AN ORIGINAL APPROACH TO THE DESIGN OF BANDPASS CAVITY FILTERS WITH MULTIPLE COUPLINGS

A novel approach to the design of general cavity filters with each cav ity coupled to an arbitrary number of other cavities is presented, Thi s approach is based on a suitable characterization of the filter struc ture which does not require to model separately the cavities (resonato rs) and the coupling elements, Suitably defined equivalent admittances are associated with each cavity allowing to design the filter structu re once the parameters of a suitable low-pass prototype are given; an efficient procedure for the synthesis of such a prototype with equirip ple passband response is also presented which allows to arbitrarily pr escribe transmission zeros placed in the complex plane (even asymmetri cally). The described design approach is particularly convenient when the filter structure does not allow a simple modeling of the resonator s and coupling elements separately, This is the case of slot-coupled c avity filters and of filter structures based on arrays of coupled tran smission lines, It is also shown that the simplified design approach o ften adopted in the past, where only two coupled cavities at a time ar e considered, can produce large errors even in the case of filters wit h all attenuation poles at infinity (i.e., two couplings per cavity).