Modeling of TE01d Filters

Student: Michelle Anne Emery

Supervisor: Dr. Aleks Rakic

Category: Electrical Engineering Thesis Project

Dielectric resonator (or ceramic) filters are a particular type of analog filter. There are various resonant modes present for a given resonator geometry. Typically, the resonator is designed to resonate in the TE01d mode. The spurious or higher order modes, which also resonate in the ceramic filters, interfere with the frequency response. This has previously contributed to difficulty in modeling these filters for use in the design process. Proprietary software does not have a readily available model for this type of filter. Usual practice in the industry was to use the model for another filter technology known as the combline filter, which gives incorrect levels of rejection on the ceramic filter passband edges.

This project has therefore pursued two different techniques of modeling these dielectric resonator filters to give a more accurate filter response for use by design engineers. The outcome of the project is a transmission line model and corresponding design process, which can be used to give very close to the exact measured response of these TE01d filters. The numerical analysis tool, HFSS, is also useful to this modeling process, as it is used to calculate field distributions and gives a reasonable approximation to the resonant mode frequencies for any desired geometry.

The project, investigates resonant frequencies of different geometries and the usefulness of the numerical analysis software, HFSS, for solving these, given the possibility that models may be derived from the resulting S11/S21 data. Three different geometries were simulated using this software, outputting an S11 response, which allows reasonable estimates of resonant frequencies to be determined. This will be useful in design work, allowing an estimate of the filter centre frequency to be determined, for a given resonator geometry, without the need to manufacture a test resonator. Also useful to design work, HFSS, models the fields between coupled cavities and shows trends of spurious modes.

The method of modeling the filter with transmission line stubs and admittance inverters, with the aid of proprietary design software, proved to give very good results. Three prototype filters, each with a different method of cross-coupling, were built and the responses measured during the project. The optimiser on this software optimised parameters in the model for the three filters, iteratively driving the responses towards the measured responses. One of these filters gave an inconsistent model, which was adapted to be more consistent with the other two. The effects of tuning screw positions and input transformer tap point positions to the spurious modes, which may effectively influence these models, was also investigated and results are included within this report.

The usefulness of these models to engineers is unquestionable. The work provided in this thesis provides a good solution for the design of ceramic filters, which will surely prompt an increase in the use of this filter technology.

Poster Presentation (PDF)

Thesis Document (PDF)