Design of Miniaturized Dual-Band Low-Pass–Bandpass and Bandpass Filters

In this paper, a lumped-element structure with four potential transmission zeros (TZs), one zero-value transmission pole (TP), and three nonzero TPs is first proposed to design dual-band low-pass-bandpass filter (DB-LBF). Then, a pair of capacitors to block zero-value TP and an inductor to generate another nonzero TP are introduced in the DB-LBF structure to transform the low-pass passband into bandpass passband, so that a dual-band bandpass filter (DB-BPF) is constituted. Closed formulas and guidelines are given for two types of filter design. As examples, a 0.9/2.4-GHz DB-LBF with compact size of 0.05λ
_{g_lb}
× 0.079λ
_{g_lb}
and a 0.395/1.575-GHz DB-BPF with compact size of 0.032λ
_{g_bb}
× 0.027λ
_{g_bb}
are designed and fabricated. Both DBLBF and DB-BPF have two independently controlled passband frequencies and also exhibit wide passband, low insertion loss, good return loss, and sharp passband selectivity. Moreover, they are also compatible with integrated circuit process.