A Pendulum-like structure for Design of Oscillators

A novel MEMS resonator employing “capacitance-shaping” principle to generate sinusoidal signals is proposed. FEM simulations are done to study the structure. Preliminary characterizations on devices fabricated through SOI-MUMPS are carried-out to study the frequency tuning of the resonator by electrostatic actuation. Variants of the proposed structure are presented. Second order dynamical models of the devices are derived. Introduction Quartz crystal oscillators have been the preferred oscillators in most communication circuits. This is due to their high quality factor (Q) and stability characteristics. However quartz crystal oscillators are bulky, cannot be integrated with electronics and are currently off-chip. This increases the size and weight of overall system. In the past decade developments in micromachining technology has enabled design of high Q MEMS resonators and oscillators [1]. Resonators that can operate in gigahertz range with high Q have been demonstrated [2]. Commercial MEMS oscillators are beginning to appear in markets [3] Most flexural mode MEMS resonators are based on comb-drive actuators. They require very high operating voltages and are not easily scalable. Nonlinear stiffness characteristics of beams can lead to distorsion of signal [4] We propose a novel resonator that can be operated at low voltages. The resonator can use either comb-drive or parallel-plate for actuation. By using a “capacitance-shaping” principle the nonlinearities are cancelled out. The resonator can be used to build oscillators that can generate pure sinusoids as well as other waveforms. Proposed structure Fig.1 shows the 3-D view of proposed resonator. It consists of a pendulum like structure placed horizontally. A cantilever supports a mass at the end of the beam. Two combs (or plates) are placed on either side of the cantilever. The cantilever and the combs form a set of capacitors as indicated in the figure. Another set of combs (or plates) are placed at the end of cantilever. Various electrical connections are as shown in the figure. The device can be modified with combs C3 attached to end of cantilevers as shown in Fig.1b. By varying the dimensions of cantilever and placement of C1 and C3 the actuation voltage can be varied. Application of DC voltage on C1 and C2 will change the resonant frequency, enabling frequency-tuning. When a voltage is applied on actuation capacitors the cantilever moves towards one side. This will result in some capacitance variation in sense capacitor. When an alternating voltage with 180degree phase difference is applied on actuating capacitors the cantilever oscillates. This results in periodic capacitance change in sense capacitor. When a DC voltage is applied on C3 the current that flows through the sense capacitor is given by Advanced Materials Research Vol. 74 (2009) pp 207-210 online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 134.148.5.104-08/05/09,14:04:50)