Research On Coupling Theories And Transmission Zero Design Technologies Of Planar Microwave Filter

With the increasingly intensive use of wireless communication frequency band,the utilization rate of spectrum resources becomes higher.However,the mutual interference between frequency bands is becoming more serious.As the hardware controller of allocating spectrum resource,microwave filter will effectively solve the contradiction between the effectiveness of spectrum resource utilization and the mutual interference of frequency bands.At the same time,improving the performance of filter and reducing the physical size have always been the hot topics.In this dissertation,with the study of cross coupling,mixed electromagnetic coupling and other coupling theories as the starting point,the design theory of transmission zeros(TZs)and the improvement of out-of-band suppression performance in ultra-wideband(UWB),narrowband and multi-band filters are systematically studied based on the planar microstrip structure.The main research works of this dissertation are as follows:1).In order to improve the band-edge rejection ability of UWB filters,the coupling theory and TZ design method of planar microwave UWB filters are presented.Firstly,in the UWB band-pass filter,the odd-even mode and multi-stub physical parameter analytical approaches are used to analyze the generation principle and the position control theory of the transmission poles(TPs)in the passband and the TZs out of the passband.And multiple TZs are produced to realize the UWB band-pass filter with excellent outof-band suppression characteristics.Secondly,in the low-pass filter,a new method of cascading cross-coupled LC resonant units to introduce TZs is also proposed,and an ultra-wide stopband is obtained by adjusting the distribution of TZs at the far end outside the passband.2).In order to reduce the complexity of designing and debugging the TZs of filters with cross couplings,a new filter topology structure is constructed based on the traditional CQ cross-coupled configuration to realize the adjustment and control of multi-pair TZs.In this dissertation,a perturbation unit is introduced between CQ units to achieve the coordinated control of multi-pair TZs.In particular,this method can realize the separation and adjustment of overlapping TZs in symmetrical physical structure filters,which means that a symmetrical filter can present the frequency response of an asymmetrical one,thus reducing the design difficulty and improving the design efficiency.Finally,the physical design,fabrication and test of a symmetric and an asymmetric eighth-order CQ structure filters are completed,and the theoretical method is verified.3).In order to improve the sideband rejection ability of dual-mode dual-pass filter,a method of introducing transmission zeros of dual-mode dual-pass filter by combining hybrid electromagnetic coupling and cross coupling is proposed.First of all,a multifolded and miniaturized multi-mode resonator is designed,and the coupling characteristics between resonators under different conditions of mixed electromagnetic coupling or cross coupling are analyzed.These two kinds of coupling are applied together to generate multi-pair TZs near the two passbands.Then,the TZ position control principle and method of mixed electromagnetic coupling and cross coupling are analyzed.Finally,the design of multi-TZ dual-mode dual-band filter with excellent out-of-band suppression characteristics is realized.4).In order to reduce the design complexity of bandpass filters with multiple TZs,a new filter design method without cross coupling structure to introduce finite frequency TZs is proposed.Firstly,by loading the second-order extracted-pole unit(EPU)composed of two mutual coupled resonators at the input and output of the filter,a topology configuration without cross coupling is designed.Secondly,in the case of no cross coupling between resonators,the new topology filter can generate even finite frequency TZs;moreover,by adjusting the resonant frequencies of the two resonators in EPU or the coupling strength between them,arbitrary distribution of TZs on the imaginary axis can be realized.Finally,the design,fabrication and test of an eight-order quasi-elliptic function filter with two pairs of TZs are achieved.5).The synthesis and design of an extended cul-de-sac configuration filter which can produce the finite frequency TZs in pairs is proposed.First of all,the TZs and TPs can be introduced in pairs by cascading resonators on the diagonal corners of a core quartet of the proposed cul-de-sac structure filter.And the designs of two seven-order culde-sac structure filters with single TZ and two TZs are completed.Secondly,the characteristics of TZs and TPs of this type of filter with different orders are summarized.Finally,a cul-de-sac structure filter with two proposed units is achieved.Each unit can introduce a pair of TZs separately,and can realize the independent control of TZ positions,thus providing a new idea for the design of filters with multi-pair TZs.