FUNDAMENTAL NOISE LIMITS FOR MINIATURE ACOUSTIC AND VIBRATION SENSORS

Recent technological advances in microfabrication and fiber optics hav e made practical the construction of very small, sensitive sensors for acoustic or vibration measurements. As the sensitivity is increased o r the size is decreased, a sensor becomes more susceptible to mechanic al noise resulting from molecular agitation. Traditional noise analysi s is often focused exclusively on electrical or optical noise; consequ ently, mechanical-thermal noise may not be considered in new types of sensors until the prototype testing reveals an unexpectedly high noise floor: Fortunately, mechanical-thermal noise is relatively easy to es timate early in the design process because the equivalent noise force is only a function of the temperature and the mechanical losses in the sensor. There are a number of specific techniques that are applicable for evaluating either the total mechanical-thermal noise or the spect ral distribution of that noise for simple or complex sensors. These te chniques are presented and, in addition, a summary of other noise comp onents is given in the context of design guidelines for high-sensitivi ty sensors.