Thomas B. Gabrielson
Naval Air Warfare Ctr., Code 5044, Warminster, PA 18974
Since the introduction of the micromachining process, wherein small mechanical structures are etched from blocks of silicon, a number of very small acoustic and vibration sensors have been built. This size reduction is attractive for many applications but the small moving parts are especially susceptible to mechanical noise resulting from molecular agitation. For sensors designed for small-signal applications (microphones and hydrophones, for example), this mechanical-thermal noise is often one of the limiting noise components. While often neglected in design and analysis, this component is relatively easy to estimate, since, like electrical-thermal noise, the magnitude of mechanical-thermal noise depends only on temperature and the magnitude of mechanical damping. This paper will review several techniques for evaluating noise in acoustic and vibration sensors in general and in micromachined sensors in particular: (1) addition of a white-noise force generator for each component of mechanical resistance; (2) distribution of the equipartition noise power according to the frequency response of the sensor; and (3) application of a software electronic-circuit simulator to the mechanical equivalent circuit of a device. In addition, the complementary relationship of shot noise (nonequilibrium) and thermal noise (equilibrium) will be discussed. Several case studies including capacitive and electron-tunneling micromachined sensors will be used to illustrate the analysis procedures and to unravel some of the misconceptions about noise limits in sensing systems. [Work supported by Office of Naval Technology.]