Research Of Miniaturized Planar Bandpass Filters

With the development of wireless communication,radio frequency microwave circuits tend to be miniaturized,planar and integrated,which are becoming more and more popular among researchers of electronic communication equipment.In the field of wireless communication system,high-performance filters play important and indispensable roles.In particular,some filters with miniaturized size,high-frequency selectivity and high stopband suppression can greatly improve the performance of communication systems,including reducing transmission power loss and the overall size.Therefore,miniaturized filters with excellent performance have gradually become a hot research direction for scientists.In this paper,several different filters are designed and fabricated,including microstrip filters with high frequency selectivity and ultra-wideband filters based on spoof surface plasmons polaritons.The details are as follows:(1)A wideband filter based on parallel-coupled lines loaded with open/shorted stubs is proposed.Firstly,two identical shorted stubs are loaded between two pairs of parallel-coupled lines,three resonance modes and four transmission zeros can be obtained.Then,two open stubs and shorted stubs are loaded at the input and output terminals,respectively.Therefore,four more resonance modes and two more transmission zeros can be obtained.Then the seven resonant modes of the filter are calculated by odd-and even-mode analysis,and the in-band characteristics are analyzed.Finally,the filter is simulated,tested,and study the influence of various parameters on the selectivity.(2)Several bandpass filters with high stopband rejection are designed.Stubs loaded are used to increase transmission zeros for improving stopband rejection.Firstly,a high stopband rejection filter based on the series-cascaded of three pairs of parallel-coupled lines is proposed.Secondly,two open or shorted stubs are loaded at the end of input and output parallel-coupled lines of the basis filter,therefore two other filters with high-stopband rejection are obtained.Five transmission poles and eight transmission zeros can be acheived by series-cascaded three pairs of parallel-coupled lines.When two open or shorted stubs are loaded at the end of input-output parallel-coupled lines,four or two more transmission zeros in the stopband can be generated,respectively.Then,the in-band and out-band characteristics of the three filters are calculated by impedance matrix analysis,and the influence of each parameter on the filters is simulated and analyzed.Finally,a filter with high stopband rejection,which consist of six pairs of parallel-coupled lines is designed,analyzed by odd and even mode analysis method.(3)Based on the theory of spoof surface plasmons polaritons(SSPPs),a terahertz waveguide and an ultra-wideband bandpass filter are designed,respectively.The cut-off frequencies of the terahertz waveguide and bandpass filter are generated by the dispersion curve of SSPP cells,while for the ultra-wideband bandpass filter,the lower stopband is caused by the microstrip to coplanar stripline(CPS)transition,thus forming a bandpass filter.