Microwave filter design based on coupling topologies with multiple solutions

Extraction techniques of circuital coupling parameters from measured or simulated scattering data have become a cornerstone of modern microwave filter synthesis. These procedures allow to compare the extracted parameters to an ideal coupling matrix (golden goal) and to infer from it precise and localized corrections on the filter. Circuital extraction techniques relying on optimization, lead by construction to a unique coupling matrix. This is in contradiction with the fact that numerous coupling topologies admit multiple solutions for the coupling matrix synthesis problem. Identifying an erroneous circuit can however destroy the whole tuning process, leading to incoherent corrections on the filter. In this paper we present a filter tuning method that systematically extracts all admissible coupling matrices and eventually identifies the physical one, that is the one that the DUT implements. The method is then illustrated and validated by the synthesis of a filter with a cascaded quadruplet topology, in microstrip technology.