Research And Design Of Multi-band Microstrip Filters

With the progress of communication technologies,especially the gradually maturation andcommercially available of 5G,new strict demands have been proposed to the design of high frequency filters.As the communication productions occupy multi-bands,not only the high-powered narrow band filters,but also the multi-band filters are needed.On the other hand,balanced circuits have attracted substantial attention and have been more and more utilized because of their high immunity to the environmental noise,interference and crosstalk between different elements,compared with the single-end counterparts.The multi-band bandpass filters are obtained more and more interesting.Therefore,three dual-band bandpass filters with high selectivity based on asymmetric open stub-loaded resonators and a balanced tri-band bandpass filter with high selectivity based on asymmetric short stub-loaded resonators are designed.The main contents of this thesis mainly include the following aspects:1.Aiming at the low selectivity of dual-band filter,the first bandpass filter proposed in this paper is achieved by employing a pair of asymmetric open stub-loaded resonators and conventional interdigital coupling lines.By adjusting the above parameters of each asymmetric open stub-loaded resonators,the center frequencies of the two passbands can be controlled easily.In the meantime,interdigital coupling line can enhance the coupling between the source and load,and introduce multiple transmission zeros,which can conduce to a higher selectivity.2.To obtain superior selectivity,the novel interdigital coupling lines is based on the traditional one,which further enhances selectivity since transmission paths are doubled.In order to achieve miniaturization and enhanced coupling,asymmetric open stub-loaded resonators are folded.In addition,four transmission zeros are generated to improve the selectivity of two passbands.3.Aiming at the problem that narrow of the two dual-band passband filters bandwidth,a dual-band broadband band-pass filter based on asymmetric open stub-loaded resonators is designed.Two broadband passbands are generated by applying two pairs of asymmetric open stub-loaded resonators and interdigital coupling lines.Therefore,a dual-band broadband band-pass filter is designed.4.To avoid electromagnetic interference,a balanced tri-band bandpass filter with high selectivity is proposed.Three differential-mode passbands are formed by applying one pair of asymmetric short stub-loaded resonators and uniform impedance resonators into the design.Meanwhile,the frequencies and bandwidths of the differential-mode passbands can be quasi-independently controlled by the lengths of resonators and the gaps between them.In addition,broadband common-mode suppression is achieved intrinsically without affecting the differential-mode parts based on the balanced stepped-impedance microstrip-slotline transition structures.