In this paper, a second-order tri-band balanced bandpass filter (BPF) with multiple transmission zeros (TZs) and compact size is presented. The structure consists of novel stepped impedance square ring loaded resonators (SI-SRLRs), which can excite six resonance modes. For design of SI-SRLR, we analysed the odd-mode equivalent circuit and obtained the electrical lengths from the design graph. Meanwhile, the wider frequency distances between differential modes (DMs) and common modes (CMs) are realized by selecting the proper admittance ratio of SI-SRLR. Then for design of BPF, six TZs are introduced by source-load coupling, which lead to band-to-band isolation of 23 dB. Additional T-shaped stubs and open stubs are loaded on the symmetric plane of SI-SRLR, which result in high CM suppressions of 43 dB, 25 dB and 37 dB at three DM centre frequencies. Finally, a tri-band differential BPF operating at 1.46 GHz, 4.45 GHz and 5.48 GHz is fabricated and measured. The measured 3-dB fractional bandwidths of three passbands are 6.8%, 7.4% and 5.6%. A wide DM and CM stopband suppression of 20 dB is achieved to 14.6 GHz (10f0). The measurements verify well the proposed structure and the design method.
Li Tian Wang,
"Compact Differential Tri-Band Bandpass Filter with Multiple Zeros Using Sext-Mode Stepped-Impedance Square Ring Loaded Resonator," Progress In Electromagnetics Research C,
Vol. 117, 73-87, 2021. doi:10.2528/PIERC21092404
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