Wideband bandpass filters with high passband selectivity using SAW-like resonators

A wideband bandpass filter (BPF) using two cascaded sections, each consisting of a SAW-like resonator and one or two transmission lines, is proposed. The lower and higher transitions of the passband are separately defined by the SAW-like resonators. The roll-off at the band edges can be controlled by properly designing the SAW-like resonators to achieve a highly selective passband. As an example, a prototype BPF centered at 0.42 GHz is implemented on PCB, in which the SAW-like resonators are realized by lumped elements. The measured filter has a 3-dB bandwidth of 47% and high passband selectivity is observed. To obtain a wideband rejection, one transmission line in the filter is loaded with an open stub and a low pass filter is added. Experimental results show good high out-of-band rejection up to more than 20 GHz.