Research On High Performance Microwave Planar Filters And 3-D Printed Waveguide Filters

Microwave filter is one of the most important devices in modern communication system.In the past decades,wireless communication technology has developed rapidly,and various communication standards have emerged.In order to accommodate various communication standards,communication systems are required to operate in multiple frequency bands.Therefore,multi-band bandpass filters?BPFs?and tunable BPFs that can support multi-band communication have become hot research topics.The study of high-performance planar multi-band BPFs and tunable BPFs can reduce the cost and volume of the communication system,and improve the system performance,thus it is worthing to study these two types of filters.On the other hand,in comparison with the planar BPFs,waveguide BPFs exhibit lower in-band insertion loss and higher power handing capacity.They are usually fabricated by using Computer Numerical Control?CNC?milling process.However,conventionally,CNC process needs to divide waveguide filter into several parts to fabricate individually,and then assemble them together,inevitably resulting in performance degradation due to assembly tolerance.Moreover,it is difficult to fabricate waveguide filters with complex geometries using CNC process.In recent years,3-D printing process has provided a new solution for fabricating microwave and millimeter-wave waveguide devices.It can not only fabricate waveguide devices monolithically,but also can produce devices with complex structures that are difficult to make using CNC process.Therefore,research on 3-D printed waveguide filters is of great significance.This thesis mainly focuses on the study of high-performance planar multi-band BPFs,electrically tunable BPFs,and 3-D printed waveguide BPFs.The main work of this thesis is arranged as follows:Firstly,based on common resonator feeding structure,dual-band,tri-band and quad-band BPFs are designed using?/4 stepped impedance resonators and dual-mode short stub-loaded resonators.These three filters feature compact size,high selectivity,wide stopband,and independently controllable centre frequencies and bandwidths,simutaneously.Secondly,a fourth-order cross-coupled dual-band BPFs based on zero-degree feed structure is proposed.The filter consists of a pair of electrically coupled open-circuit stub-loaded dual-mode resonators and two different pairs of magnetically coupled?/4short-circuit resonators.The proposed dual-band BPF offers seven transmission zeros?TZs?in the stopband,and three of them are located between the two passbands,enhancing the frequency selectivity and passband isolation significantly.Moreover,the filter features compact size and independently controllable bandwidths.Then,based on the same resonator combination structure,a feed structure including inductive source-load coupling is introduced to design a fully-canonical dual-band BPF.The newly developed filter maintains compact size and independently controllable bandwidths,in addition,eight TZs are generated in the stopband.Thirdly,by loading an adjustable capacitor on the center of open-circuit stub loaded dual-mode resonator,a capacitor loaded dual-mode resonator is proposed.The even-mode resonant frequency of the resonator can be independently adjusted by changing the capacitance of the loaded capacitor.Using the resonator,a wideband bandpass filter with tunable bandwidth is designed,the lower and upper passband edges can be adjusted independently.Compared with other bandwidth tunable filters,the number of the tunable devices and bias circuits is significantly reduced,i.e.,only two varactor diodes and two bias circuits,which reduces the design cost and circuit complexity.In addition,a dual-mode resonator loaded with two adjustable capacitors is proposed.The odd-mode and even-mode resonant frequencies of the resonator can be independently controlled.Using this resonator,and based on zero-degree feed structure and meandered feed line,a compact independently tunable dual-band bandpass filter with high selectivity is designed and implemented.Furthermore,through introducing capacitive source-load coupling between the input and output feed lines,a compact tri-band bandpass filter with high selectivity using varactor-loaded inductive coupled?/4 short-circuited resonators is presented.Fourthly,a TM₁₀₁-mode slotted spherical resonator is proposed.The proposed resonator can suppress the high-order modes TM₂₁₁ and TE₁₀₁without significantly reducing the unloaded quality factor?Q_u?of the fundamental TM₁₀₁ mode.Based on the slotted resonator,a third-order X-band waveguide BPF with spurious suppression is designed,and is fabricated using selective laser melting?SLM?and stereolithography?SLA?3-D printing process.The measured results show that the filter exhibits excellent passband performance and wide stopband.In addition,a TM₂₁₁-mode slotted spherical resonator is proposed to suppress the fundamental mode TM₁₀₁ and the high-order modes TE₁₀₁ and TM₃₁₁ without significantly reducing the Q_u of the TM₂₁₁ mode.Compared with the TM₁₀₁-mode spherical resonator,the TM₂₁₁-mode resonator offers higher Q_u and larger volume,thus the resultant filter can gain lower passband insertion loss and better tolerance to fabrication.Using the proposed resonator,a fourth-order Ka-band waveguide BPF with enhanced stopband is developed.The designed filter is manufactured by using the SLM 3-D printing process.The measured results are in good agreement with the simulated ones.