Programmable Oscillator Implementation Using 2.5 GHz Mirror-encapsulated BAW Resonator to Achieve $\pm 20$ PPM Overall Stability

This work presents a programmable oscillator using a 2.5 GHz mirror-encapsulated BAW resonator as the clock source. Unlike low-frequency crystal- or MEMS-based oscillators which require fractional PLLs (phase-locked loops) to generate higher output frequencies, proposed DBAR (dual-Bragg acoustic resonator) oscillators utilize an FOD (fractional output divider) to enable programmability and active compensation. The DBAR operation requires no cavity on either side of the resonator yet still maintain the immunity to humidity and mass loading from assembly and contamination, enabling cost-effective system integration. The CMOS die contains a cross-coupled differential oscillator core, FOD, buffer, as well as the required compensation blocks. Both dies are stacked and integrated in a $\boldsymbol{3.2}\ \mathbf{mm}\times \boldsymbol{2.5}\ \mathbf{mm}$ QFN package, achieving $\boldsymbol{\pm 20}$ ppm overall stability (including compensation error, aging, solder shift, etc.).