Novel Filter Topologies and Synthesis Techniques for Addressing the Challenges of Future Communication Systems
Final Report Abstract
In the this project, different multi-mode filters were simulated, fabricated and measured. All filters are based on low-cost and commercially available ceramic TM-mode resonators with a cylindrical shape. By combining these cylindrical ceramics in different ways with the use of prepared copper cores, novel dual- and triple-mode resonators can be realized. This is possible because the ceramics have a silver coating at their ends and can therefore be soldered to other components. Two different dual-mode approaches were considered, based on a X- and Y-shaped arrangement of the ceramics, respectively. In addition, a previously unpublished arrangement of the ceramics for a triple-mode realization has been investigated. Different filters were evaluated and fabricated for all three concepts. The X-shaped filter can be synthesized by classical design techniques. However, subsequent measurements reveal a fabrication defect that could not be clearly identified. However, based on previous publications, the principle operation could be demonstrated. With the described prototype, it is therefore possible to achieve quasi-elliptical filter responses compared to the previous work, where an all-pole response was obtained. The subsequently discussed Y-shape offers some remarkable advantages. For example, only three instead of four cylindrical ceramics are required to realize a dual-mode filter. In addition, the structure exhibits better spurious mode performance since the next higher mode is a cavity resonance (which is comparably far away from the desired modes) and is not guided by the ceramic rods, as it is the case of the X-shaped resonator. Within this project a two-pole filter and two fourth order filters were fabricated successfully. For this purpose, basic fabrication steps are discussed and difficulties encountered in the process are pointed out. The fourth order filter can be used to realize phase-equalized filter responses as well as filter responses with transmission zeros on the real frequency axis. The latter version exhibits a high degree of flexibility: Besides the fabricated demonstrator, it could be shown by parameter studies that the available transmission zeros can be moved flexibly and are also suitable for the realization of highly asymmetric filter responses, e.g. by positioning both transmission zeros above the passband. Furthermore, theoretical investigations such as the frequency dependence of the eigenmodes as well as a coupling matrix representation based on global eigenmodes were carried out. Analogous to the X-shaped filter, triple-mode filters can be designed by appropriate symmetrical alignment of four cylindrical ceramics, which allows for an increased filter order while using the same number of ceramics. For this purpose, different geometries for triple-mode resonators were investigated with respect to their suitability for filter design. The chosen shape of a truncated octahedron has advantages with respect to volume and the property of being space-filling compared to the other geometries investigated. Based on the field distributions of the first three eigenmodes of the resonator and results from simulations with weak excitation, it could be derived analytically how all three degenerate eigenmodes can be excited specifically. Furthermore, the corresponding „Cul-de-Sac“ filter topology could be derived. Within the scope of the investigations, a three-pole filter was manufactured and measured. The basic manufacturing steps correspond to those of the investigated dual-mode filters. In conclusion, it can be stated that the objectives of the research project to investigate different dual- and triple-mode resonators with the goal of achieving a reduction in volume and mass compared to conventional filters based on single-mode resonators have been achieved. However, as has been shown, there are restrictions with respect to the flexibility concerning the design of filters based on multi-mode resonators. This is valid especially when the number of modes used increases.
Publications
- "Complex Eigenmodes for Modelling of Dielectric Dual-Mode Resonators,"2018 IEEE MTT-S International Conference on Numerical Electromagnetic and Multiphysics Modeling and Optimization (NEMO, Reykjavik, Island), 2018, pp. 1-4
M. Höft and D. Miek
(See online at https://doi.org/10.1109/NEMO.2018.8503102) - "Realization of Cross-Coupled X- and Y-Shaped Dual-Mode Dielectric Resonator Filters,"2018 Asia-Pacific Microwave Conference (APMC, Kyoto, Japan), 2018, pp. 506-508
D. Miek, S. Salzer and M. Höft
(See online at https://doi.org/10.23919/APMC.2018.8617282) - “Dielectric TM Dual-Mode Filters with Y-shape,” In:2021 IEEE MTT-S International Microwave Filter Workshop (IMFW, Perugia, Italien), 2021, pp.69-72
D. Miek, P. Boe, F. Kamrath und M. Höft
(See online at https://doi.org/10.1109/IMFW49589.2021.9642323) - “Triple-Mode Bandpass Filter Based on TM Dielectric Rod Resonators,” In:2021 IEEE MTT-S International Microwave Symposium (IMS, Atlanta, GA, USA), 2021, pp. 499–502
P. Boe, D. Miek, F. Kamrath und M. Höft
(See online at https://doi.org/10.1109/IMS19712.2021.9574862)
