| Recent years, as the result of booming development in the information technol-ogy industry and rapid improvement of the mobile telecommunication techniques, the mobile terminal devices (MTDs) have been required to support multiple communica-tion protocols simultaneously so as to provide customers with various services as well as convenient wireless network accesses anytime and anywhere. Meanwhile, the in-tensive market competitions lead to continuous demands for MTDs with more com-pact volumes and lower power consumptions. Under such circumstances, designing of functional blocks in radio frequency (RF) front-ends for mobile communication ap-plications should not only adapt to multi-band and multi-protocol operations, but also consider size, power consumption as well as the complexity and reliability of entire system. It is well known that filters are essential components in RF front-ends, and among which, the bandpass filters are also facing the challenges of multi-band, multi-protocol and miniaturization requirements. In particular, as a reasonable balance be-tween performance and complexity, the dual-band bandpass filter designs are of spe-cial practical value.The status quo of academic research in RF filter design is firstly discussed in general. The basic concepts and theories are also addressed. A new sort of H-shaped microstrip resonator design is then introduced and applied in dual-band filter design. Using transmission line modeling, numerical analyses and electromagnetic simula-tions, the third chapter adequately investigated the unique frequency property, cou-pling strategy and terminal excitation of the H-shaped resonator. For instance, one2.45GHz/5.25GHz dual-band filter example utilizing H-shaped resonators is given to illustrate the design process. The prototype circuit board of this design example has been fabricated and tested. The measured responses match well with the simulation results hence verifies the design theory. In addition, further exploration focused on the harmonic suppression of dual-band filters is performed. By simplifying available de- sign methods and taking advantage of the H-shaped resonators, an alternate coupling strategy is proposed to enhance the harmonic suppression of the dual-band filters. Two dual-band filter examples with harmonic suppression property are given along with simulated results. It is shown that the proposed approach is efficient to reject harmonic responses with in20GHz through a relatively simple process, hence im-prove the selectivity of the dual-band filters.In order to pursue miniaturized dual-band filter design, a modified ring resonator is proposed in the fourth chapter. This resonator is used in dual-mode dual-band filter design to achieve flexible passband ratios. The concept of degenerate modes in ring resonator is introduced along with the methods to split these modes. Based on qualita-tive analyses of the electromagnetic field distribution of the ring resonator at reso-nances, equivalent boundary conditions are introduced to divide the entire ring. It simplifies the ring resonator design to two-port network issues. T-junctions are used in replace of the arc sectors of original ring. This modified ring resonator can not only guarantee one pair of degenerate modes under either first or second order resonance, but also provide freedom to achieve flexible passband ratios. Based on theoretical analyses, a dual-mode dual-band filter design example is illustrated, fabricated and tested, the measured responses match well with transmission line predictions and simulated results, hence verifies the design method.Finally, the work is summarized and the deficiencies are considered. According to the latest research trends, some study aspects such as improving design adaptability, device miniaturization and accuracy enhancement are considered as future work which may have potential values. |
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