Miniaturized quad-band bandpass filter using quad-mode stepped impedance resonator

A miniaturized quad-band bandpass filter (BPF) using compact quad-mode stepped impedance resonator (IQMSIR) is proposed. Based on the symmetric presented by the resonator, four modes are deduced using even-and odd-mode technique. Design formulas are also derived and they are used to guide the filter design. Multiple coupling circuit technique is employed to realize the four passbands. Independent design for four passbands are achieved due to the physical dimensions and additional cross slots. Upper stopband is performed by adding open loop SIR at I/O ports. Transmission zeros among each passbands are generated, resulting in high isolation and selectivity. A quad-band filter is designed, fabricated and measured. The operating central frequencies of the fabricated filter are at 2.54/3.36/5.24/6.6 GHz. The measured 3-dB fractional bandwidths of the four passbands are about 3.9/6.4/4/4.6% and the minimum insertion losses are about 2.3/0.89/3.2/2.3 dB. Measurements correlate well with the simulation results.