Investigations On Novel Microwave Wideband Passive Filtering Circuits

In modern microwave wideband communication systems,the development of wideband filtering technology has been playing an important role.In order to meet the application requirements,modern RF/microwave circuit systems generally require low profile,miniaturization and integration.With the rapid development of wireless communication technology,as important units of RF/microwave system front-end,passive filtering circuits are also gradually developing towards miniaturization and multifunction.Single-end filters have been the most widely used filtering circuits.The traditional implementation can not meet more and more complex application requirements.The integration of novel technology and novel materials has provided some possible approaches,thus opening up a new research direction.However,more and more intensive physical layout,complex and variable external environment factors in circuit systems bring serious electromagnetic interference for signal transmission.Therefore,wideband balanced filtering circuits with common mode noise suppression and anti-jamming characteristics have gradually become the focus of researches in the industry.To satisfy the application requirements of microwave wideband communication system,aiming at the hot research issues and challenges of wideband passive filtering circuits,some in-depth researches have been conducted in this thesis,especially the design theory and implementation methods of wideband bandpass filters and wideband balanced filtering circuits.The main contents of this work can be summarized as below:1.Microstrip line stub-loaded technology and its application.Novel multi-mode resonators called T-type resonators with open or shorted stub loaded in the center are proposed.Its resonant mode is analyzed and their wideband frequency responses are given;a novel model of stub-loaded improved coupled lines is proposed,and the general expression of ABCD transmission matrix is derived.The band-pass and all-stop characteristics of the cascaded structure for three kinds of improved coupled lines are emphatically analyzed;a input/output port stub-loaded technology is proposed and the locations of transmission zeros for different types of loading stubs is analyzed.At the same time,the potential application of above structures in the design of microwave wideband passive filtering circuits in this paper is introduced.2.Investigation on high selectivity wideband bandpass filters.Based on the side-coupled ring resonator and the stub-loaded improved coupled line,high selectivity planar wideband bandpass filters are realized.The numbers of out-of-band transmission zeros are increased by loading open/shorted coupled-line stubs at ports,and the passband selectivity is further improved.In order to achieve miniaturization,the design principle and modeling method from planar coupling to coupling between layers are proposed.Combined with stub-loaded improved coupled lines and port stub-loaded technology,high selectivity wideband bandpass filters are realized through strong coupling between layers by using LTCC multilayer technology.The feasibility and validity of the proposed microstrip line stub-loaded technology for designing high performance wideband bandpass filters in planar circuits and LTCC circuits design are verified by the simulated and measured results.3.Investigation on high selectivity wideband balanced bandpass filters.Based on the proposed T-type resonator and the input/output port stub-loaded technology,high selectivity wideband balanced bandpass filters with multiple fixed transmission zeros or position-controllable transmission zeros are obtained by loading different open stubs or coupled-line stubs.The common mode suppression is good in the passband.Based on the proposed stub-loaded improved coupled lines,high selective wideband balanced bandpass filters with multiple transmission zeros are also designed by combining the input/output port stub-loaded technology,which can achieve high common mode suppression in the whole frequency band.The feasibility of the proposed microstrip line stub-loaded technology for designing wideband balanced bandpass filters is verified by the simulated and measured results.4.Investigation on novel wideband balanced-to-unbalanced filtering power dividers.Based on the coupled line,the wideband response is achieved.According to the principle of balanced-to-single-ended power division,the impedance condition of the coupled line and the impedance relation under the matching condition are obtained.Finally,a wideband balanced-to-unbalanced filtering power divider is realized.Combining with the proposed input/output port stub-loaded technology in this paper,the passband selectivity of this kind of filtering power divider is further improved.A half-wavelength coupled line is introduced to make the differential mode circuit obtain position-controllable transmission zeros,while fixed transmission zeros can be obtained by loading half-wavelength open stubs at the output ports.This part of work provides new technical solutions for the design of the wideband balanced-to-unbalanced filtering power divider.5.Investigation on wideband bandpass filters based on Spoof Surface Plasmon Polaritons(SSPPs).A kind of novel planar SSPPs transmission line loaded with H-shaped cells is proposed.Transmission zeros can be introduced by the spiral DGS(Defected Ground Structure)resonators etched on the CPW ground surface for feeding.Combining with its cutoff frequency,a high selectivity wideband bandpass filter with wide upper stopband is finally realized;a kind of novel double-row staggered double-sided SSPPs transmission line is proposed.By loading U-shaped cells symmetrically,a transmission zero can be introduced into the original operating frequency band,which forms a high out-of-band suppression wideband bandpass filter.The structure is analogical with double-sided parallel stripline,and the equivalent circuit model of microstrip line is proposed to assist analysis and design.The passband selectivity can be further improved when the number of U-shaped cells is increased from 3 to 5.This part of work provides new design thoughts for the design of microwave filters and provides optional technical solutions for the application of planar SSPPs transmission line structures in millimeter wave and terahertz integrated circuits.