Novel microstrip realization and straightforward design of fully canonical Cul-de-Sac coupling bandpass filters

A novel planar-filter realization of a fully canonical Nth-order Cul-de-Sac coupling matrix that can produce the maximum number of transmission zeros (TZs) is proposed in this paper. It is known that in general, a transversal array coupling topology that offers a generalized Chebyshev function response is hard to design in a physical structure, owing to simultaneous parallel excitation of resonators. To overcome such a drawback, this paper introduces a fully canonical Cul-de-Sac coupling that can be obtained with mathematical transformation from a transversal array coupling. The proposed filter structure for the Cul-de-Sac coupling has two resonator blocks, which are excited by two-branch lines: one is a block of even-mode half-wavelength resonators arranged in parallel to input/output lines, and the other is a block of odd-mode ones. The filter can be easily designed by coupling coefficients between two adjacent resonators without any cross couplings. The effectiveness of the proposed filter is verified with synthesis, design, and test of a 2-GHz fourth-order microstrip filter with four TZs.