Ultralow-Power Class-C Complementary Colpitts Crystal Oscillator

Zule Xu,Noritoshi Kimura,Kenichi Okada,Masaya Miyahara

doi:10.1109/lssc.2020.3014048

Abstract

This letter presents a 24-MHz ultralow-power and low-phase-noise complementary Colpitts crystal oscillator. Class-C operation is realized by biasing the oscillator always in the weak-inversion region with the proposed circuit. A Langasite-type CTGS (Ca3TaGa3Si2O14) crystal resonator, with the characteristics different from those of quartz (SiO2), helps to reduce the startup time to less than 1 ms. An injected- ${g} _{m}$ -booster is implemented to speed up the oscillation further. Prototype chips in 65-nm CMOS were evaluated with CTGS and quartz resonators. With each type, the lowest steady-state power consumption and the best jitter FoM have been achieved. With the CTGS resonator, the startup energy as low as 10 nJ has also been achieved.

Highlights

Crystal oscillators (XOs) are widely used to provide reference clocks to phase-locked loops (PLLs) for frequency synthesis and clock generation
Since an XO critically contributes to the phase noise of a PLL, this technique is unsuitable to an IoT radio
Instead of the conventional inverter-based Pierce oscillator, we propose a complementary Colpitts topology performing the class-C operation in the weak-inversion region

Summary

INTRODUCTION

Crystal oscillators (XOs) are widely used to provide reference clocks to phase-locked loops (PLLs) for frequency synthesis and clock generation. Along with the rapid deployment of Internet-ofThings (IoT) radios which operate intermittently, most recent works on MHz-level XOs focus on shortening startup time (Ts) and lowering startup energy (Es) [1]–[3], but few on lowering steady-state power consumption (Pss) [4], [5]. We adopt a Langasite-type CTGS (Ca3TaGa3Si2O14) crystal resonator that may have a smaller quality factor (Q) but much smaller inductance (Lm) as well, compared with quartz (SiO2) Such characteristics enable significantly short Ts. While Ts can be further reduced via previous solutions, we implement a less-complex injected-gm-booster to accelerate the startup by about three times, resulting in about 10-nJ Es, which is the lowest in the comparison table.

COMPLEMENTARY COLPITTS CRYSTAL OSCILLATOR

Biasing Circuit and Class-C Operation

CTGS Crystal Resonator and Startup Booster

EXPERIMENTAL RESULTS

CONCLUSION