EMC barriers : measurement methodologies and model verification
Abstract: The subject of Electromagnetic Compatibility (EMC) is introduced. A short overview of the regulatory EMC requirements in Europe and in the United States is presented. An explanation is given why it is important for people in the electronics business to understand the need for EMC regulations. Electromagnetic shielding is a often used as an electromagnetic barrier for attenuation of disturbances from radiating sources. An orientation over the concept of electromagnetic barriers is presented. Ways to characterize barriers are presented and measurement methods are reviewed. Results from transfer impedance measurements show that conductive gaskets made of tin coated stainless steel, can give as good shielding performance as gaskets made of beryllium copper in the entire frequency interval of the investigation. The results show that the difference in shielding performance with gaskets made of tin coated stainless steel and of beryllium copper can be insignificant compared to the influence of a very small change in contact pressure. The difference between gaskets with and without tin coating is small with beryllium copper as base material. With stainless steel as base material the difference is about 10dB at 2MHz and less at higher frequencies. A method to describe electromagnetic barriers such as filters, shielded cables and connectors etc. with circuits consisting of linear discrete components is presented. By comparing a barrier with a multi-conductor transmission line a lumped circuit can be constructed where component values are designated by the per-unit length transmission line parameters. For barriers that cannot be viewed as transmission lines a method for determining equivalent circuits outgoing from measured S-parameters has been developed. Different measurement fixtures were constructed in order to obtain accurate S-parameter values. Derived models have been used in SPICE simulations and validated by comparison with measurements. The current distributions on the surface of a printed log periodic dipole antenna (LPDA) have been investigated. A position scanned magnetic field probe was used to obtain values of the magnitude and phase of the magnetic field at each point of the scan. From the measurements the time variant instantaneous values of the magnetic field and the instantaneous currents on the LPDA were derived. The wave nature of the current distribution can be readily observed and problems with the design such as standing waves on the feeder lines are highlighted. Measured current distributions are compared with predicted distributions obtained from Method of Moments (MOM) analysis of the LPDA structure. Measured and predicted far field radiation patterns are also compared.
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