Split-type quad-band bandpass filter with quarter-wavelength resonators

In this paper, a split-type sharp rejection microstrip quad-band bandpass filter (BPF) with X/4 resonators has been presented. Two split-type dual-band BPFs are studied and combined to form quad-band responses. The quadruplet coupling topology is introduced to generate source-load coupling, resulting in one pair of transmission zeros at each side of all the four passbands. Eventually, a quad-band BPF centered at 2.1, 2.4, 3.9 and 4.5 GHz with respectively fractional bandwidths of 8.5%, 3.4%, 5.5% and 4.9% has been designed and implemented, whose measured results provide a good verification for the simulated ones.