Self-Equalized Generalized Chebyshev Microwave Filters in Inline Topology Using Double-Zero Single-Pole Frequency-Variant Coupling Networks

The objective of this article is to demonstrate the applicability of frequency-dependent couplings (FDCs) to the design of self-equalized, generalized Chebyshev microwave bandpass filters (BPFs) in inline coupled-resonator circuit topologies. To this aim, a family of frequency-variant reactive coupling (FVRC) networks with double-zero single-pole (DZSP) characteristics is exploited, where the zeros can be either positioned at the imaginary axis or as a pair of real zeros. Thus, flattened-group-delay sharp-rejection microwave BPFs with in-band equi-ripple-type response and close-to-passband transmission zeros (TZs) can be realized while avoiding more-complex cross-coupling structures. The theoretical foundations of the proposed class of DZSP FVRC networks for the flexible allocation of the two zeros in the complex plane, as well as different circuit variants for their implementation, are presented. Two design examples of self-equalized fifth-order microwave BPFs in lumped-element/transmission-line and 3-D technologies centered at 1.5 and 9.98 GHz, respectively, are also shown, in which different structures for the DZSP FVRC networks are adopted. In both BPF designs, their coupling-matrix-based-synthesis responses obtained from solving an inverse structured nonlinear eigenvalue problem and electromagnetically (EM)-simulated results are provided. Furthermore, for practical validation purposes, a proof-of-concept microstrip prototype of the first BPF design example is built and tested. To the best of the authors’ knowledge, this is the first time that self-equalized, generalized Chebyshev microwave BPFs in inline schemes—i.e., without cross couplings—are experimentally verified.