Magnetodynamic modes in ferrite spheres for microwave filter application

A magnetically tunable microwave filter structure is described using magnetodynamic modes in ferrite spheres. The tuning behavior and the loss properties are investigated. Advantageous tuning behavior is found in the unsaturated state of magnetization, where frequency shifts of 1.5GHz are obtained with tuning fields between 0 and 600Oe. To calculate the resonance frequency as a function of the tuning field, a first-order perturbation theory is developed, based on the variation of the magnetic-field energy stored in the ferrite. In the vicinity of the isotropic resonance frequency, good quantitative agreement between theory and measurements is obtained. Unloaded quality factors up to 20 000 at 3.5GHz are achieved with polycrystalline garnets. It is shown that the dielectric losses of the ferrite material mainly determine the losses of the filter device.