Termination Effects on Attenuation Properties of Ferrites Installed Around Cable Conductors

A Ferrite installed around an electrical conductor attenuates the flow of RF power. The ferrite suppresses the flow of RF power at specific frequencies due to its inductive and core loss characteristics. The core loss is the heat dissipated within the ferrite and is represented as an equivalent resistance. A new technique that accurately measures the electrical properties of ferrites using a RF Current Probe Calibration Fixture (CPCF) is described. RF amplitude attenuation and phase angle properties for several ferrites were investigated for different precision terminations connected to the CPCF. Radiated emissions measurements were recorded for a product using a RJ45 cable having different terminations while operating both, with and without ferrites. The RF attenuation data obtained using the CPCF measurements and the product measurements were compared and demonstrated good agreement. Variations in the cable terminations significantly affect the RF attenuation provided by a ferrite. The relevance of these termination variations on actual product operation is discussed.