Low-Phase Noise Oscillator and Physics of Scaling
Clark Nguyen and Ashkan Borna
To date, micromechanical resonators have been realized using MEMS technology with resonance frequencies in the UHF band and with tremendous quality factor on the order of 11,000, making them potentially suitable as on-chip frequency reference elements for ultra stable oscillators needed in wireless communications. This project aims to realize a UHF oscillator with long- and short-term stability commensurate with the needs of communication applications. Indeed, a UHF oscillator with long- and short-term stability on par with that of quartz crystal oscillators would be highly desirable. Before such an oscillator can be achieved, however, a number of device issues must be overcome, such as high impedance, limited linearity, and noise sources, all of which can get worse as device dimensions are scaled to achieve higher frequency. To combat such scaling-induced issues, their mechanism must first be modeled, which should then reveal the most effective strategies for nulling them, such as arraying for high power handling and noise suppression.
This work will likely need to investigate and model scaling induced mechanical noise sources such as derived from adsorption/desorption, thermal fluctuation, and Brownian motion, and then determine the degree to which they are correlated/uncorrelated. A modeled understanding of intrinsic scaling-induced noise sources would reveal strategies for nulling them via engineered surfaces, materials, and environments, or via circuit techniques all of which will be explored pursuant to attaining on-chip oscillators and filters with unprecedented stability.
An analytical model for phase noise has been developed and evaluated. The predictions of this model have been compared with phase noise data obtained previously for an oscillator referenced to a 60-MHz, Q = 48,000 wine-glass disk resonator. The measured phase noise of -110 dBc/Hz at 1-kHz offset, and -132 dBc/Hz at far-from-carrier offsets, is fairly closely predicted by the model. Beyond verification of theory, a new Pierce oscillator topology for a 433 MHz oscillator has been designed that has a simpler layout compared to the previously designed transresistance one. The design of a UHF micromechanical resonator with noise and linearity characteristics that maximize oscillator performance is ongoing.
Figure 1: Phase noise density versus carrier offset frequency plots for the 60-MHz wine-glass disk resonator array oscillator (left) and SEM picture (right) of the fabricated wine-glass disk resonator array