A Filtering Balun Utilizing Multi-Ring Loaded Vertical Transition Structure With 161% Fractional Bandwidth and >5.8f₀ Stopband

An ultra-broadband filtering balun with microstrip line-slotline (ML-SL) vertical transition structure and multi-ring slotline termination is proposed in this brief. The proposed multi-ring structure consisting of three closed slotline loops connecting in series with each other is stretched beyond ML-SL transitions as a termination load and helps introduce five transmission poles (TPs). Furthermore, a hybrid structure integrating backside coplanar waveguide (CPW) and topside microstrip-line stepped-impedance resonators (SIRs) are employed to achieve a high harmonic rejection within a wide frequency range. In addition, a low insertion loss can be obtained owing to the reduced radiation loss brought by the fully symmetry of the ring loops. The fabricated balun filter demonstrates a 161% fractional bandwidth (FBW) centering around f
₀
of 3.3 GHz, a minimum in-band insertion loss of 0.37 dB, and a wide harmonic rejection band up to 20 GHz (5.8f
₀
) with a minimum rejection level of 18.9 dB. To the best of the authors’ knowledge, the achieved fractional bandwidth and stopband are among the best.