Dielectric resonators. Materials and components

In the article [E 1 920] , dielectric resonators were presented as replacements for metal cavities in microwave circuits, serving as resonant elements. They are used in the construction of oscillators and filters and operate in various electromagnetic modes, such as TE, TM, and TEM. This article describes the characterization of the properties of dielectric resonators. Conducted by the manufacturer, this characterization enables them to improve these properties for the benefit of the user, namely: high permittivity for greater miniaturization, high quality factor to achieve high spectral purity in an oscillator or minimal insertion loss in a filter, and an adjustable temperature coefficient. This is achieved by studying the chemical compositions and the ceramic process, which involves the thorough mixing of the elements, raw materials, and powdered oxides or carbonates—generally via a liquid-phase method—followed by the sintering conditions, including temperatures and the oxidizing atmosphere. The formulations presented here were developed using simple oxides such as alumina (Al ₂ O ₃ ) or titanium oxide (TiO ₂ ), with permittivities of approximately 10 and 100, respectively, as well as other oxides of barium, zirconium, tin, tantalum, niobium, and rare earth elements such as samarium and neodymium. These non-trivial formulations provide a good compromise between improved thermal frequency stability, of the order of 10, ^–6, /K, and sufficiently low dielectric losses, less than 10, ^–3, , or, equivalently, quality factors greater than 1,000 up to tens of thousands. Some guidelines for implementation in circuits are provided.