Parametric modeling of the coupling parameters of planar coupled-resonator microwave filters

The design of planar coupled-resonator microwave filters is widely based on coupling matrix theory. In this framework a coupling matrix is first obtained during the synthesis step. Next this coupling matrix is physically implemented by correctly dimensioning the geometrical parameters of the filter. The implementation step is carried out using simplified empirical design curves relating the coupling coefficients and geometrical parameters. The curves typically only provide initial values and EM optimization is often needed such that the filter response meets the specifications. This paper proposes to extract parametric models that relate the filters design parameters directly to the coupling parameters. The advantage of such models is that they allow to tune the filter in a numerically cheap way and that they provide physical insight in the filters behavior. This paper explains how such models can be extracted from EM simulations and illustrates the technique for the design of an 8 pole cascaded quadruplet filter in a microstrip technology.