Design of Compact Quasi-Elliptic Bandpass Filters Based on Coaxial Inset Resonators

This article reports a novel inset resonator configuration for coaxial filter applications with quasi-elliptic responses. The design and analysis of the inset resonator are discussed in detail and accurately modeled as a capacitively-loaded stepped-impedance half-wavelength resonator featuring more compactness, high quality factor, and enhanced spurious responses in comparison with conventional half-wavelength and combline resonators. Additionally, the operating frequency can be tuned intrinsically through the displacement of the coaxial resonator, eliminating the need for any additional tuning elements and maintaining a stable quality factor. Two quasi-elliptic inset resonator type filters are implemented in planar and longitudinal coupling configurations, respectively. The first takes the form of a folded four-pole 2.93 GHz filter with two symmetrical transmission zeros. The fabricated filter has a compact structure of 29.76 cm3, an insertion loss better than 0.73 dB, a return loss better than 18 dB, and a wide spurious-free band up to 3.5⋅f0 . The second inset-type quasi-elliptic filter is realized in a longitudinal inline arrangement. An example of a 2.53 GHz three-pole filter is presented with a closely-positioned transmission zero, wide spurious-free band ( ≈3⋅f0), and a very compact structure of 55.7×33×33 mm3.


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