Synthesis And Application Of Tightly-Coupled Microstrip Lines

With the rapid development of mobile communication,microwave devices are developing towards miniaturization,broadband and multi-function integration,which are important components of the wireless communication system.The coupled transmission lines can be used to realize miniaturization and broadband of microwave devices such as directional couplers,phase shifters and filters,as well as negative group delay(NGD)characteristics of microwave circuits.Due to the limitation of processing technology,the traditional structure of parallel coupled microstrip lines(PCML)can not realize tight coupling.Thus,PCML with floating ground-plane conductor(FGPC)and trans-directional(TRD)coupled microstrip lines with periodical capacitors were proposed to realize tight coupling.However,there is no synthesis method for FGPC coupled microstrip lines and open-and short-circuited TRD coupled microstrip lines,which only can be designed by using electromagnetic simulation software to repeatedly adjust the structure size.The design method using electromagnetic simulation software has a long design cycle,and the optimal performance often cannot be obtained.In order to solve this problem,this dissertation is devoted to the research on the synthesis method of tight-coupling microstrip lines and the design of high-performance,miniaturization and multi-function microwave devices using tight-coupling microstrip lines.The main innovation achievements are as follows:(1)The synthesis method of PCML with FGPC based on artificial neural network and conformal transform analysis is proposed.By adjusting the width of the FGPC strip,the physical size of the PCML having equal even-and odd-mode phase velocities can be calculated directly,which improves the microwave device performance and shortens the design cycle.The experimental results show that the synthesis method can be adopted to enhance the isolation of the broadband directional coupler,improve the return loss of the broadband Schiffman phase shifter and suppress the second-order spurious band of the PCML bandpass filter.(2)An NGD implementation method based on the interference is proposed to reduce the NGD performance degradation due to the temperature-dependent resistance variation.Using the method,a novel NGD microwave circuit consisting of Wilkinson power dividers and PCML with FGPC is presented.There significantly decreases the effect of the temperature-dependent resistance variation on the performances of the NGD microwave circuit.Insertion loss can be controlled by adjusting the power division ratio of Wilkinson power divider,and a larger coupling factor of the PCML result in a larger absolute value of NGD time.The proposed NGD circuit has inherent input and output matching characteristics without the need for additional matching networks,which can be applied to compensate group delay in various microwave circuits and systems.(3)A 900 Schiffman phase shifter with the TRD coupled microstrip line is proposed to realize miniaturization.A?/4 open-circuited TRD coupled microstrip line is introduced to replace the 3?/4 transmission line of the traditional 900 Schiffman phase shifter.The coupling factor of the TRD coupled microstrip line can be used to control the bandwidth,which adds an additional variable to the design of the Schiffman phase shifter and improves the design flexibility.Compared to the existing 900 Schiffman phase shifter,the reference line of the proposed phase shifter is reduced by two thirds in length.(4)A miniaturized rat-race coupler with harmonics suppression and a miniaturized rat-race coupler with a tunable power dividing ratio are proposed.In both multi-function couplers,the ?/4 short-circuited TRD coupled microstrip lines are introduced for miniaturization.For the harmonics-suppression rat-race coupler,T-type transmission line networks are adopted to further reduce the circuit size and realize the harmonic suppression.For the rat-race coupler with the tunable power dividing ratio,varactor diodes are used to realize the adjustment of power dividing ratio.And the short-and open-circuited TRD coupled microstrip lines having inherently 180° phase differences between them are used to improve the isolation and the bandwidth of anti-phase output with±10° phase deviation.Furthermore,synthesis formulas of short-and open-circuited TRD coupled microstrip lines are derived to attain perfect matching and isolation for any coupling factor of the TRD coupled microstrip lines,which are validated by simulations and measurements.