Computer-aided design of a compact dual-band bandpass filter

A compact dual-band bandpass filter (BPF) is designed using stub loaded resonator. The resonator has a circular via at the mid-point of the resonator, which not only allows dual-mode implementation but also shift the harmonics so that the first spurious response occurs at frequency higher than the second passband. The structure is then improved for three modes in the second passband, a better selectivity and smaller overall circuit size. The filter is designed to have center frequencies at 2.4 GHz and 5.2 GHz. The measured 3dB bandwidths for the first and second passbands are 9.9% and 12.1%, respectively. The measured insertion losses for the two passband frequencies are 0.9 dB and 1.5 dB.