Dual-band bandpass filter with improved performance in extended upper rejection band

Stepped-impedance resonators with different dimensions are used to design bandpass filters with a dual-passband response, as well as good rejection levels in the extended upper stopband. To achieve the goal, the resonators are designed to have two identical leading resonant frequencies, but dispersed higher order ones to make spurious peaks have low levels and small bandwidths. The stopband is then extended and the rejection levels are enhanced by collocating transmission zeros with the unwanted peaks. The zeros are tuned by adjusting the coupling lengths of the coupled stages and sliding the tap positions of the dual-band transformers along the end resonators. Measured results of two experimental circuits show a rejection level of 30 dB up to more than eight times the first passband frequency can be obtained. The measured data have good agreement with the simulation.