  We present the first group of acoustic delay lines (ADLs) at 5 GHz, using the
first-order antisymmetric (A1) mode in Z-cut lithium niobate thin films. The
demonstrated ADLs significantly surpass the operation frequency of the previous
works with similar feature sizes, because of its simultaneously fast phase
velocity, large coupling coefficient, and low-loss. In this work, the
propagation characteristics of the A1 mode in lithium niobate is analytically
modeled and validated with finite element analysis. The design space of A1 ADLs
is then investigated, including both the fundamental design parameters and
those introduced from the practical implementation. The implemented ADLs at 5
GHz show a minimum insertion loss of 7.94 dB, an average IL of 9.1 dB, and a
fractional bandwidth around 4%, with delays ranging between 15 ns to 109 ns and
the center frequencies between 4.5 GHz and 5.25 GHz. The propagation
characteristics of A1 mode acoustic waves have also been extracted for the
first time. The A1 ADL platform can potentially enable wide-band high-frequency
passive signal processing functions for future 5G applications in the sub-6 GHz
spectrum bands.
