| Summary: | 博士 === 國立臺灣大學 === 電信工程學研究所 === 102 === This dissertation aims to design novel varactor-tuned reconfigurable microstrip bandpass filters (BPFs) that are capable of being adjusted to different center frequencies or passband widths for multi-functional operations in microwave and radio frequency communication systems. To achieve this goal, we propose two novel design schemes: The first design scheme is a conceptual structure. It consists of a cascade of two series stub-based structures in shunt with a varactor-tuned microwave resonators to adjust the bandwidth, and realizing passband edges tunabilities. The second design scheme is developed by including a tunable coupling coefficient for inter-resonator coupling tuning as well as a pair of controllable immittance inverters at the I/O ports for an external quality factor. Hence, this scheme can be modified slightly according to the design specification, but its general flow is kept. The two novel design schemes can facilitate shaping prototypes, and guide the design of reconfigurable BPFs. By applying these novel design schemes when necessary, we designed four reconfigurable BPFs with advanced responses: The first design applies the former design scheme along with a simple ordinary procedure to design and fabricate a BPF with a low passband insertion loss less than 2 dB, a 21.8 % higher band edge tuning range, a 18.2 % lower band edge tuning range, as well as wideband suppression greater than 17 dB at frequencies up to 5.5 GHz. The second design adopts a quasi-lumped circuit topology and a varactor-based mixed electric and magnetic coupling mechanism, in which the second design scheme is used in designing a prototype. The BPF implemented for demonstration is a compact microstrip BPF of a very high passband bandwidth tuning ratio (5-to-1 ratio) centered at 1 GHz. The third design chooses a simple T-shaped dual-mode resonator which is developed according to a slight modification on the second design scheme and is externally coupled and perturbed with varactor diodes. In addition, an iterative procedure corresponding to this modified scheme is also presented to guide the design. The implemented BPF has a 1-dB bandwidth adjustable from 55 to 175 MHz for center frequency ranging from 750 to 1240 MHz with a low insertion loss less than 2.9 dB. The final design uses a simple loop-shaped dual-mode resonator tapped and perturbed with varactor diodes, in which the modified second design scheme and the proposed iterative procedure are also exploited in developing the BPF design. The fabricated microwave BPF is with an up to 33% center frequency tuning range, an excellent bandwidth tuning capability, as well as a freedom to change the filter response, such as the ordinary Butterworth, Chebyshev, and all-reject specifications.
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