Investigations On Filter Design By Coupling Matrix Synthesis Technology

With the rapid development of modern wireless communication services and the rapid evolution of communication system standards,the demand for various information services is also growing.At the same time,limited by the available spectrum resources,in the multi-system and multi-standard wireless communication environment,the compatibility of communication system,the flexibility of channel switching and the anti dry ability of communication devices are facing greater challenges.As one of the important core components of RF front-end transceiver link in wireless communication system,RF filter has a very important impact on system performance,structure complexity and manufacturing cost.With the development trend of miniaturization,lightweight and portability of communication system,the research on miniaturized band-pass filter with high selectivity,compact structure and flexible bandwidth control has important theoretical significance and engineering application value..In this dissertation,based on the defective structure and parallel coupled microstrip structure,a constant bandwidth frequency tunable bandpass filter with enhanced passband performance is designed and fabricated.On the basis above,the behavior of defective structure is further studied,and a switchable fixed constant bandwidth frequency-tunable bandpass filter is created.Then,based on the coupling matrix network synthesis theories,a duplexer with hybrid coupled filters by cavity and microstrip structure is proposed and constructed.By applying the coupled network matrix with parameter reqirements of each independent channel in the diplexer,the resonance frequencies of each resonant-mode and the coupling coefficients between each two resonators in the mixed-coupling conditions are extracted.The main innovative work and achievements of this paper are listed as follows:1)According to the coupling matrix network synthesis method and the unload coupling transmission lines extraction method,the frequency tuning characteristics of the uniform transmission line structures with varactor diodes as resonant segments were analyzed and predicted.Based on the coupling network topology,tunable filters with preset performance were designed and manufactured under the fixed tuning frequency range could be designed.2)Based on the modified parallel coupled transmission line structure,the added transmission zero positions in the frequency response were determined by the even and odd mode analysis of the main mode and higher order resonance modes in the transmission lines.By utilizing the characteristics of the transmission zero positions of the MPCL structure,a highly selective triple resonant mode tunable filter with constant bandwidth is proposed and constructed,which achieved better filter out-of-band suppression.3)A constant bandwidth bandpass tunable filter with enhanced pass-band performance based on parallel coupled microstrip with defective ground structure is presented.The influence factors of the coupling coefficients in the parallel coupled transmission line structures are also studied.In this paper,the electric field and magnetic field of parallel coupled transmission lines are fully analyzed and calculated by full-wave electromagnetic simulation.The influences of defective ground structure size of inductance and capacitance values in LC equivalent circuits of transmission lines were explored and discussed.Though the variations in inductance coupling and capacitance coupling coefficients between resonance lines,the total coupling coefficient K was effectively controlled.On the basis of work above,the behaviour defective ground structures were further analyzed.Based on the structure of absolutely constant bandwidth frequency-tunable filter,bandwidth switchable tunable filter is designed and validated by defective ground structures terminated with varactors to achieve bandwidth switching controls.4)Based on the synthesis algorithm of coupling matrix network,a filter implemented by hybrid coupling structure with planar microstrip and coaxial cavity is proposed and created,and the three-mode mixed coupling characteristics was illustrated.By employing the vertical space above the microstrip-line filter,the spatial structure of the coaxial cavity is effectively utilized though adding an inner conductor in the metal shielded cavity,resulting in a new harmonic resonant mode between the inner conductor and the planar microstrip resonators.During filter design procedure,the central frequencies of the three resonant modes in mixed coupling states are determined by calculation though filters' coupling matrices.Then,on the basis of the planar dual-mode resonance microstrip line structure,by controlling the gap distance between the metal inner conductor and the planar circuit,or by changing the radius of the inner conductor,the resonant frequency of the coaxial cavity could be changed.Design goals are achieved by adjusting the resonant frequency of the coaxial cavity to approach the resonant frequency of the third mode in the hybrid filter.The additional transmission zero and corresponding transmission pole of the bandpass filter were determined by the coupling effect between the planar circuit and the coaxial cavity space,and the physical dimensions of the inner conductor are fine-tuned to obtain better filter performance.Based on the designed hybrid coupling structure,a duplexer with microstrip-coaxial cavity hybrid structure is fabricated.