Filtering Multifunctional Circuits Using The Fusion Of Three-Dimensional And Planar Transmission Lines

RF transmission lines consists of the three-dimensional and planar transmission lines,among them the three-dimensional one represented by the waveguide possesses high Q value and high power capacity,which is widely used in microwave and millimeter wave circuit design.However,the waveguide circuit suffers from large volume and monotonous construction.The planar transmission line,such as the microstrip line,has the advantages of design flexibility,low profile and easy integration.Therefore,the fusion of the three-dimensional and planar transmission lines could improve the design flexibility and functional diversity of the circuits,while maintaining high performances.To this end,by using the fusion construction of the three-dimensional waveguide and planar microstrip line in this dissertation,a series of tunable filters and multifunctional filtering transmission circuits are designed,achieving high-performance and miniaturized RF devices.The main research work includes:(1)Aiming at the problems of monotonous feeding network and construction of the waveguide circuit,the cavity-microstrip integrational topology is proposed.The planar microstrip line is employed to feed the multi-mode rectangular cavity,and the direct-coupling and gap-coupling constructions of the cavity-microstrip topology are studied.By using the single-port and two-port differential feeding networks of the two coupling constructions,independent excitations of the three fundamental modes in the cavity are obtained.(2)Aiming at the problem of the complex design in multi-band circuit architecture,a series of tunable dual-band balanced filter and frequency-and bandwidth-tunable filter are designed.For dual-band frequency-tunable balanced filter,based on the direct-coupling cavity-microstrip construction,through adding the dielectric posts and the metal screws into the cavity,the resonant frequencies and the inter-resonator couplings of the two modes are independently regulated,resulting in wide frequency tuning ranges of the two passbands.During the tuning process,low-loss filtering response and below 40 dB common-mode suppression are maintained.In addition,for the bandwidth control,by changing the height of the metal ground of the microstrip feeding line,the external coupling coefficients of the two modes are independently manipulated,then independent controls of the frequency and bandwidth within a wide tuning range are achieved,and meanwhile within 1 dB insertion loss of the passband is maintained during the tuning range,a series of methods of the tunable filters with low losses and wide tuning ranges are formed.(3)Aiming at the problems of large volume and high cost of the high-performance passive devices,the fusion design of the filter and unbalanced-balance circuit is studied,and a series of passive multifunctional circuits such as filtering balun and filtering magic T are designed.For the filtering balun,based on the gap-coupling cavity-microstrip topology with the single-port and dual-port differential feeding network,out-of-phase and equal currents for the two modes are obtained at the two output ports.Meanwhile,a dielectric-filled rectangular cavity is used to realize low-loss filtering response and high-performance balun characteristic.For the filtering magic-T,based on the direct-coupling cavity-microstrip topology,the mechanisms of out-of-phase and in-phase power dividers and port isolation are deeply studied,and the filtering magic T with low loss and high isolation is realized.In this series of passive multifunctional circuits,the resonant modes and cavity resonators are shared by the two outputs,which reduces the volume and cost of the circuits.(4)Aiming at the problem of high losses caused by the cascaded devices in the RF front-end circuit,the fusion circuit of the filter and switch is studied,and a series of single-band and dual-band filtering switches are designed.Based on the direct-coupling cavity-microstrip topology,the external coupling coefficient of the two modes is controlled by switching the diode inserted in the microstrip feeding line,then the synchronous control of the excitations and suppressions of the two modes are achieved.When the diode is reversely biased,the filtering switch is in the ON-state,and the parasitic effect of the diode makes little influence on the passband,then low-loss dual-mode filtering passband is realized.When the diode is forward biased,the switch is in the OFF-state,and high isolation is obtained.For the dual-band filtering switch,the mechanism of independent regulations of the dual-mode external coupling and inter-stage coupling are further studied,0.4 dB insertion loss of the two passbands are achieved in the ON-state and high isolation are obtained in the OFF-state.In addition,this series filtering switches do not need additional PCB circuit to integrate the diode,which simplifies the circuit construction.