Journal
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES
Volume 68, Issue 1, Pages 102-110Publisher
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TMTT.2019.2945768
Keywords
Resonant frequency; Passband; Impedance; Dual band; Couplings; Frequency control; Stripline; Controllable frequency ratio; dual-passband filter; self-coupled; stepped-impedance resonator (SIR)
Categories
Funding
- National Natural Science Foundation of China [61771247, 61571468]
- State Key Laboratory of Millimeter Waves [K201921]
- University of Macau [MYRG2017-00007-FST]
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Dual-band stripline bandpass filters (BPFs) with widely controllable frequency ratio and high selectivity is proposed based on a new type of self-coupled resonator. The presented self-coupled resonator evolves from a symmetrical open-ended stepped-impedance resonator (SIR) by folding their two open-ended arms coupled with each other. Due to different capacitance characteristics under even- and odd-mode operation in coupled-line, the two fundamental resonant frequencies of self-coupled SIR bring out dual passbands that can be separated in a wider spacing than the ones generated by noncoupled resonator. In addition, due to the inherent coupling loop, two additional transmission zeros (TZs) are generated in the middle stopband between dual passbands, which greatly improves the isolation between dual passbands. For design guidance, theoretical responses are provided with numerical solutions of each resonant frequency and TZ. For demonstrative purposes, two filters with dual passbands centered at 1 and 4 GHz (frequency ratio of 4:1), as well as 2 and 3 GHz (frequency ratio of 1.5:1) are designed, fabricated and tested. In our designs, edge-coupled striplines are used for the final implementation attributing to its good performances of no frequency dispersion in wideband and easy package. Ultimately, experimental results are displayed to convincingly verify the feasibility of proposed concept.
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