High Q Tunable Filters
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Microwave tunable filters are key components in radar, satellite, wireless, and various dynamic communication systems. Compared to a traditional filter, a tunable filter is able to dynamically pass the required signal and suppress the interference from adjacent channels. In reconfigurable systems, tunable filters are able to adapt to dynamic frequency selection and spectrum access. They can also adapt to bandwidth variations to maximize data transmission, and can minimize interferences from or to other users. Tunable filters can be also used to reduce size and cost in multi-band receivers replacing filter banks. However, the tunable filter often suffers limited application due to its relatively low Q, noticeable return loss degradation, and bandwidth changing during the filter tuning. The research objectives of this thesis are to investigate the feasibility of designing high Q tunable filters based on dielectric resonators (DR) and coaxial resonators. Various structures and tuning methods that yield relatively high unloaded Q tunable filters are explored and developed. Furthermore, the method of designing high Q tunable filters with a constant bandwidth and less degradation during the tuning process has been also investigated. A series of novel structures of dielectric resonators have been proposed to realize in a high Q miniature tunable filters. The first type of TME mode DR filter is designed to be tuned by piezoelectric bending actuators outside the cavity, and has achieved a tuning range from 4.97 to 5.22 GHz and unloaded Q better than 536 over the tuning range. The second type of TME mode tunable filters are integrated with various tuning elements: GaAs varactors, MEMS switches, and MEMS capacitor banks are employed. The designed filter with MEMS switches operates at 4.72 GHz, and has achieved a tuning ratio of 3.5% with Q better than 510 over the tuning range. The designed filter with GaAs varactors operates at 4.92 GHz, and has achieved a tuning ratio of 2% with Q better than 170 over the tuning range. Finally, the designed filter with MEMS capacitor bank operates at 5.11 GHz, delivering a tuning ratio of 3.5% with Q better than 530 over the tuning range. Cavity combline/coaxial resonators are also used in the design of high Q tunable filters. This thesis presents a novel approach to design a tunable cavity combline filter tuned by a MEMS switched capacitor bank. Instead of mechanically moving the tuning disk, the cavity combline filter is tuned with capacitances loading on the tuning disks, which are electrically adjusted by MEMS switched capacitor bank. The assembled 2-pole filter operates at 2.5 GHz with a bandwidth of 22 MHz, a tuning range of 110 MHz and a Q better than 374 over the tuning range. The assembled 6-pole filter operates at 2.6 GHz with a bandwidth of 30 MHz and has a tuning range of 44 MHz. Finally, the design of high Q tunable filter with constant bandwidth is explored. A 4-pole high Q cavity combline tunable filter with constant bandwidth is demonstrated. The tuning has been realized manually and by using a piezoelectric motor respectively. The designed filter operates at 2.45 GHz and has achieved a stable bandwidth of 30 ±1.1 MHz over a tuning range of 400 MHz and an unloaded Q better than 3000. This design method for a constant bandwidth filter is applicable to both cavity combline filters and dielectric resonator filters.