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

Abstract

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 cm 3, an insertion loss better than 0.73 dB, a return loss better than 18 dB, and a wide spurious-free band up to $3.5\cdot f _{0}$ . 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 ( $\approx 3\cdot f _{0}$ ), and a very compact structure of $55.7\times 33\times 33$ mm 3.

Keywords

• Coaxial filter
• inset resonator
• miniaturization
• passive components
• tunable filters
• waveguide technology