Design Of Electric Tunable Radio Frequency Filter With Cross-coupling

RF bandpass Filter,which is mainly used to suppress signal out of band,is the essential component for RF front-end in the modern wireless communication systems.RF tunable filter is proposed as a novel filter design theory to meet the development requirement of RF systems being broadband and miniaturized.It introduces new tunable components into the circuits so that the center frequency,bandwidth or position of transmission zeros becoming reconfigurable,which can realize dynamic deployment of RF spectrum resources.Tunable circuit design theory effectively reduces the size and complexity of the RF front-end circuits,hence can be used in multiple application scenarios such as broadband receivers,multimode/multi-frequency communication,and software radio.On the other hand,crosscoupled filter topology introduces extra transmission zeros by designing multiple energy coupling paths,so that frequency selectivity of the filter is significantly enhanced.Crosscoupled tunable filters absorb the advantages of above two,and the related research work is of great significance in improving RF front-ends of broadband miniaturized receivers and radio frequency communication technology.Firstly,a novel fourth-order CQ tunable RF bandpass filter with cross-coupling is designed in this thesis.The design is based on electric and magnetic coupling mechanism of half-wavelength tunable resonator,and cross-coupling matrix theory is used to determine the target curves of coupling coefficient between the resonators.The electromagnetic field energy distribution of the varactor-loaded microstrip resonator is analyzed,so that independent electric dominant and magnetic dominant coupling region are selected to construct energy coupling paths and realize cross-coupled topology.A pair of transmission zeros is generated at both sides of the passband due to the fourth-order cross-coupled topology,and then adjacent band rejection of the tunable filter is enhanced.The measurement shows that the filter achieves a tunable center frequency range from 1.36 GHz to 1.78 GHz,and the rejection of transmission zeros is greater than 20 d B.In addition,nearby by the both edges of passband,the 3d B to 20 d B attenuation slope is better than 515 d B/GHz.The absolute bandwidth keeps in the range of 93±7MHz,with insertion loss varying from 2.8d B to 5d B.Secondly,a novel third-order constant bandwidth tunable cross-coupled RF bandpass filter based on two dual-mode resonators is proposed in this thesis.The design theory is also presented to employ odd-and even-resonant mode of the dual-mode resonators to realize CT cross-coupled filter topology.And the control methods of electric coupling and magnetic coupling in this topology is researched.Detailed derivation and control methods of the coupling coefficients are given,with extracted coupling coefficients from simulation used to validate these conclusions.Center frequency of the passband is tuned due to the loaded varactors,and finally tunable cross-coupled topology with constant absolute bandwidth is realized.The measurement shows that the filter achieves a tunable center frequency range from 1.64 GHz to 2.16 GHz.A transmission zero is generated at the lower side of the passband due to the third-order cross-coupled topology,with the rejection greater than 20 d B.Nearby by the low side of passband,the 3d B to 20 d B attenuation slope is better than 340 d B/GHz.The absolute bandwidth keeps in the range of 119.5±3.5MHz,with a variation rate of bandwidth less than 2.9%.The insertion loss of the filter is varying from 3.9d B to 4.5d B,and the return loss is better than 10 d B during the all frequency tuning range.