Study Of Electrically Tunable Dielectric Resonator Bandpass Filter

Electrically tunable filters have been widely used in anti-tracking frequency hopping communication,multiband communication,radar,electronic countermeasure and software defined wireless system.Their performance directly affects the quality of the whole wireless communication system.Dielectric resonator bandpass filters exhibit the advantages of small size,excellent thermal and structural stabilities.Nowadays they have become one of the most favored microwave components for base stations,mobile phones and vehicle mounted stations.Despite the wide and rising applications of electrically tunable dielectric resonator bandpass filters,there has been very little research on them to date.The research work in this dissertation is an attempt to contribute to the field and to fill in some gaps in research in the field.Indeed,in this dissertation,several tunable filters with different characteristics have been developed at the first time in the literature,including the tunable dielectric filters with constant bandwidth,the tunable dielectric filters with both tunable bandwidth and center frequency,and dielectric resonators&filters tuned by piezoelectric transducers.Tunable dielectric resonator bandpass filters have been develop in this dissertation,including the design and theory as well as verification by experiments.The novel work and results are listed as follows:1.A composite step-impedance resonator(CSIR)composed of coaxial dielectric resonator and microstrip line is proposed for the first time in the literature,where the novel CSIR can control the electrical coupling and magnetic coupling,and realize the mixed coupling to get the desired coupling coefficient which is satisfied with the constant bandwidth.The coupling mechanism of a pairs of CSIRs coupled to each other is analyzed.The tunable dielectric filters with constant absolute bandwidth(CAB)as well as with constant fractional bandwidth(CFB)are realized.The structure parameters of tunable filters are optimized by using aggressive space mapping(ASM)technique.A tunable dielectric filter with CAB is designed and fabricated by using CSIR with the impedance ratio more than one,where the results showed that the absolute bandwidth varied from 42 to 47MHz in the center frequency tuning range of 0.67?0.98GHz with variable bias voltage being 1?10V.A tunable dielectric filter with CFB is designed and fabricated by using CSIRs with the impedance ratio less than one,where the results showed that the fractional bandwidth varied from 6.4%to 6.8%in the center frequency tuning range of 0.43?0.78GHz with variable bias voltage being 1?10V.Both filters agree with their designs.Performance comparison of the proposed work with the State-of-Art approach has also been undertaken.2.The coupling structures for dielectric filters with tunable bandwidth are searched,and a method which a variable capacitor are inserted between two quarter-wavelength coaxial dielectric resonators to tune bandwidth is proposed.The relationships of passband bandwith and coupling coefficient of dielectric filters with tunable center frequencies against variable capacitance are induclated.Three types of electrically tuned dielectric filters are designed and realized,one with increasing bandwidth and tunable bandwidth at each center frequency by using capacitors as external coupling elements,others with constant absolute bandwidth and fractional bandwith by using inductors as external coupling elements.The parameters of coupling elements desired for constant bandwidth are optimized by using space mapping algorithm.Three types of electrically tuned filters are farbicated and measured,the results show that the range of the center frequencies is from 550MHz to 850MHz,tunable bandwidth is in the ranges of 42-70MHz and 72?100MHz with the bias voltage of the varactors for the frequency tuning fixed,constant bandwidth are 81MHz in the center frequency tuning range of 550-806MHz and 15%in the center frequency tuning range of 537-860MHz,respectively,by controlling simultaneously the bias voltages of varactors for the frequency and bandwidth tuning.Performance comparison of the proposed work with the State-of-Art approach has been undertaken.3.Theoricially the tuning mechanism of a dielectric resonator tuned by a piezoelectric transducer is analyzed.A tunable TE mode dielectric resonator and a dielectric filter are designed and realized by using bilayzer piezoelectric transducer and dielectric resonator with dielectric constant of 45.The measured results are listed as follows:the dielectric resonator tuned by a piezoelectric transducer resonates at 4.1073GHz,the variation of the resonant frequency ranges from negative 16MHz to positive 24MHz while a voltage from negative 27V to positive 27V is applied to the transducer and the resonant frequency varies linearly,the loaded Q value varies from 2850 to 2400,the tuning by transducer is compared with that by a metal rod.While a voltage from 0V to 50V is applied to the transducer,the tuning range of the center frequency of the dielectric filter tuned by a piezoelectric transducer is 4.155?4.205GHz and in-band performance varies slightly.4.The parameters of tuable dielectric filter structures and coupling compnents are optimized to realize electrically tuning filters with various bandwidth characteristic by using ASM technique.The center frequency of a tunable filter is regarded as reference variable is proposed in coarse models,a apporiate optimization algorithm is selected to make the bandwidth(absolute or fractional bandwidth)of the tunable filters constant at each center frequency,and the fine models only verify the extracted parameters.In the parameters extraction processs the variation of frequencies needn't be extracted,which accelerates the optimization.In modeling LC parallel resonant circuits represent the coarse models of dielectric resonators loaded by tunable components,optimization is fast,the most optimized parameters of fine model can be obtained by only 2?4 iterations.