Self-Equalized Microstrip Filers Using Frequency-Variant Couplings

This work presents a design method for self-equalized microstrip filters using nonlinear frequency-variant couplings (FVCs). Some types of such couplings allow one to produce a complex-conjugate pair of transmission zeros (TZs) and introduce an additional transmission pole, effectively increasing the filter order by one. A distinctive feature of the employed type of FVC is its ability to generate complex TZs even in an inline topology. The concept is validated through two experimental filters as follows: one providing a nearly flat in-band group-delay response and the other exhibiting an in-band rippled group-delay profile. The obtained results confirm the practical feasibility of the engineered approach and its potential for compact, high-performance microwave filter applications with low phase-distortion requisites.