Practical Demonstrations of FR3-Band Thin-Film Lithium Niobate Acoustic Filter Design

This article presents an approach to control the operating frequency and fractional bandwidth (FBW) of miniature acoustic filters in thin-film lithium niobate (TFLN). More specifically, we used firstorder antisymmetric (A1) mode lateral-field-excited bulk acoustic wave resonators (XBARs) to achieve efficient operation at 20.5 GHz. Our technique leverages the thickness-dependent resonant frequency of A1 XBARs, combined with the in-plane anisotropic properties of 128. Y-cut TFLN, to customize filter characteristics. The implemented three-element ladder filter prototype achieves an insertion loss (IL) of only 1.79 dB and a controlled 3-dB FBW of 8.58% at 20.5 GHz, with an out-of-band (OoB) rejection greater than 14.9 dB across the entire frequency range 3 (FR3) band, while featuring a compact footprint of 0.90 × 0.74 mm2. Moreover, an eight-element filter prototype shows an IL of 3.80 dB, an FBW of 6.12% at 22.0 GHz, and a high OoB rejection of 22.97 dB, demonstrating the potential for expanding to higher order filters. As frequency allocation requirements become more stringent in future FR3 bands, our technique showcases promising capability in enabling compact and monolithic filter banks toward next-generation acoustic filters for 6G and beyond.