Moorthi Palaniapan

High-Q bulk-mode SOI square resonators with straight-beam anchors

In this paper, the performance of 6.35 MHz Lame-mode square resonators with different dimensions of straight-beam anchor supports is presented, with quality factor values exceeding one million in ambient pressures as high as 150 Pa. A maximum Q value of 1.70 million was experimentally measured for some of the square resonators at a vacuum pressure of 36 µTorr. The Q values of square resonators were relatively independent of pressure at levels below 100 Pa, which suggests that Q is pressure limited due to air damping only when pressures become higher than 100 Pa. Dimensions of straight-beam anchors placed at the four corners of the square resonator lead to tradeoffs among achievable Q, power handling capabilities and motional resistance. Longer anchor beams generally provide good signal-to-noise performance of a square resonator at lower dc bias; however, the resonator goes into the nonlinear regime at lower ac–dc drive amplitudes, which means reduced power handling capability. The benefit of shorter anchors is that the resonator is able to operate in a linear mode under high drive conditions. Depending on the type of application, anchor dimensions can be chosen such that the resonators performance is optimal in terms of a quality factor, motional resistance and power handling. The resonators were fabricated using the silicon-on-insulator multi-user MEMS process from MEMSCAP.

A CMOS Readout Circuit for SOI Resonant Accelerometer With 4-

A fully differential CMOS readout circuit for SOI resonant accelerometer is reported. The readout circuit is essentially an oscillator, consisting of an oscillator and a low noise automatic amplitude control (AAC) loop. A differential sense resonator is proposed to facilitate fully differential circuit topology and improves the SNR under a 3.3-V supply. A second-order AAC loop filter and a novel chopper stabilized rectifier are employed in the AAC loop to remove the noises, in particular, the 1/f noise, and to minimize the phase noise caused by the amplitude stiffening effect. The strong driving feedthrough is avoided by separating the drive and sense operation in the time domain, while using the same electrodes. The complete resonant accelerometer operates under a 3.3-V supply and achieves 140-Hz/g scaling factor, 20 mug/radicHz resolution and 4 mug bias stability. The readout circuit draws 7 mA under 3.3-V supply.

\mu \rm g

A novel inductor voltage control (IVC) method cable of achieving any input power factor including unity is being proposed for buck-type AC-DC pulsewidth modulation (PWM) converters. In this method, the input inductor voltage is kept within hysteresis band about a sinusoidal template, thus ensuring sinusoidal input current. This control is much less sensitive to parameter and control signal changes than the existing delta modulation control (DMC). In this paper the IVC method is applied to single-phase buck-type converter. Useful design results based on steady-state analysis have been presented. Simulation and experimental results have been provided to verify the theoretical results. The companion paper extends the applicability of the IVC control method to three-phase converters also. The proposed IVC method has potential in applications requiring AC-DC rectifiers with over-current/short-circuit current limit.

Bias Stability and 20-

In this paper, we present comprehensive analysis of the nonlinearities in a micromechanical clamped-clamped beam resonator. A nonlinear model which incorporates both mechanical and electrostatic nonlinear effects is established for the resonator and verified by experimental results. Both the nonlinear model and experimental results show that the first-order cancellation between the mechanical and electrostatic nonlinear spring constants occurs at about 45 V dc polarization voltage for a 193 kHz resonator in vacuum pressure of 37.5 µTorr. Our study also reveals that the nonlinearity cancellation is helpful in optimizing the overall resonator performance. On top of improving the frequency stability of the resonator by reducing its amplitude-frequency coefficient to almost zero, the nonlinearity cancellation also boosts the critical vibration amplitude of the resonator (0.57 µm for the beam resonator with 2 µm nominal gap spacing), leading to better power handling capabilities. The results from the clamped-clamped beam resonator studied in this work can be easily generalized and applied to other types of resonators.

\mu\rm g/\sqrt{{\hbox{Hz}}}

Characterization of nanomechanical graphene drum structures is presented in this paper. The structures were fabricated by mechanical exfoliation of graphite onto pre-etched circular trenches in silicon dioxide on a silicon substrate. Drum structures with diameters ranging from 3.8 to 5.7 µm and thicknesses down to 8 nm were achieved. Mechanical characterization of the devices was then carried out by using atomic force microscopy (AFM) to measure their electrostatic deflection. The structures were found to have linear spring constants ranging from 3.24 to 37.4 N m−1 and could be actuated to about 18–34% of their thickness before exhibiting nonlinear deflection. An analytical framework was formulated to model the deflection behaviour which was verified through finite element simulations (FEM). The experimental measurements agree well with analytical and finite element results using Youngs modulus of 1 TPa. The resonance characteristics of the structures were derived by both plate theory and FEM simulations. It was found that our drum structures could potentially vibrate at frequencies in excess of 25 MHz. The small size and high operating frequencies of our nanomechanical graphene devices make them very promising for resonant mass sensing applications with 10−20 g Hz−1 sensitivity, a two order of magnitude improvement over other reported silicon structures.

Resolution

In this paper, we report a 6.3 MHz Lame-mode square resonator with fully differential drive and sense electronics, exhibiting quality factor, Q values exceeding 1 million in ambient pressures as high as 100 Pa. A maximum Q value of 1.6 million was experimentally measured at vacuum pressure of 36 muTorr. It was also experimentally observed that the Q value for the bulk mode resonator was relatively independent for pressures below 100 Pa suggesting that the Q is pressure limited for pressure higher than 100 Pa. This resonator was fabricated using SOIMUMPs process from MEMSCAP.

Inductor voltage control of buck-type single-phase AC-DC converter

A novel inductor voltage control method capable of achieving any input power factor including unity is proposed for both the single and three phase buck type AC-DC PWM converters. In this method, the input inductor voltage is kept within a hysteresis band about a sinusoidal template, thus ensuring sinusoidal input current. This control is much less sensitive to parameter and control signal changes than the existing delta modulation technique. Detailed analysis has been performed for a single phase converter. A new switching logic scheme is then proposed which enables the inductor voltage control (and delta modulation control) to be extended to a three phase converter also. For both single phase and three phase converters, simulation and experimental results have been provided. The proposed inductor voltage control method has potential in applications such as DC motor drives and AC-DC rectifier with current limiting.

The nonlinearity cancellation phenomenon in micromechanical resonators

This paper presents a new phase-noise model for nonlinear microelectromechanical-system (MEMS) oscillators. Two widely recognized existing phase-noise models, namely, the linear time-invariant and time-variant models, are first reviewed, and their limitations on nonlinear MEMS oscillators are examined. A new phase-noise model for nonlinear MEMS oscillators is proposed according to the state-space theory. From this model, a closed-form phase-noise expression that relates the circuit and device parameters with the oscillator phase noise is derived and, hence, can be used to guide the oscillator design. The analysis also shows that, despite the nonlinearity in the MEMS resonator, the phase noise is still governed by its linear transfer function. This finding encourages the designers to operate the MEMS resonator far beyond its Duffing bifurcation point to maximize the oscillation signal power and put more emphasis on the low-noise automatic-amplitude-control-loop design to minimize the noise aliasing through amplitude-stiffening effect without concerning the nonlinear chaotic behavior.

Characterization of nanomechanical graphene drum structures

In this paper, we present a systematic characterization and modeling technique for the micromechanical free?free beam resonator to analyze its nonlinear vibration behavior. Different from the conventional FEM-based approach whose simulation accuracy is usually limited around 60?70%, the proposed modeling method is able to accurately identify both the mechanical and electrostatic nonlinear parameters from just a few preliminary experimental observations. The nonlinear equation of motion is then numerically solved, demonstrating both the spring hardening and softening effects in the system. The simulated nonlinear behavior of the resonator under different driving conditions is validated by comparing them with the experimental data. In addition, based on the verified nonlinear model, design guidelines such as the nonlinearity cancellation are also highlighted. Although this work focuses on the free?free beam resonators, the proposed modeling approach can be applied to any other electrostatically driven microresonator to reveal different intrinsic nonlinear properties of the device.

6Mhz Bulk-Mode Resonator with Q Values Exceeding One Million

In this paper, a simple and effective experimental approach has been used to extract the mechanical properties of suspended nanomechanical graphene devices using atomic force microscopy (AFM). The main objective of this work is to study the deflection behaviour of graphene devices as a function of layer number (1–5 layers) and anchor geometry which has not been widely investigated so far. Elastic and nonlinear responses of the devices were obtained using AFM nanoindentation. The estimated linear (2.5 N m−1 to 7.3 N m−1), nonlinear spring constants (1 × 1014 N m−3 to 15 × 1014 N m−3) and pretension (0.79 N m−1 to 2.3 N m−1) for the monolayer (3.35 A) to five layer (16.75 A) graphene devices of diameter 3.8 µm show an obvious increasing trend with increase in graphene thickness. The effect of anchor geometry on the force versus deflection behaviour of these devices has also been investigated. The Raman spectroscopy results confirm the absence of defects in the pristine and indented devices. Using the continuum mechanics model, the Youngs modulus and 2D elastic modulus of a monolayer graphene device have been found to be 1.12 TPa and 375 N m−1 respectively. The high stiffness and low mass of these devices make them well suited for sensing applications.