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Abstract

A bandpass filter includes a combination of a BAW filter and a patterned planar filter with stubs. The BAW filter is composed of a plurality of piezoelectric resonators to give a specific frequency bandpass, while the planer filter is configured to attenuate frequencies near and outside the bandpass. The resonators are connected in a ladder configuration between a first signal transmission path and a ground. The planar filter includes a strip line formed on a dielectric layer to define a second signal transmission path. The BAW filter and the planar filter are formed on a common substrate with the first and second transmission paths connected to each other. The BAW filter, in combination with the patterned planar filter added with the stub, can improve a deep near-band rejection inherent to the BAW filter, exhibiting an excellent out-of-band rejection over certain adjacent frequency ranges outside of the bandpass, and therefore give a sharp and wide bandpass.

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