A High-Selectivity Quad-Band Bandpass Filter Utilizing Asymmetrical Interdigital Coupled Lines and Circular Spiral Lines

This letter proposes a novel high-selectivity quad-band bandpass filter (BPF) with nine transmission zeros (TZs) using asymmetrical interdigital coupled lines and circular spiral lines. By deploying five asymmetrical circular interdigital coupling lines, a quad-band BPF with seven TZs is achieved. With adding two open circular spiral lines, two additional independent TZs are obtained. The nine controllable TZs contribute to selectivity improvement significantly. The fabricated BPF demonstrates four passbands located at 2.4, 3.5, 4.5, and 5.8 GHz, which are suitable for Wi-Fi and 5G communication. Within the four passbands, the minimum insertion losses are 1.3, 0.9, 2.2, and 1.3 dB, and the shape factors are 2.4, 1.8, 1.9, and 1.9, respectively. The overall size of the fabricated filter is 15.5 × 22.5 mm
²
. Compared with the other double-layered works, this letter demonstrates significantly enhanced selectivity and competitive insertion loss.