Dual-band Bandpass Filter Using Two Ring Structures With Good Out-of-band Suppression

In this paper, a novel dual-band bandpass filter (BPF) using two ring structures with good out-of-band suppression is proposed, which is constructed by three pairs of coupled lines and four transmission lines. Synthesis theory of dual-band BPF is introduced, and nine examples with different passband fractional bandwidth (FBW) and frequency ratio (FR) are presented for demonstration. In order to verify the proposed design theory, an example centered at 3 GHz with frequency ratio 3.36:1 and fractional bandwidth 5% is designed and fabricated. The measured out-of-band suppression performance in the experiment maintained -41.1 dB at low frequency, which matched well with the simulated result.