Integrated Frequency References

Electronic devices, including mobile phones and IoT gadgets, rely on accurate frequency references for their operation. The de facto standard in the industry for frequency references is Quartz crystal oscillators (XOs) due to their high accuracy across variations in process, voltage, temperature (PVT), and lifespan. However, XOs are costly, bulky, and require protective packaging to shield the resonator from air damping. The work in this thesis aims to develop a fully integrated frequency reference within a standard CMOS process, targeting a low-cost replacement for XOs.

In this thesis, the challenges specific to CMOS-based frequency references are explored, particularly sources that result in short- and long-term frequency variation such as PVT effects and degradation. Additionally, existing fully integrated frequency reference designs are reviewed. Achieving XO-level accuracy and stability (±100 ppm) without costly, multiple trimming steps poses a major challenge.

This work shows that the frequency temperature coefficient of a Colpitts LC oscillator is ten times less sensitive to process variations than that of a cross-coupled LC oscillator, resulting in a stable frequency across temperature based on a cost-effective single temperature trim (1T-trim). A prototype using a 130 nm high-voltage CMOS SOI process achieves ±70 ppm frequency accuracy from −50 °C to 170 °C across 48 samples from three different wafers. While the Colpitts oscillator generates a PVT-stable frequency, it has considerably higher power consumption compared to XOs.

To address this power penalty, a frequency reference system is designed to combine the high-frequency accuracy of the Colpitts oscillator with the low-power consumption of a ring oscillator. A prototype of this design achieves ±93 ppm frequency accuracy from −63 °C to 165 °C while consuming only 210 μW. This design maintains accuracy and reduces power consumption, demonstrating that CMOS-based frequency references can match XO performance without extensive trimming or high power requirements.