Mechanical superheterodyne and its use for low frequency vibrations sensing

Mechanical micro and nano scale devices are being increasingly used for realizing signal processing and logic or computing functions, which until recently were traditionally performed by electronics. Extremely low-power consumption and robustness make mechanical micro structures attractive for implementation in autonomous distributed sensing systems. Heterodyning is one of the most important and indispensable signal processing techniques. In microelectromechanical heterodynes, the frequency mixing is commonly achieved using nonlinear forces, which often limits scalability and the operational range of the device and may lead to undesired dynamic behavior. In this paper, we introduce an approach allowing purely mechanical realization of the superheterodyne principle. The frequency mixing is based on the inertial coupling between two vibratory modes of the device. The architecture is distinguished by linearity of the mixing term with respect to the input inertial and the local oscillator signals. We demonstrate the mixing effect both theoretically and experimentally, using the devices fabricated from a silicon on insulator (SOI) wafer by deep reactive ion etching (DRIE). We show the applicability of the device as a mechanical low frequency vibration sensor. The substrate vibration frequencies down to 25 Hz were measured using a device with a fundamental mode frequency of 4700 Hz.