A Switchless Quad Band Filter Bank Based on Ferroelectric BST FBARs

An intrinsically switchable ferroelectric barium strontium titanate (Ba( x )Sr(1– x )TiO3, BST)-based film bulk acoustic resonator (FBAR) quad channel filter bank is presented for the first time. The design incorporates a matching network free of tunable elements and provides a high isolation >30 dB across all four filter paths when the resonators are switched off. The measured insertion loss for the four 2.5 stage BST FBAR filters range from 6 to 7 dB with an out-of-band rejection >30 dB, which is an improvement over previously reported BST ladder-type structure. The filter bank, realized through T-networks, occupies an area of 840 $\mu \text {m} \times 340\,\,\mu \text{m}$ and operates across four different bands solely through the BST-based acoustic resonators.