Paul A. Ward

Low-noise MEMS vibration sensor for geophysical applications

The need exists for high-sensitivity, low-noise vibration sensors for various applications, such as geophysical data collection, tracking vehicles, intrusion detectors, and underwater pressure gradient detection. In general, these sensors differ from classical accelerometers in that they require no direct current response, but must have a very low noise floor over a required bandwidth. Theory indicates a capacitive micromachined silicon vibration sensor can have a noise floor on the order of 100 ng//spl radic/Hz over 1 kHz bandwidth, while reducing size and weight tenfold compared to existing magnetic geophones. With early prototypes, we have demonstrated Brownian-limited noise floor at 1.0 /spl mu/g/Hz, orders of magnitude more sensitive than surface micromachined devices such as the industry standard ADXL05.

Experimental study of thermoelastic damping in MEMS gyros

We present new experimental data illustrating the importance of thermoelastic damping (TED) in MEMS resonant sensors. MEMS gyroscopes have been used to demonstrate that both the choice of materials and variations in device design can lead to significant differences in the measured quality (Q) factors of the device. These differences in the Q-factor can be explained by including the contribution of thermoelastic damping, which varies strongly between the different silicon etch-stop compositions used in this study. Known damping mechanisms, such as fluid damping, anchor damping, and electronics damping are minimized and held fixed in this experiment so that materials effects can be isolated.

Oscillator phase noise: systematic construction of an analytical model encompassing nonlinearity

This paper offers a derivation of phase noise in oscillators resulting in a closed-form analytic formula that is both general and convenient to use. This model provides a transparent connection between oscillator phase noise and the fundamental device physics and noise processes. The derivation accommodates noise and nonlinearity in both the resonator and feedback circuit, and includes the effects of environmental disturbances. The analysis clearly shows the mechanism by which both resonator noise and electronics noise manifest as phase noise, and directly links the manifestation of phase noise to specific sources of noise, nonlinearity, and external disturbances. This model sets a new precedent, in that detailed knowledge of component-level performance can be used to predict oscillator phase noise without the use of empirical fitting parameters.

Low frequency mechanical antennas: Electrically short transmitters from mechanically-actuated dielectrics

Antennas that operate in the low-frequency (LF) band and below are useful for a number of applications. However, the long wavelengths result in very low efficiency for antennas that could be made portable. This has motivated the need for novel approaches for electrically short antenna design. Here, we present the concept of an electromagnetic transmitter that operates by mechanically moving bound static charge. The resulting motion induces electromagnetic fields that are similar to a short dipole antenna. However, the voltage, current, and resistance of a conventional antenna are replaced by force, velocity, and damping in a mechanical system. The mechanical system offers very high efficiency at low frequencies where impedance matching naturally occurs and mechanical structures have very low losses. We present a basic proof-of-concept demonstration by rotating a charged electret material up to 167Hz and measuring the resulting time-varying magnetic field. This work is intended to lay the foundation for future tests involving the implementation of efficient, small form-factor, mechanically-actuated antennas.