Study On The Synthesis And Design Of Filters With Frequency-dependent Couplings

New research results of passive microwave filter design are keeping appearing in relevant community,although corresponding synthesis theories and techniques have been developed for about a century.It’s also exciting to see that computer technologies and algorithms have assisted a lot in filter design process,e.g.numeric calculating methods and parameter extraction methods.However,it is still meaningful for researchers to explore flexible and accurate design theories to adapt to today’s complex and changeable communication systems.Frequency-dependent coupling（FDC）filter is very promising in wireless communication due to its more useful characteristics compared with traditional filter,for example,when introducing transmission zeros（TZs）,the filter body can be smaller,lighter and more compact.This dissertation focuses on synthesis methods and improvements of FDC filter.Firstly,two different methods for synthesizing FDC filters are given.One of them is by using matrix transformation method,including rotation and scaling,to generate FDC matrix,demonstrated as M₀+M_d（ω）.The other is based on combination of filter parameter extraction method and traditional synthesis theories,through iteration to obtain equal-ripple response in pass band.In the the latter method,this dissertation adds a new decision nodes to design process to deicide whether the filter design has been convergent,by judging convergence of inverter impedance（K）.Then,a 6 order Ku-band inline FDC filter with offset partial-height posts coupling is implemented by using iteration method.However,the offset partial-height posts structure will destroy out-of-band performance due to the spurious mode,called TE₁₀₃-like mode,which appears between two adjacent cavities.This dissertation proposed two solutions to solve this issue for the first time.Firstly,one can replace the constant coupling elements into FDC elements to generate additional TZs in the spurious pass band to suppress it,and a Ku filter designed with this method has achieved-20 d B attenuation in spurious pass band.Secondly,by changing the inductive coupling irises into capacitive coupling slots,the spurious mode can also be attenuated.At last,with initial dimensions given by using matrix transformation method,the out-of-band insertion loss is suppressed below-30 d B after a tuning process based on filter parameter extraction theory.And both the improvements of out-of-band performance are verified by simulated and measured results.