Analytical Modeling of Highly Loaded Evanescent-mode Cavity Resonators for Widely Tunable High-Q Filter Applications

In the present work, the frequency and quality factor (Q) variation of an evanescent-mode cavity resonator are predicted using analytical expressions, which give design variables for these structures. Two widely tunable narrowband evanescent-mode bandpass filters having 2.5% and 0.58% bandwidths are demonstrated, tuning from 2.7 to 4 GHz and 3 to 6 GHz respectively. A low loss Rogers Duroid® flexible substrate forms the top of the filter, acting as a deformable membrane that allows variable frequency loading. A commercially available piezoelectric actuator is mounted on the substrate for precise electrical tuning of the filter center frequency. The narrow band filter demonstrates the potential of the tuning mechanism to compensate for fabrication tolerances, which are especially important for such a narrow bandwidth filter. The high-Q and the sensitive tuning combine to allow for high quality preselect filtering for a front-end receiver.