A Study Of Dual-band Reconfigurable Bandpass Microstrip Filter

With the rapid development of modern wireless communication technology, the spectrum resources inevitably suffer from congestion problems and become more and more serious. In order to alleviate the crisis, a variety of information processing technology and spectrum saving technology are widely used at present. As one key component to realize these techniques, reconfigurable filters become a focus of attention and research. Therefore, it is significant to study reconfigurable filter with fast tuning speed, wide variation range, low cost and small size.Considering the application requirements and development trend of reconfigurable filters, this paper focuses on the research of tunable dual-band bandpass filter(BPF) on the basis of the comprehensive theory of traditional filter and the technology of tunable single-band BPF. Varactor is adapted as the tuning device in this thesis. Using stub-loaded stepped impedance dual-mode resonators, two types of microstrip BPFs, reconfigurable filter with two independently controllable passbands and a dual-band reconfigurable filter with controlled frequency and bandwidth, are designed.For the design goal of independently tunable two passbands, there are two kinds of dual-band reconfigurable filters to be designed with single or double loops. First of all, this thesis presents a dual-mode stub loaded ring resonator(SLRR) to achieve a tunable dual-band BPF with two independently controllable passbands. The proposed resonator principally comprises a stepped-impedance ring resonator(SIRR) loaded with three stubs and two varactor diodes. Two independently tunable passbands are implemented by employing two varactors to control the dominant even-mode resonant frequency and odd-mode resonant frequency, respectively. Moreover, a new stub loaded double-ring resonator(SLDRR) is proposed to design the second tunable dual-band filter by shorting two stubs of the SLRR.With the same tuning method, the new filter can achieve two independently controllable passbands. In order to suppress the harmonics, defected ground structures(DGSs) are introduced at input and output feeding lines without degrading the passbands characteristics. The measured results are in good agreement with the simulated ones.Simultaneously, this thesis proposed a resonator loaded two stubs on the center. Based on the hybrid electromagnetic coupling theory and the method of tuning bandwidth, adual-band BPF with adjustable bandwidth is designed. And an additional varactor is added at the center to tune the second passband. Then, by loading another two varactors to the terminal of the main transmission line, the microstrip BPF can achieve controllable bandwidth and two passbands at the same time. The simulation of the two filters verifies the validity of design structures.