Integrated circuit design to enable quartz free miniature wireless sensing systems

Abstract

Emerging technologies such as Internet of Things (IOT) and Wireless Body Area Network (WBAN) demand wireless sensors with ultra low power consumption, low cost and previously impossible form factors. Moore's law is exponentially scaling down the size of CMOS circuits making them faster and cheaper. However, frequency references, antennas and power sources which are essential for wireless sensors do not benefit from such scaling. These components are increasingly becoming a bottleneck in achieving size/cost/power reduction in wireless sensors. Quartz crystal is the most commonly used frequency reference in radios. Quartz crystal is bulky, needs a complicated manufacturing process increasing cost and does not yield itself to integration with CMOS circuits. This has prompted circuit designers to look for MEMS based alternatives to Quartz crystals. While MEMS resonators are successfully replacing quartz in timing applications, a quartz alternative with stability / power consumption suitable for radio applications has not been reported so far. The first part of this thesis addresses this problem and develops circuits and systems to demonstrate an FBAR (thin-Film Bulk-Acoustic-Resonator) based quartz replacement. Circuit solutions to improve phase noise and temperature stability of FBAR oscillators are discussed. These efforts culminate in a 750 MHz FBAR frequency reference with a ± 3 ppm stability and 1.1 mW power consumption. The second part of the thesis addresses the challenge of power delivery to miniature subcutaneous sensors. At mm scale inductive power transfer efficiencies drop below 0.1 %. In applications where the implant is just below the skin optical powering becomes a viable option due to high transmission of skin at infrared wavelengths. We demonstrate an operational mm scale tag with optical power and data links across a pig skin barrier.