Reference clock design for low power and low phase noise with temperature compensation

Thesis (Ph.D.)--University of Washington, 2016-03

Low power and low phase noise RF frequency references are essential for applications such as high performance ADCs, high speed serial data links, and low power radios. They constitute a multi-billion dollar market in today’s electronic industry. Quartz crystal is the most commonly used mineral for generating a reference clock. However, it needs a complicated manufacturing process, which increases cost, and it cannot be integrated with CMOS circuits. This is reason why wafer scale high-Q MEMS resonators are becoming attractive alternatives to quartz owing to their small size, low cost and integration potential. However, oscillators using MEMS resonator perform poorly compared to quartz based oscillators in terms of close-in phase noise. Close-in phase noise is an important performance metric for a reference oscillator as it dominates the in-band phase noise of a frequency synthesizer in a radio. In addition, highly miniaturized MEMS resonator based oscillators have exhibited poor frequency stability over temperature. This characteristic is an issue, which limits the choice of the oscillator type in wireless application such as Bluetooth, Wi-Fi and GPS. The first part of this thesis addresses the close-in phase noise issue and proposes circuits with MEMS resonator such as AlN contour mode resonator and FBAR (thin-Film Bulk-Acoustic Resonator) to demonstrate solutions for improving the phase noise and lowering the power consumption. The proposed oscillator with FBAR culminates in achieving more than 10dB lower phase noise than that of conventional oscillator with 350uW power consumption. The following part of this thesis addresses the frequency drift of the reference clock when the temperature changes. The wireless application requires stringent and challenging spec. for the oscillator to generate a stable clock signal. For example, GPS needs to have less than 2ppm frequency drift over temperature. The first prototype of fully integrated oven-controlled temperature compensation system is thus introduced. This effort aims to have a ±1.6ppm stability reference clock with 150uK temperature resolution.