| With the development of human society,communication system plays an indispensable role in human production and life.Microwave filter is an important device in the modern communication systems,and its in-band and out-of-band characteristics play crucial roles in the systems.The parasitic passband of a bandpass filter seriously restricts its applications,and thus,a filter with good out-of-band suppression is one of the research hotspots.In addition,size of filter also limits the system to integrate more function components,and thus miniaturized size is one of the important indicators for filters’ design.Microstrip filters are widely used in communication systems because of the advantages of simple structure,easy processing,and low cost.In this thesis,we focuse on the research of microwave filters with wide stopband and miniaturized size.Filters with half wavelength(λ/2)resonators and quarter wavelength(λ/4)resonators coupled by short connecting lines are proposed and studied.The main study works and innovations are as follows:1.In order to achieve wide stopband characteristics of band pass filter,this work designed λ/2 resonator filters based on coupling of short connecting line.Firstly,the relationship between the coupling coefficient and the position of the short connection investigated,and the short connection position that inhibit the generation of parasitic passband are found.Based on a series of optimization and exploration results,We designed and manufactured λ/2 uniform impedance microstrip width stopband filter,λ/2stepped impedance microstrip width stopband filter and λ/2 mixed impedance microstrip ultra-wide stopband filter.Results show that the introduction of short connecting line can effectively suppress the spurious passband,and the spurious passband of these three kinds of filters can be suppressed at 3,3.5 and 5.5 octaves,respectively.According to the optimized dimension parameters,the corresponding third-order and fourth-order filters are fabricated.The simulation results are in good agreement with the test results,which further proves that the introduction of short connecting line can improve the wide stopband characteristics of the band-pass filter.2.In order to realize miniaturization and ultra-wide stopband characteristics of the filter,the λ/4 resonator band-pass filter based on short connecting line is proposed.Firstly,the internal and external coupling laws of λ/4 uniform impedance microstrip filter unit are explored.Secondly,the wide stopband filter is designed by using uniform impedance,stepped impedance and mixed impedance resonator structure,and the upper stopbands of the designed filters is extended to 4 octaves,5 octaves and 6.5 octaves respectively,the circuit size is reduced by 42% compared to the first work.In order to further reduce the sizes of the filters,we fold the high impedance resonator into L-type,and the designed folded hybrid impedance filter can suppress the spurious passband at 6 octaves,the corresponding circuit size has been reduced by 66% compared to the first work.In this work,the electromagnetic simulation and machining test of the above four kinds of filters are carried out.The test results further verify the correctness of the assumption,which proves the reliability of the scheme. |
