Single-Trim Highly Accurate Frequency Reference Techniques

This article describes a CMOS frequency reference system that achieves quartz-crystal-oscillator-rivaling frequency accuracy and power consumption across a temperature range of 228 °C, with low degradation over lifetime. This system uses only a batch calibration and a sample-specific single-temperature production-time calibration. The reference frequency is generated by a tunable RC-based oscillator (RCO) at relatively low power. The RCO’s significant process and temperature dependencies, and degradation over lifetime, are eliminated by periodically recalibrating it to a co-integrated inherently robust LC-oscillator (LCO). The resulting hybrid RC/LC CMOS frequency reference system combines the low-power properties of the RCO and the single-trim highly accurate and low-aging properties of an LCO. This article reviews circuit and system aspects of this frequency reference system, complemented with measurement results and insights for key devices and for many effects that limit system accuracy over wide temperature ranges and over lifetime.