Compact Stripline Dual-Band Bandpass Filters With Controllable Frequency Ratio and High Selectivity Based on Self-Coupled Resonator

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.