Effects of Resonator Volume on The Oscillator Near-Carrier Phase Noise

This paper reports the first study of near-carrier phase noise (PN) dependency on the resonator volume/harmonic-number in thin-film piezoelectric-on-substrate (TPoS) MEMS oscillators. Two lateral bulk-acoustic TPoS resonators with different volumes (3
^rd
and 5
^th
harmonic orders) were designed to operate at the same ~25.6 MHz and fabricated on a 25μm silicon on insulator (SOI) substrate. The devices chosen for this study have different volumes but exhibit identical Quality factor ( Q) and insertion loss (IL). The measured PN of the oscillators built based on these two resonators are measured and it is observed that the one based on the 5
^th
order resonator exhibits ~6dB lower PN @1KHz frequency offset compared to the 3
^rd
order device. Although our preliminary results are not conclusive enough, it is hypothesized that an inverse proportionality to the resonator volume/mass predicted by earlier models could also describe this observation.