Low-Power MEMS-Based Pierce Oscillator Using a 61-MHz Capacitive-Gap Disk Resonator

A 61-MHz Pierce oscillator constructed in 0.35-µm CMOS technology and referenced to a polysilicon surface-micromachined capacitive-gap-transduced wineglass disk resonator has achieved phase noise marks of −119 dBc/Hz at 1-kHz offset and −139 dBc/Hz at far-fromcarrier offsets. When divided down to 13 MHz, this corresponds to −132 dBc/Hz at 1-kHz offset from the carrier and −152 dBc/Hz far-from-carrier, sufficient for mobile phone reference oscillator applications, using a single MEMS resonator, i.e., without the need to array multiple resonators. Key to achieving these marks is a Pierce-based circuit design that harnesses a MEMS-enabled input-to-output shunt capacitance more than 100× smaller than exhibited by macroscopic quartz crystals to enable enough negative resistance to instigate and sustain oscillation while consuming only 78 µW of power—a reduction of ∼4.5× over previous work. Increasing the bias voltage of the resonator by 1.25 V further reduces power consumptionto 43µW at the cost of only a few decibels in far-from-carrier phase noise. This oscillator achieves a 1-kHz-offset figure of merit (FOM) of −231 dB, which is now the best among published chipscale oscillators to date. A complete linear circuit analysis quantifies the influence of resonator input-to-output shunt capacitance on power consumption and predicts further reductions in power consumption via reduction of electrode-to-resonator transducer gaps and bond pad sizes. The demonstrated phase noise and power consumption posted by this tiny MEMS-based oscillator are attractive as potential enablers for low-power “set-and-forget” autonomous sensor networks and embedded radios.