Design of Tunable Coaxial Bandpass Filters With Constant Absolute Bandwidth Using One Tuning Element

• Published in: IEEE transactions on circuits and systems. II, Express briefs Express briefs, 2024-04, Vol.71 (4), p.1-1
• Main Authors: Yan, Jun-Jie, Chen, Fu-Chang, Xiang, Kai-Ran, Tu, Zhi-Hong
• Format: Article
• Language: English
• Subjects: Band-pass filters; Bandpass filters; Bandwidth; Bandwidths; Cavity resonators; Coaxial resonator; Configurations; Constant bandwidth; Coupling structures; Couplings; Delays; Resonant frequencies; Resonant frequency; Resonator filters; Tunable filer; Tunable filters; Tuning
• ISSN: 1549-7747; 1558-3791
• DOI: 10.1109/TCSII.2023.3331445

This paper describes a method of designing coaxial wide-range tunable filter with constant absolute bandwidth (CABW). The slots placed on the top of the coaxial resonator and the tuning element form an interdigital structure to enlarge loading capacitance, which extends the tuning range of the coaxial cavity resonant frequency. Based on this resonator, novel input/output (I/O) and inter-resonator (IR) coupling structures are proposed to meet the requirements of the CABW of the tunable filter. A fourth-order inline bandpass filter is designed for verification. Furthermore, methodology of designing cascaded-quartet (CQ) configuration filter with two transmission zeros (TZs) is proposed. The fabricated inline filter exhibits a wide tuning range from 590 MHz to 765 MHz (25.8%), over which the absolute bandwidth (ABW) variation is within 16.2±1.2 (±7.4%) MHz while the CQ-configuration filter provides a tuning range from 565 MHz to 765 MHz (30.1%) over which the ABW variation is within 15.285±0.335 MHz (±4.4%).