory.
4 DescrIption of the test set-up
4.1 Reverberation chamber
The reverberation chamber is a shielded enclosure having any shape, provided that its smallest dimension exceeds three wavelengths at the lowest test frequency. A perfect cubic shape should be avoided for optimum performance at lower frequencies. It shall be made of conductive materials (copper, aluminium o steel) and shall not contain lossy materials.
The upper frequency limit depends only on the screening attenuation, which shall exceed 60 dB for the whole frequency range- However, this value is not critical if the spectrum analyzer and the connecting devices of the test set-up are sufficiently screened and If the quality factor of the cavity remains sufficiently high. The quality factor shall be checked to verify that during one revolution of the mode stirrer the ratio between the maximum and the minimum power at the output of the reference antenna exceeds 20 dB. As a minimum, the test chamber and the test instrumentation shall have a combined screening attenuation at each test frequency that is ID dB greater than the screening requirement of the DUT.
The shielded enclosure is drilled with four coaxial feed-throughs: two for the output of the antennas and Iwo for the output of the measuring loop. For furl her details see annex A.
4.2 Mode stirrer
The mode stirrer shall be large with respect to wavelength and be bent at angles to the walls of the chamber. The mode stirrer shall be at least two wavelengths from tip to tip at the lowest test frequency. An example of a mode stirrer is described in annex B
4.3 Antennas
The reverberation chamber Is equipped with Input and reference antennas. Both antennas shall present limited resonances in the frequency range and shall not introduce losses; their return loss shall be better than B dB.