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Abstract
A dual-band filter consisting of two cascaded triplet (CT) units is presented in this paper. Two sets of λ /4 resonators are employed to construct the CT unit. The utilization of λ /4 resonators not only makes the filter compact, but also allows the inductive cross coupling to be realized conveniently. This results in a transmission zero of finite frequency on the high side of each passband. The bandwidth of each passband can be tuned independently by adjusting the coupling via hole. The source-load coupling is also proposed to introduce two tunable transmission zeros, which can improve the selectivity of the filter. A trial dual-band filter is designed and measured, and the filter centered at 2.45 and 5.25 GHz occupies an area of only 33 × 15 mm2. Measured results show that a rejection level of 20 dB up to more than four times the first passband frequency can be obtained.