Compact Tunable Balanced Bandpass Filter With Constant Bandwidth Based on Magnetically Coupled Resonators

• Published in: IEEE microwave and wireless components letters Vol. 29; no. 4; pp. 264 - 266
• Main Authors: Deng, Hong-Wei, Sun, Liang, Liu, Fei, Xue, Yi-Fan, Xu, Tao
• Format: Journal Article
• Language: English
• Published: IEEE 01.04.2019
• Subjects: Band-pass filters; Bandwidth; Constant bandwidth; Couplings; high common-mode (CM) suppression; high selectivity; magnetically coupling; Microstrip; Passband; Resonators; tunable balanced filter; Varactors
• ISSN: 1531-1309; 1558-1764
• DOI: 10.1109/LMWC.2019.2902328

In this letter, a compact high-selectivity and common-mode (CM) suppression tunable balanced microstrip bandpass filter (BPF) with constant bandwidth is designed by adopting the magnetically coupled resonators with step-impedance parallel coupling feedline (SIPCF) structure. The desired coupling coefficient <inline-formula> <tex-math notation="LaTeX">K_{12} </tex-math></inline-formula> and external quality factor <inline-formula> <tex-math notation="LaTeX">Q_{e} </tex-math></inline-formula> can be obtained by the mixed coupling between the two resonators and SIPCF structure, respectively. Two differential-mode (DM) transmission zeros (TZs) near the lower and upper passband edges are created by the mixed coupling and source-load coupling, respectively. Furthermore, intrinsic high CM suppression in the DM tunable operating passband can be maintained because of the magnetically coupled resonator structure. Finally, a compact tunable balanced BPF prototype with constant fractional bandwidth of 6.5% is implemented and fabricated. The measured results agree well with the simulated ones.