摘要
This work investigates an AT-cut quartz crystal oscillator 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. 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 (Rm) 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 ~95,000 with a decent Rm. The Q-factor ratio attains 170-180 between the third overtone and fundamental mode. With the feature of low Q-factor on the fundamental mode, the oscillator system could successfully oscillate at higher frequency without the help of bandpass filtering. Finally, the closed-loop oscillation measurement is performed for the phase noise characterization. The phase noise performance could reach -154.5 dBc/Hz at 1 kHz offset at the carrier frequency of ~60 MHz. This technique has proven its potential as a future solution of the high-frequency quartz oscillator technology.