| With the rapid development of wireless communication systems,highly selective communication systems require more microwave circuits and components to accurately screen RF signals in all different frequency bands.Therefore,in order to reduce the size,weight and cost of these systems,tunable or reconfigurable compact devices are required,and in this case,the study of tunable filters is even more important.Microstrip tunable filters are widely used in radio frequency communication fields such as satellite communication,radar,aerospace and other fields due to their simple structure,easy processing and integration,low manufacturing cost,simple operation,and fast response.Therefore,this type of filter is widely used in communication systems.It is more common and has better application prospects.The main research contents and experimental results of this thesis are as follows:1.In this thesis,a second-order dual-passband tunable filter is designed based on the Uniformity Impedance Resonator(UIR)structure.The filter adopts source-load coupling feeding and independent electromagnetic hybrid coupling to introduce four transmission zeros at the same time.Any one of the dual passbands can be independently controlled by the loaded capacitor,and the transmission zeros on the left and right sides of the passband generate synchronous displacement with the shift of the center frequency,so the filter has the transmission zero point with excellent out-of-band rejection and the advantage that the center frequency can be independently regulated.The simulation results show that when only the first passband is tuned,the center frequency of the first passband will move within 1.17～1.72 GHz and the insertion loss will remain at about-1 d B.The transmission zero changes with the center frequency and has better out-of-band rejection,The second passband does not shift significantly,but the insertion loss of the passband will increase to a certain extent.When only the second passband is tuned,the center frequency of the second passband will move within 2.13～2.76 GHz and maintains good out-of-band rejection,but the insertion loss of the second passband gradually increases.When the dual passbands are tuned simultaneously,the passbands have frequency shifts of 1.18 to 1.64 GHz and 1.65 to 2.45 GHz,respectively.The experimental results show that the center frequency and transmission zero point of the dual-pass band filter based on the UIR structure can be adjusted,and its center frequency and transmission zero point can be controlled to shift to both sides of the passband according to the value of the capacitance,and its insertion loss deteriorates slightly during offset,but the tunable filter always maintains normal filtering performance.2.On the basis of capacitor and UIR structure,a multi-pass band-pass filter based on Stepped Impedance Reaonator(SIR)structure and varactor diode whose transmission zeros can be adjusted independently is proposed.The traditional UIR filter design method has been relatively complete,but due to the defects of limited design parameters and spurious response of the UIR structure,the filter of the SIR structure is derived.In this thesis,the independent regulation of the transmission zero is realized by loading a varactor diode on the basis of the Fold Open-loop Stepped Impedance Resonators(FOLSIR).We first analyzed the resonant characteristics of the SIR structure and the generation principle of the multi-pass band,and established the relationship between the resonance frequency,impedance ratio and electrical length.Then,a multi-passband filter is proposed based on the symmetrical folded open-loop step impedance resonator,and better passband characteristics are obtained.Finally,in order to improve the roll-off characteristics of the filter and suppress the passband,a varactor diode is loaded at the end of the resonator to control the coupling strength of the open end and the coupling amount of the odd-even mode of the resonator to realize the independent regulation of the transmission zero point. |
