ME PhD Oral Examination: Hyung Kyu Lee (Monday, May 16th, 2:00 pm)

Stanford University Ph.D. Dissertation Defense

Title: "Frequency Stability of Micromechanical Resonators with Nonlinearities"

Hyung Kyu Lee

Department of Mechanical Engineering

Advisor:  Prof. Thomas W. Kenny

Date:  Monday, May 16th, 2011

Time: 2:00 pm (Refreshments at 1:45 pm)

Location: Mechanical Engineering Research Lab (MERL, bldg. 660), Conference room (203)

http://campus-map.stanford.edu/index.cfm?ID=02-660

Abstract: 

Frequency references are an essential component of electronics, and there is increased interest in miniaturized frequency references. This interest has heightened the need for MEMS resonator-based oscillators because they can provide frequency references at low cost in small packages. Of particular interest is the oscillators' the temperature stability and the phase noise performance: temperature stability must be low enough for the resonator to provide a stable reference signal, and the frequency references in RF devices must satisfy stringent phase noise specifications. 

The first part of the presentation discusses the effect of nonlinearities in a resonator on the temperature stability of the resonator-based timing reference. To achieve good temperature stability , various temperature compensation techniques have been developed. Some of these compensation techniques tend to demonstrate resonator performance by using open-loop network analyzer measurements. However, most applications require a closed-loop system, with the resonator embedded in a feedback loop to form an oscillator. Thus, temperature stability measured in a closed-loop is more accurate for real-world applications. If the temperature stability decreases when the resonator is part of a closed-loop, it is very undesirable. In this part, I explain how this undesirable change commonly occurs, and I demonstrate how they actually affect the performance of a resonator. Most importantly, I demonstrate a method to prevent these changes. 

The second part of the presentation discusses the phase-noise performance of a resonator-based timing reference. The phase noise, especially the far-from-carrier phase noise, often scales inversely with the carrier power. Hence, researchers have developed various approaches to improve the power-handling capabilities of a resonator-based timing reference. However, these approaches have relatively limited applications since they often require special resonator designs, optimized operating conditions, or non-standard fabrication processes. In terms of general approaches, having a large input-driving amplitude Vac directly improves the power-handling performance. This is because Vac is proportional to the electrical-current amplitude I in an oscillator. Hence, the power-handling performance improves when Vac increases. However, this approach has not been utilized because of the preconception that instability occurs at large Vac, where nonlinearities arise and cause the indeterministic frequency-to-amplitude relation. In this part, I explain why this preconception is not applicable to closed-loop oscillators. More importantly, I experimentally demonstrate stable operation of a closed-loop oscillator beyond the limit dictated by the critical vibration amplitude. This means that one can improve the power-handling performance of MEMS oscillators by operating them in the nonlinear regime.