Quad-band Balun Filter using Dual-mode Spiral Slotline Notched Structure with Enhanced Selectivity

A quad-band balun bandpass filter (BPF) based on dual-mode notched-band structure is presented in this paper. The proposed notched structure is composed of two spiral slotline stubs (SSSs) loaded along the main slotline path. Each SSSs pair introduces two transmission zeroes (TZs) with enhanced roll-off skirt selectivity. By utilizing microstrip and SSSs structures on both side of the substrate respectively, the proposed balun BPF achieves competitive selectivity over four operating passbands, as well as good balance performance. Moreover, the harmonics of introduced multiple TZ pairs help to suppress high frequency resonance, resulting in a wide stopband. A prototype quad-band balun BPF operating at 1.6, 2.5, 3.5 and 5.3 GHz is fabricated and measured results demonstrate an amplitude imbalance of less than 0.45 dB and a phase imbalance of less than 5.1°.