A Frequency-Locked Oscillator Using Complex RC Impedance IQ-Balancing

This article presents a frequency-locked loop (FLL) oscillator using an impedance-sensing frequency detector for use in fully-integrated, on-chip frequency references. By detecting the real and imaginary components of a complex RC impedance, the FLL can be locked into the pole frequency where both components are balanced. A phase-mixing technique is also introduced to reconfigure the output frequency without altering the amplifier operating voltage. A prototype chip demonstrating this IQ-balanced approach was designed and fabricated in a general-purpose 0.18- $\mu \text{m}$ CMOS process. From measured results across multiple dice, the prototype integrated circuit (IC) chips achieve a temperature stability of 22 ppm/°C from −40°C to 80 °C and 10-ppm Allan deviation at an average output frequency of 647 kHz with 1.27 pJ/cycle energy efficiency, and 34.4 ppm/°C from −40°C to 80 °C and 15-ppm Allan deviation at an average output frequency of 1.39 MHz with 0.89 pJ/cycle energy efficiency.

I. Introduction

The growing use of portable and wearable Internet-of-Things devices increasingly necessitates energy-efficient and small form-factor sensing electronics to enable energy autonomy and to improve mobility [1], [2]. The clock generation circuit at the heart of most sensing electronics is one of the most active blocks that functions across sleep modes as well as normal modes for timely wake-up and precise operation of the system. Hence, it is important to reduce both energy consumption of the clock generator as well as variation and drift of the clock frequency. Though quartz crystals and MEMS-based oscillators can achieve acceptable frequency stability at low power, they require additional component integration and increases device size and form-factor [3], [4]. Scalable on-chip oscillators that can generate process-, voltage-, and temperature-invariant clock frequencies using less energy per cycle are critically needed for portable sensing applications.