Investigation Of Hign-Performance Commensurate-Line Filters

With the rapid development of wireless networking and communication systems,microwave filters have received tremendous challenge due to ever-increased stringent requirements.On the one hand,emerging wireless communication systems and applications require high selective microwave filters to select wanted signal frequencies efficiently from ever more crowded frequency bands.On the other hand,the requirement of high-data access promotes the development of wideband and ultra-wideband filters.Meanwhile,the demands of compact,lightweight and low volume filters enable planar filters to be researched extensively.Thus,a method of achieving better performance with less elements tends to be one key issue to solve these problems.Commensurate-line filters are inclined to be ignored by nowadays designers because of the simple structure and well-established design theory.However,this type of filters still plays an important role in transmission line filters for that their frequency dependence can be well described by Richards? transformation over the entire spectrum.Taking advantange of such a property,this dissertation investigates and proposes many high-performance commensurate-line filters;put forward a systematic and exact method to design cross-coupled commensurate-line filters using the lumped lowpass prototypes.The main academic contributions of this dissertation are as follows:1.Two bandstop sections with electric/magnetic couplings are investigated and analysed from the viewpoint of lumped equivalent circuits.It is found that the two sections are third-order filters in dual circuits.Higher order bandstop filters comprised of these third-order sections can be exactly designed from the lumped lowpass prototypes.2.A set of lumped lowpass bridged-T sections is proposed.It is found that these sections are equivalent or dual circuits of each other and equivalent to fifth-order lowpass ladder filters.Based on these bridged-T lowpass sections,a series of cross-coupled or mixed electric-magnetic coupled commensurate-line bandstop filters are proposed and exactly designed using the extensive design tables for lowpass prototype ladder filters.Explicit formulas are derived from equivalent network transformation.These bandstop filters typically features elliptic-function or generalized Chebyshev responses over wideband or ultra-wideband spectrum.3.A set of lumped highpass bridged-T sections is proposed from their lowpass counterpart.Two groups of commensurate-line bandpass filters,which are equivalent or dual with each other,are designed from the highpass sections.These bandpass filters typically have mixed electro-magnetic couplings from the viewpoint of lumped equivalent circuit and can also be designed directly from elliptic-function or generalized Chebyshev lowpass ladder prototypes based on explicit formulas.