Sharp-rejection dual ultra-wideband bandpass filter using sextuple mode resonator

In this paper, a novel sextuple mode resonator (SMR) is proposed to design a compact ultra-wide dual-band bandpass filter (BPF) with high selectivity. An even- and odd-mode analysis method is employed to analyze the proposed SMR, and the SMR can excite six resonant modes simultaneously. In addition, a cross shape and several open-circuit stubs are used in the proposed SMR for generating multiple transmission zeros (TZs). The TZs can divide the transmission poles into two groups to form the two passbands. With properly tuned the TZs and the transmission poles, the proposed dual-band ultra-wideband BPF is of high selectivity. The center frequencies (CFs) of the BPF are centered at 1.6 GHz and 5.2 GHz, respectively. The 3-dB fractional bandwidths (FBW) of the two passbands are larger than 145% and 25%, respectively. To demonstrate the performance of the proposed method, an ultra-wide dual-band BPF is fabricated. Measurement results show that the proposed BPF has high band-to-band isolation, wide stopband, low insertion losses (ILs), controllable bandwidth, and compact size. The stopband is up to 12.5 fc1 (fc1 is the center frequency of passband 1st), and the circuit size is only 0.1λg × 0.12λg.