This letter reports the design techniques for a class of inline waveguide bandpass filters with sharp-rejection capabilities at the lower stopband based on a novel nonlinear-frequency-variant-coupling (NFVC) structure. The proposed NFVC consists of a modified-T-shaped metallic post (MTP) that is placed at the center of the waveguide broad wall with its open arms lying along the waveguide width. The engineered NFVC structure produces a first-order bandpass filtering transfer function with a pair of transmission zeros (TZs) located below the passband range. To demonstrate the usefulness of the proposed NFVC inverter, a 9.9-GHz third-order inline waveguide bandpass filter prototype with two TZs is developed and tested. It consists of two half-wavelength cavity resonators coupled together via the conceived MTP coupling structure. The measured results are in close agreement with the EM simulated ones, thus validating the devised waveguide filter design principle.

This work presents the design techniques to realize quasi-elliptic-type inline waveguide bandpass filters based on mutually-coupled rotated-dual-post (MCRDP) nonlinear frequency-variant coupling structures. A pair of closely-spaced partial-height posts placed inside the waveguide allows to pro- duce a first-order bandpass filtering transfer function with two transmission zeros (TZs). The pole can be moved either below or above the TZ pair by separating the posts laterally along the length of the waveguide and by controlling the sign of the mutual coupling between the posts. The mutual-coupling sign can be reversed by placing both posts either at the same or at opposite broad walls of the waveguide. EM simulated results for two third-order filters with a pair of TZs located above and below the passband are provided to validate the proposed design theory.

This work presents the design techniques to synthesize a class of compact inline quasi-elliptic-type waveguide cavity bandpass filters based on novel nonlinear frequency-variant coupling (NFVC) inverters. These highly dispersive frequency variant couplings are realized by means of a pair of partial height posts that are placed at the junctions between each two cavity resonators. Each NFVC produces a transmission pole in between a pair of independently adjustable transmission zeros (TZs). Whereas the pole is added to the overall filtering function to augment its order, the TZs can be placed at each side of the filter passband to attain sharp rejection capability and increase the stopband attenuation levels. To synthesize these filters, two coupling-routing-diagram (CRD) approaches for the NFVC are presented that either consider (i) an arbitrary frequency-variant coupling (AFVC) or (ii) two resonating nodes interacting with a zero-susceptance non-resonating node through constant inverters. An equivalent lumped-element circuit model associated with both CRD approaches is provided. It is demonstrated that both CRD models can be exploited for the theoretical synthesis of this type of filters, whereas the equivalent lumped-element circuit model can provide a deeper insight for the systematic dimensioning of the posts. For experimental validation purposes, three design examples are synthesized, and 10-GHz proof-of concept filter prototypes of two of them are EM-simulated, fabricated, and characterized. The measured results agree well with the simulations and the design theory, thus verifying the concept of inline waveguide cavity filters with TZs using NFVCs.

A novel, general circuit-level description of coupled-resonator microwave filters is introduced in this paper. Unlike well-established coupling-matrix models based on frequency-invariant couplings or linear frequency-variant couplings (LFVCs), a model with arbitrary frequency-variant coupling (AFVC) coefficients is proposed. The engineered formulation is more general than prior-art ones and can be treated as an extension of previous synthesis models, since constant or linear couplings are special cases of arbitrary frequency dependence. The suggested model is fully general, allows for AFVCs with highly nonlinear (even singular) characteristics, loaded or unloaded non-resonating nodes (NRNs), frequency-dependent source-load coupling, multiple frequency-variant cross-couplings, and{/}or multiple dispersive couplings for connecting the source and load to the filter network. The model is accompanied by a powerful synthesis technique that is based on the zeros and poles of the admittance or scattering parameters and the eigenvalues of properly defined eigenproblems. In the most general case, the zeros and poles of the admittance or scattering parameters are related to solutions of nonlinear eigenvalue problems. The synthesis is defined as an inverse nonlinear eigenvalue problem (INEVP) where the matrix is constructed from three sets of eigenvalues. This is accomplished by optimization using an iterative nonlinear least-squares solver with excellent convergence property. Finally, third- and fifth-order examples of bandpass filter topologies involving AFVCs are shown, and the experimental validation of the proposed theory is presented through the manufacturing and characterization of a microstrip filter prototype with transmission zeros (TZs)