Research On High Isolated Diplexer Without Extra Matching Network

Nowadays,with the rapid development and change of the wireless communication technology,the requirements for miniaturization,integration,and high performance(isolation,sharp factor)of radio frequency front-end circuits in communication systems are increasing.As a three-port device in the RF front-end,the diplexer plays an important role in seperating and filtering the signals in the transmitting and receiving channels.Commonly,the matching network and filtering structure are designed separately,and the matching network outside the filtering structure may occupy an additional circuit of the diplexer.Therefore,it is of great significance to design a diplexer without additional matching network.Based on composite resonator and improved coupling structure,the design method for high isolated diplexer is studied.Moreover,a design method based on open-circuited stubs,shorted-stubs,J-inverters and impedance transformation theory for diplexer without extra matching network is proposed.Three types of diplexers are designed to verify the proposed design method.The main work and innovations of this thesis are including:(1)A method for designing diplexer without external matching network is proposed in view of the transmission zeros produced by multiple open stubs in the composite resonator.The detials are using open stub to generate the transmission zero at the center frequency of the other channel of the diplexer.After the impedance transforming derivation of open stubs and J-inverters,the designed filter can be directly connected to the common port,and good filtering characteristic and isolation can be obtained,simutaneously.Simulated and measured results show that the isolation of the proposed diplexer is better than 40 d B.In addition,a wide-stopband switchable diplexer with incorporating PIN diodes and low-pass filters has been also studied.(2)In order to further improve attenuation of the lower stopband and upper stopband of a diplexer composed of traditional composite resonator,modified composite resonator is proposed.By replacing the quarter-wavelength microstrip line with gap coupling of the J-inverter between composite resonators,and improving the uniform impedance open stub with stepped-impedance open stub,a novel diplexer is realized.By using the modified composite resonator and design method of diplexer,second and third order diplexers are proposed.The isolation of the diplexer is better than 40 d B and 59 d B,respectively.In addition,PIN diodes are loaded into the J-inverters of second-order diplexer to achieve independently switchable performance of each channel.The application of the proposed diplexer is extended.(3)Based on the embedded shorted-stub resonator,a resonator coupled diplexer without extra matching network is proposed.Novel coupling method is used and three transmission zeros are observed in each channel of the diplexer,where the transmission zeros are generated by open and short stubs.According to the theory of impedance transformation,the conditions for constructing a diplexer without additional matching network are derived.Therefore,a diplexer without additional matching network was designed by using the open stub and short stub and impedance transformation theory.Rich transmission zeros and good passband selectivity are observed in each channel from the simulation and measurement results.Finally,the isolation level is higher than 45 d B.