Full band 41 filter with high Wi-Fi rejection - design and manufacturing challenges

In this article we present a full band 41 filter using state-of-the-art BAW (bulk-acoustic wave) technology. Due to the unusually large bandwidth requirement (7.8% fractional bandwidth) low insertion loss in the passband cannot be maintained by typical design strategies used for narrower bands. We discuss how to cope with the challenges arising due to the filter´s wide bandwidth and Wi-Fi coexistence requirement on the design side as well as on the manufacturing side. The steepness of the filter´s lower edge plays a major role to fulfill the stringent rejection requirement in the Wi-Fi band, where the use of high-Q BAW technology is indispensable. Additional manufacturing challenges arise to achieve the accuracy needed and guarantee the filter selectivity as well as its low loss performance over the whole band 41.