Design Of Tunable Planer Filter Based On Multimode Resonator

Nowadays,the wireless communication technology is developing rapidly,and many kinds of communication standards coexist.The matter of frequency crowd has become worser day by day.In order to improve the utilization of spectrum resource,tunable microwave filters have been widely used.The tunable microwave filters are so reusable that they can be tuned to the desired frequency band according to the actual needs.Microwave tunable filters can not only improve the utilization ratio of spectrum resource,but also reduce overall volume and complexity of the microwave device,getting more and more attention.This paper firstly introduces the research significance and the research status at home and abroad of the tunable filter,then elaborates the basic principle of the traditional filter and the integrated design method,which provide a theoretical basis for the design of the tunable filter.Besides,this paper proposes three tunable filters which based on dual-mode resonators,a triple-mode resonator and a quadruple-mode resonator.The content arrangement of the paper is as follows:Firstly,the resonant modes of the dual-mode resonator with open stub loaded at the center are analyzed and a dual-band bandpass tunable filter with independently tunable center frequencies and bandwidths by loading varactor diodes at the ends of the resonators are designed.The center frequencies of the passbands can be changed by adjusting the varactor diodes loaded at the end of the resonators,the bandwidths of the passbands can be changed by adjusting the varactor diodes loaded at the end of the open stub.The two passbands of the filter are independent of each other,and tuning one of the passbands has little effect on the other passband.Because of the coupling between source and load is introduced in the structure of this filter,there are four transmission zeros exist at the both sides of the passbands,which improves the filter selectivity and stopband rejection.The results show that the first passband can be tuned in a frequency range from 1.08 to 1.19 GHz with the bandwidth varies from 112 to 152 MHz,whereas the second passband can be tuned in a frequency range from 2.07 to 2.22 GHz with the bandwidth varies from 132 to 189 MHz.Then,the resonant modes of the cross multimode resonator are analyzed,based on this resonator,a single bandpass tunable filter is designed.Adjusting the varactor diodes loaded at the ends of resonator can change the center frequency of the passband.Adjusting the varactor diodes loaded at the ends of the two open stubs can change the upper cutoff frequency and lower cutoff frequency of the passband and control the passband bandwidth.From the experimental results,it was found that the proposed filter exhibited the center frequency tunable range from 2.04 to 2.15 GHz with the bandwidth variation from 229 to 387 MHz.Lastly,a simple quadruple-mode resonator is introduced and the resonant modes of the resonator are analyzed.A single bandpass tunable filter is designed with varactor diodes loaded at the ends of the quadruple-mode resonator,and the SIR structure is added to reduce the size of the filter.By adjusting the internal and external varactor diodes of the resonator,we can tune the lower cutoff frequency and the upper cutoff frequency of the passband respectively.Thus,the center frequency of the passband is adjusted and the bandwidth is controlled.The proposed filter exhibited the passband center frequency tunable range from 1.18 to 1.31 GHz,and the bandwidth varies from 358 to 417 MHz.These three filters all verified the effectiveness of the designs.