The design of 2-port microwave filters relies either on an exact synthesis procedure in the t-plane involving one kind of element separated by UEs or on an approximate s-plane technique involving one kind of element separated by immitance inverters. This sort of topology enters in the realisation of directly coupled bandpass filters using half-wave long cavities connected by metal or inductive posts. The circuit is not canonical since the impedance inverters do not contribute to the overall amplitude response of the filter but only provide a practical layout of the circuit elements. The design is completed by physically realising the impedance inverters. The original immittance inverter took the form of a simple quarter-wave impedance transformer. Such filters are known as quarter-wave coupled. The modern version, in which the inverter is realised by a step discontinuity, is known as a directly coupled arrangement. Since the notion of the immittance inverter is central to the design it is given special attention notwithstanding that it is a classic topic in the literature.

Inspec keywords: microwave filters; impedance convertors; coupled circuits; invertors; electric immittance

Other keywords: directly coupled arrangement; quarter-wave impedance transformer; half-wave long cavities; bandpass filters; directly coupled filter circuits; impedance inverters; immittance inverters; 2-port microwave filters

Subjects: Waveguide and microwave transmission line components