Compact Tri-Band Bandpass Filter Using Open Stub Loaded Tri-Section $\lambda /4$ Stepped Impedance Resonator

A modified tri-mode
$\lambda /4$
stepped impedance resonator (SIR) is proposed, in which a shunt open stub is loaded in the conventional
$\lambda /4$
tri-section SIR. The resonant frequencies and frequency ratios can be properly controlled by adjusting the loaded open stub. A compact second-order tri-band bandpass filter (BPF) based on two coupled proposed open stub loaded
$\lambda /4$
tri-section SIR is presented. A pair of transmission zeros (TZs) are generated at the lower and upper stopband of each passband for high selectivity and excellent band-to-band isolation. A tri-band BPF with the central frequencies of 1.92, 3.50, 5.75 GHz, and respective fractional bandwidths of 5.4%, 4.0%, and 2.4% is designed and fabricated. Measured results show good agreement with simulations. Moreover, the circuit only occupies
$0.09 \lambda _{\rm g} \times 0.10 \lambda _{\rm g}$
.