A Wideband Balun Filter Based on Folded Ring Slotline Resonator and Dual-Feedback Stubs With >17.8 f₀ Stopband Rejection

This brief proposes a wideband balun filter with ultrawide stopband utilizing a shunt slotline coupled with a folded ring slotline resonator (FRSR) and dual-feedback stubs (DFS). Utilizing the FRSR, there are four transmission poles (TPs) within the passband and ten transmission zeros (TZs) in the range of < 20 GHz generated, which contribute to large bandwidth, high selectivity, and >10-dB harmonic suppression. In addition, the two additional TZs generated by the DFS can improve the harmonic suppression to >18 dB. In the meantime, the folded slotline design effectively reduces the size of the proposed balun filter to 33.2% of the conventional round slotline ring. The fabricated balun filter demonstrates a 111% fractional bandwidth (FBW) centering around $f_{0}$ of 2.25 GHz, a measured stopband up to 40 GHz (17.8 $f_{0}$ ) with a minimum rejection level of 18 dB, and a shape factor of 1.3. To the best of the authors’ knowledge, both the fractional bandwidth and stopband range achieved by the balun filter are among the best.