Abstract
A 61-MHz Pierce oscillator constructed in 0.35- [Formula: see text] CMOS technology and referenced to a polysilicon surface-micromachined capacitive-gap-transduced wine-glass disk resonator has achieved phase noise marks of -119 dBc/Hz at 1-kHz offset and -139 dBc/Hz at far-from-carrier 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 [Formula: see text] of power-a reduction of ∼4.5× over previous work. Increasing the bias voltage of the resonator by 1.25 V further reduces power consumption to [Formula: see text] 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 chip-scale 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.
Highlights
I N RECENT years, reference oscillators based on high-Q MEMS resonators have become viable alternatives to traditional quartz-based oscillators
The micromechanical disk resonator used as the frequencyselecting tank circuit in this work, shown in Fig. 1, comprises a 3-μm-thick, 32-μm-radius polysilicon disk supported at quasi-nodal points [12] by four beams and surrounded by electrodes spaced by 80-nm gaps from its edges
The demonstrated 61-MHz capacitive-gap transduced wineglass disk Pierce oscillator capable of meeting mobile phone specifications while using only 78 μW of power marks a milestone for MEMS-based frequency control technology
Summary
I N RECENT years, reference oscillators based on high-Q MEMS resonators have become viable alternatives to traditional quartz-based oscillators. Standard circuit for traditional crystal oscillators due to its low complexity and high performance Not surprisingly, it has found use in previous capacitive-gap transduced flexuralmode MEMS resonator oscillators [14], [15]. Oscillators referenced to much higher Q capacitive-gap transduced wine-glass disk resonators have not used the Pierce topology, opting instead to employ balanced transresistance amplifier designs [12] more resistant to common-mode noise sources, e.g., from the power supply and its dependence on temperature and acceleration. The power consumption shrinks to just 43 μW via bias adjustments that sacrifice just a few decibels of far-from-carrier phase noise
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