Tristan O. Rocheleau

A 78-microwatt GSM phase noise-compliant pierce oscillator referenced to a 61-MHz wine-glass disk resonator

A 61-MHz Pierce oscillator referenced to a single polysilicon surface-micromachined wine-glass disk resonator has achieved phase noise marks of -119dBc/Hz at a 1-kHz offset and -139dBc/Hz at far-from-carrier offsets. When divided down to GSMs 13MHz, this corresponds to -132dBc/Hz at 1-kHz and -152dBc/Hz at far-from-carrier offsets, both of which satisfy GSM reference oscillator phase noise requirements. This Pierce oscillator achieves such performance using a single disk, not an array, while only consuming 78 microwatts of power, a reduction by a factor of ~4.5 compared with previous work. When power consumption is considered, this performance marks the best figure of merit at 1-kHz carrier offset among published on-chip oscillators to date. Such low phase noise and power consumption posted by a tiny MEMS device may soon become key enablers for low power “set-and-forget” autonomous sensor networks with substantial communication capability.

2.97-GHz CVD diamond ring resonator with Q >40,000

A capacitive-gap transduced micromechanical ring resonator based on a radial contour vibration mode and constructed from hot filament CVD boron-doped microcrystalline diamond has achieved a Q of 42,900 at 2.9685GHz that represents the highest series-resonant Q yet measured at this frequency for any on-chip room temperature resonator, as well as the highest f·Q of 1.27×1014 for acoustic resonators, besting even macroscopic bulk-mode devices. Values like these in a device occupying only 870μm2 may soon make possible on-chip realizations of RF channelizers and ultra-low phase-noise GHz oscillators for secure communications.

Enhancement of mechanical Q for low phase noise optomechanical oscillators

A self-sustained Radiation-Pressure driven MEMS ring OptoMechanical Oscillator (RP-OMO) attaining an anchor-loss-limited mechanical Q-factor of 10,400 in vacuum has posted a best-to-date phase noise of -102 dBc/Hz at a 1 kHz offset from a 74 MHz carrier, more than 15 dB better than the best previously published mark [1]. While enhanced optical and mechanical Q both serve to lower the optical threshold power required to obtain oscillation, it is the mechanical Q that ends up having the strongest impact on phase noise [2], much as in a traditional MEMS-based oscillator [3]. This motivates a focus on increased mechanical Q-a challenge in previous such devices measured in air-and requires measurement in the absence of gas-damping using a custom optical vacuum measurement system. The improved phase noise performance of these RP-OMOs is now on par with many conventional MEMS-based oscillators and is sufficient for the targeted chip-scale atomic clock application.

Acoustic whispering gallery mode resonator with Q > 109,000 at 515MHz

A capacitive-gap transduced micromechanical resonator design based on an acoustic whispering gallery mode (WGM) constructed in micro-crystalline diamond has achieved a Q >;109,000 at 515MHz, posting an f·Q product of >;5.6×1013. The key to this performance is anchor-loss nulling by the WGM, as evidenced by comparison of Q values between radial-contour modes and WGM ones on the same disk device, where a 2-3× enhancement of Q is observed. Qs exceeding 100,000 should enable unprecedented RF front-end frequency selectivity and low phase noise oscillators for future communications.

A multi-material Q-boosted low phase noise optomechanical oscillator

A Radiation Pressure driven Optomechanical Oscillator (RP-OMO) comprised of attached concentric rings of polysilicon and silicon nitride has achieved a first demonstration of a mixed material optomechanical device, posting a mechanical Qm of 22,300 at 52 MHz, which is more than 2× larger than previous single-material silicon nitride devices [1]. With this Qm, the RP-OMO exhibits a best-to-date phase noise of -125 dBc/Hz at 5 kHz offset from its 52-MHz carrier - a 12 dB improvement from the previous best by an RP-OMO constructed of silicon nitride alone [1]. The key to achieving this performance is the unique mechanical Q-boosting design where most of the vibrational energy is stored by the high-Qm polysilicon inner ring which in turn boosts the overall Qm over that of silicon nitride, all while retaining the high optical Qo >190,000 of silicon nitride material. Simultaneous high Qo and Qm reduces the optical threshold power for oscillation, allowing this multi-material RP-OMO to achieve its low phase noise with an input laser power of only 3.6 mW.

A real-time 32.768-kHz clock oscillator using a 0.0154-mm 2 micromechanical resonator frequency-setting element

A capacitive-comb transduced micromechanical resonator using aggressive lithography to occupy only 0.0154-mm2 of die area has been combined via bond-wiring with a custom ASIC sustaining amplifier and a supply voltage of only 1.65V to realize a 32.768-kHz real-time clock oscillator more than 100× smaller by area than miniaturized quartz crystal implementations and at least 4× smaller than other MEMS-based approaches, including those using piezoelectric material. The key to achieving such large reductions in size is the enormous rate at which scaling improves the performance of capacitive-comb transduced folded-beam micromechanical resonators, for which scaling of lateral dimensions by a factor S provides an S2× reduction in both motional resistance and footprint for a given resonance frequency. This is a very strong dependency that raises eyebrows, since the size of the frequency-setting tank element may soon become the most important attribute governing cost in a potential MEMS-based or otherwise batch-fabricated 32.768-kHz timing oscillator market. In addition, unlike quartz counterparts, the size reduction demonstrated here actually reduces power consumption, allowing this oscillator to operate with only 2.1μW of DC power.

Long-term stability of a hermetically packaged MEMS disk oscillator

A low phase noise oscillator referenced to a wineglass disk MEMS resonator, hermetically vacuum packaged in a purpose-built packaging system, and measured in a double-oven, has provided a first long-term measurement of a MEMS disk oscillator over 10 months. After an initial burn-in period, the frequency can be seen to stabilize to within the short-term measurement variation of 300 ppb over a period of months, a significant improvement from previous studies on other MEMS resonator types, where frequency fluctuations were between 3.1 ppm and 1.2 ppm over similar time scales. Including burn-in, the total observed aging of 10 ppm is now on par with many consumer-grade quartz oscillators designed for timing applications and sufficient for target wireless sensor network applications.

Vibration-insensitive 61-MHz micromechanical disk reference oscillator

A low phase noise oscillator referenced to a 61-MHz vibrating wine-glass disk resonator with anchor-isolating supports designed to suppress microphonics has posted (without any compensation) a measured acceleration sensitivity at least as good as Γ ~0.2ppb/g for vibration frequencies up to 2kHz and in all directions, yielding a vector magnitude Γ less than 0.5ppb/g. Remarkably, this result is at least 30 times better than previous work using a similar wine-glass disk resonator and is the best mark among MEMS-based oscillators to date, including those aided by feedback compensation circuits. It is also more than an order of magnitude better than an off-the-shelf crystal oscillator and is now comparable with low sensitivity oven-controlled crystal oscillators (OCXOs). Such low sensitivity to environmental vibration by a tiny uncompensated MEMS-based oscillator is expected to enable harsh environment and military applications that require stable and compact reference oscillators.

Micromechanical disk array for enhanced frequency stability against bias voltage fluctuations

A 215-MHz polysilicon capacitive-gap transduced micromechanical resonator array employing 50 mechanically coupled radial-contour mode disks - the largest such array yet fabricated and measured - has achieved 3.5× better frequency stability than single stand-alone disks against fluctuations in the dc bias voltage (VP) normally applied across electrode-to-resonator gaps during device operation. The key to enhanced frequency stability is the electrode-to-resonator capacitance (Co) generated by the parallel combination of input/output electrodes overlapping each resonator in the array that in turn reduces the efficacy of the bias voltage-controlled electrical stiffness. Here, a new equivalent circuit based on negative capacitance provides improved visualization that helps to identify methods to suppress electrical stiffness induced frequency variation. The circuit model indicates that the more resonators in an array, the smaller the frequency shift imposed by a given bias voltage change. Both modeling and measurement suggest that the most stable MEMS-based oscillators (e.g., against supply noise and acceleration) are ones that utilize mechanically-coupled arrays of resonators.

High-

A vibrating micromechanical spoke-supported ring resonator employing a central peg-anchor, balanced non-intrusive quarter-wavelength extensional support beams, and notched support attachments attains high