Design of a four-pole wide stopband bandpass filter using combined quarter-wavelength resonators and stub-loaded SIR

In this paper, a four-pole microstrip bandpass filter (BPF) with excellent stopband characteristics is proposed. The proposed filter consists of two quarter-wavelength resonators and a stub-loaded stepped-impedance resonator (SIR). Higher order harmonics of the proposed filter are suppressed by using two kinds of resonators which have the same fundamental resonant frequency, but different harmonic responses. Moreover, a cross coupled-line between two quarter-wavelength resonators creates four transmission zeros which improve the selectivity as well as the stopband performance of the proposed filter. The filter is designed with the center frequency of 2.1 GHz, and the stopband with attenuations higher than 20 dB extents up to approximate 10.8 GHz, which is 5.14 times the center frequency of the filter. Experimental results show good agreement with the simulated ones, which validated the proposed design.