Research And Design Of Electronically Tunable Microstrip Filter

With the rapid development of modern wireless communication technology, the microwave / millimeter-wave systems are widely used and the problem of spectrum congestion is becoming more and more prominent. In order to improve the utilization of limited spectrum resources and make the communication systems smaller and less complicated, tunable filter was widely used as one key component in reconfigurable systems and attracted more and more attention since it could deal with the issues of high-performance and complexity at the same time.This thesis mainly focuses on the design of varactor-tuned bandpass filters with center frequency and bandwidth control. Tunable filters have been studied during the several decades, and certain achievements on frequency tuning and bandwidth control were obtained. But, there are still a lot of problems and implementation can be carried out on the tunable filter design, such as bad insertion loss, narrow tuning range, variational bandwidth, unstable filtering performance. Considering these problems, two types of tunable bandpass filters with wider tuning range, constant absolute bandwidth and more stable performance were designed in this paper. The major work is concluded as follows:Firstly, the odd- and even-mode equivalent circuits theory is introduced as the basic theory, and then coupling matrix is derived and discussed as the stability conditions of tunable filter. Meanwhile, coupling coefficient and extemal quality(Qe) factor involved in the discussion were carried out in a detailed introduction. After that, a summarization and comparison of different implementations of electronically tunable filter is presented.Secondly, study of bandpass filter with frequency and bandwidth tuned independently is carried out. By analyzing related literatures at home and abroad, the principle and different realization methods about frequency and bandwidth control were investigated respectively. Based on the study, a tunable filter consisted of two varactor-loaded asymmetric U-shaped resonators with frequency and bandwidth controlled independently is designed. This paper analyzed the filter from the structure, coupling coefficient, Qe factor, varactor Q value to gain a high performance, and then the proposed tunable filter was fabricated and measured. Good agreement between the simulated and experimental results can be observed.Finally, microstrip tunable filter with more stable and better filtering performance is investigated. Considering the influence of the Qe factor, the Qe tuning structure was applied between the resonator and I/O port for variable external coupling in the filter design. Based on the theory above, a varactor-tuned bandpass filter with center frequency and bandwidth control is designed, fabricated and measured. The excellent agreement between simulated and measured results shows that the filter has lower insertion loss and more stable filtering performance. Meanwhile, the filter realized the wide tuning range, especially the wide bandwidth tuning range, and achieved constant frequency response shape over the frequency tuning range and invariable center frequency in the overall bandwidth tuning range.