Low phase noise micromechanical reference oscillators for wireless communications

Series-resonant vibrating micromechanical resonator oscillators, using a custom-designed, single-stage, zero-phase-shift sustaining amplifier together with surface-micromachined micromechanical resonators, are demonstrated to have performance that satisfy the stringent phase noise specifications for GSM wireless communications reference oscillator standards. Since the short-term stability of a reference oscillator is dependent mainly upon the Q and power handling of its resonant tank element, this work achieves GSM performance by introducing micromechanical resonator designs with Q's exceeding 160,000 and distortionless in-band power handling on the order of -12 dBm, both substantially larger than achieved by previous designs. Pursuant to attaining this, several resonator designs were explored. The first two designs comprise two 40mum-long, 10-MHz clamped-clamped beam resonators, one of them with a much wider width than the other to allow larger power handling capability. To further maximize both Q and power handling, another resonator design, a 32mum-radius, 60-MHz wine-glass disk resonator, is also demonstrated. This disk resonator has 45X higher Q and 10X higher power handling than the previous wide-width clamped-clamped beam resonator. Finally, boost the power handling even further, a disk resonator array consisting of nine mechanically-coupled 60-MHz wine-glass disks, has been designed and is able to achieve 9X higher power handling capability than a single disk resonator while maintaining Q larger than 118,000. An oscillator utilizing a 60-MHz version of this array-composite device achieves a measured phase noise of -123 dBc/Hz at 1 kHz offset and -136 dBc/Hz at far-from-carrier offset. When divided down to 10 MHz, this phase noise performance effectively corresponds to -138 dBc/Hz at 1 kHz offset and -151 dBc/Hz at far from carrier offset, both of which now satisfy stringent GSM specifications for reference oscillators in wireless communications. This, together with its low power consumption of only 350 muW, and the potential for full integration of the integrated circuit and MEMS device onto a single silicon chip, makes the micromechanical resonator array oscillator of this work an attractive on-chip replacement for quartz crystal reference oscillators in wireless communications and other applications.