The wireless communications' revolution has led to the proliferation of handheld, wireless devices, such as mobile phones freeing the user from the cord. Ceramic materials, which can serve as dielectric resonators storing and transferring microwave communication signals, have a crucial role to play in this revolution. Very few ceramics possess the essential properties for forming thermally and mechanically stable dielectric resonators with quality factors comparable to cavity resonators. Furthermore, low loss materials are highly desirable for frequency tuneable dielectric resonator filters used in the base stations of mobile communication systems. By modifying the electric field of the dielectric resonator, their resonating frequency can be adjusted. To broaden their use in mobile communication systems, researchers at the University of South Bank in London evaluated different methods for rapidly tuning dielectric resonator filters. In an attempt to improve the tuning ability of dielectric resonators, they devised an innovative method for fast tuning. A ferroelectric element, comprising a thin ferroelectric film mounted on conductive substrate made of silver or other metal with a high melting point was added around the dielectric resonator. When applying a direct current (DC) bias, the permittivity of the ferroelectric film decreased and hence the dielectric resonator's electric field could be changed. An alternative approach included a piezoelectric element connected to the moveable conductive component of the resonator's cavity. When voltage was applied to the piezoelectric element, preferably a bimorph, its dimensions would change and subsequently the resonator's frequency would be adjusted as conductive components moved. The results were encouraging with patent applications arising from the research work on both ferroelectric and piezoelectric tuning.
Tuneable filters based on dielectric resonators
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