A Self-Equalized Waveguide Filter With Frequency-Dependent (Resonant) Couplings

This letter presents a design of a fifth-order linear phase filter with frequency-dependent couplings. The filter is composed of a triplet that is directly coupled to two resonators at the input and output. To provide group delay flattening a cross-coupling in the trisection has a strongly dispersive character with a negative slope parameter. To achieve this, an E-plane stub with a septum was used. To further improve the filter selectivity, the direct couplings connecting the triplet to the outer resonators are also frequency dependent. In this particular case, these were implemented in the form of two partial-height conducting posts to provide two imaginary transmission zeros, one on each side of the stopband. With three strongly dispersive couplings, four zeros were obtained in a fifth-order topology that generates only one imaginary transmission zero in a classical design. The filter was realized in the WR-90 waveguide and the measured characteristics match the simulated ones, confirming the validity of the concept.