Author: James Christian Salvia
Publisher:
Published: 2010
Total Pages:
ISBN-13:
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Almost all imaginable electronic devices in common use today, including cell phones, laptops, music players, cameras, televisions, automobiles, appliances, and wristwatches, rely upon timing references of some kind. Traditionally, the timing references used in all of these applications have relied upon the same technology: quartz crystal oscillators. However, Microelectromechanical Systems (MEMS) oscillators have become a viable option and are replacing quartz in segments of the timing reference market. In part, this paradigm shift is based upon the improved size, cost, and reliability of MEMS solutions. Unfortunately, the temperature stability of MEMS oscillators is inferior to that of compensated quartz oscillators, and this is one of several shortcomings that have precluded the use of MEMS references in some high precision applications like wireless communication and navigation. This thesis presents the fundamental concepts behind MEMS resonator and oscillator operation as well as an overview of previously established temperature compensation schemes for MEMS devices. Details are provided on the MEMS technology used throughout this work, including Double Ended Tuning Fork (DETF) resonators, "epi-seal" encapsulation, and a variety of associated nonideal behaviors. Measurement data from several MEMS prototypes is also provided along with an overview of the concepts of phase noise and Allan variance. Two MEMS interface circuits are demonstrated. The first is an integrated transimpedance amplifier (TIA) designed specifically to interface with MEMS devices that exhibit very large motional impedance. The TIA consists of a capacitive-feedback current amplifier that drives current into an active load to obtain a 56 M[omega] transimpedance gain, 1.8 MHz bandwidth, phase response near 0 degrees, and 65 fA/[square root]Hz input-referred noise. The TIA was fabricated in 0.18 [mu]m CMOS technology and dissipates 436 [mu]W from a 1.8 V supply. The second circuit is a printed circuit board (PCB) implementation of a fully functional 1.2 MHz MEMS oscillator, including automatic level control. This PCB-based oscillator was used to flexibly test the MEMS prototypes used throughout the remainder of the thesis. Two active temperature compensation schemes that significantly improve the temperature stability of silicon MEMS oscillators are also demonstrated. Both schemes rely on micro-oven based compensation, using micro-scale thermal isolation and heating to maintain a MEMS resonator at a constant elevated temperature. The power consumption for the micro-ovens used in this work was in the range of 9 to 15 mW for a 100 degrees C operation range. The first temperature compensation scheme, called "Q(T)-based temperature compensation, " uses resonator quality factor as a proxy for temperature in a closed loop feedback system. This system achieved frequency stability of +/-25 ppm over a temperature range of 0 to 70 degrees C with a single-point calibration or +/-1 ppm with a multi-point calibration, but suffered from the limitations of considerable calibration overhead and poor long term stability. In particular, the Q(T) system's sensitivity to the analog gain of the components in the temperature sensing feedback path proved to be a major hindrance to this system's performance. The second scheme, called "[delta]f-based temperature compensation, " uses a phase lock loop and an integrated micro-oven to achieve temperature compensation. The phase lock loop monitors the difference frequency between two resonators with different temperature coefficients. This difference frequency provides a high resolution measurement of the resonators' temperature and is compared to a reference frequency derived from one of the resonators. Negative feedback is then used to drive the difference between the difference frequency and the reference frequency to zero by applying heat to the micro-oven. This procedure ensures that the micro-oven is held at a constant temperature despite variations in ambient temperature, thereby allowing the [delta]f system to maintain sub-ppm frequency stability under transient temperature conditions from -20 to 80 degrees C and part-per-billion level Allan deviation in an uncontrolled environment. Additional calibration is shown to reduce the steady-state temperature stability to the range of +/-60 ppb. It is hoped that this novel temperature scheme may facilitate the use of low power, low cost, space saving MEMS oscillators in a new arena of high precision timing reference applications.
