A Filter-Free Third Overtone Quartz Oscillator Based on Micromachining Technology

This work investigates an AT-cut quartz crystal oscillator (XO) operating at its enhanced third overtone thickness shear (TS) mode while suppressing the fundamental TS mode by micromachining technology. A unique quartz plate structure has been proposed to alleviate the strong resonant signal from the first mode. A finite-element method (FEM) was used to characterize the first mode suppression while improving the performance of third overtone in terms of
${Q}$
-factor and motional resistance (
${R}_{m}$
) of the proposed device. In measurement, the unloaded
${Q}$
-factor of the fundamental TS mode is from several tens of thousands (the reference design) to only several hundred (the proposed design). Meanwhile, the unloaded
${Q}$
-factor of the third overtone response for the proposed design could reach
${Q} \sim 95$
000 with a decent
${R}_{m}$
. The
${Q}$
-factor ratio attains 170–180 between the third overtone and fundamental modes. With the feature of low
${Q}$
-factor on the fundamental mode, the oscillator system could successfully oscillate at a higher frequency without the help of bandpass filtering. Finally, the closed-loop oscillation measurement is performed for the phase noise (PN) characterization. The PN performance could reach −154.5 dBc/Hz at a 1-kHz offset at the carrier frequency of
$\sim $
60 MHz. This technique has proven its potential as a future solution of high-frequency quartz oscillator technology.