Research On CPW Branch Tuning Technology Based On Liquid Crystal

With the continuous development of modern electronic technology,wireless electronic devices have been increasingly used due to their flexibility and space-free advantages.The demand for wireless frequency band is also increasing.Therefore,broadband,multi-frequency point antennas,filters and other circuits have been developed rapidly to meet the demands.Tunable frequency microwave element is one of the most important part of them.In this thesis,the tunable technology of coplanar waveguide（CPW）stubs of coplanar waveguide based on liquid crystal is studied.Liquid crystal is utilized to realize the continuous tunable resonance frequency point of CPW stubs structure.The tunable CPW stubs structure based on liquid crystal is designed by adding liquid crystal material and external bias voltage on the conventional CPW stubs resonance structure.The tuning of resonance frequency points of CPW stubs is realized by using the characteristics of liquid crystal with different equivalent dielectric constant under different external bias electric fields.The contents and results of this thesis are organized as follows:1.CPW stubs structure design.Three kinds of CPW stubs structures with different structures are designed by using electromagnetic simulation software HFSS,that is,CPW series-parallel shorted stubs,open-end and shorted series stubs and opened series stubs.The characteristics are simulated and analyzed.2.Design of tunable CPW stubs structure based on liquid crystal.The liquid crystal layer is sandwiched between the top substrate with CPW stubs structure and the bottom substrate covered with Cu layer.The tunable liquid crystal CPW stubs structure is simulated by HFSS.The effect of the equivalent dielectric constant of liquid crystal to the tunable characteristics is investigated.The simulation results that the designed structure can achieve continuous tunability of the resonant frequency.3.The tunable CPW stubs based on liquid crystal was fabricated and the characteristic of the tunability was experimentally demonstrated.Three didfferent kind of liquid crystal materials are used for three different liquid crystal tunable CPW stubs structures.The tunable characteristics of CPW stubs based on liquid crystal materials are verified experimentally,and the results are analyzed.It can be seen that the liquid crystal tunable CPW series-parallel shorted stubs structure has good resonant frequency point tuning characteristics.The characteristics of CPW tunable structure are significantly influenced by different liquid crystal materials.When the liquid crystal material 5CB is used,the resonance frequency point f1 can be shifted from 3.48 GHz to 3.4 GHz while the resonance frequency point f2 from 7.2 GHz to 6.95 GHz,when the applied bias voltage is increased from 0 V to 42 V.The corresponding offset isΔf₁=80 MHz andΔf₂=250 MHz,respectively.Moreover,based on the test results,the equivalent dielectric constant of different liquid crystal materials under different bias voltages is fitted for three different structures.It provides theoretical and experimental support for the design of tunable microwave elements based on liquid crystal.