A 12-/13.56-MHz Crystal Oscillator With Binary-Search-Assisted Two-Step Injection Achieving 5.0-nJ Startup Energy and 45.8-μs Startup Time

This article reports a 12-/13.56-MHz fast-and-energy-efficient startup crystal oscillator (XO) featuring a binary-search-assisted two-step injection technique for ultralow-power duty-cycled radios. Specifically, after the first injection, a calibration module refines the auxiliary oscillator’s frequency toward the crystal’s resonant frequency in a binary-search fashion with a sub-500-ppm error for the second injection. This calibration method eliminates the closed-loop phase-locked loop (PLL) and shortens the calibration period into 48 clock cycles. Prototyped in a 65-nm CMOS process, the XO occupies an active area of 0.134 mm2. The 13.56-MHz XO can achieve a startup time ( $t_s)$ of $45.8 \mu $ s with a $V_{\mathrm {DD}}$ of 0.7 V, while consuming 5.0 nJ of energy. $t_s$ varies by ±2.2% amid temperature variations (−40 °C to 85 °C). Its steady-state power consumption is $28.4 \mu $ W, with an output phase noise of −143.7 dBc/Hz at 1-kHz offset (FoM: 241.8 dBc/Hz), manifesting its state-of-the-art performance.

I. Introduction

Crystal oscillator (XO) is the de facto frequency reference for modern electronic devices attributable to its spectral purity and stability [1], [2], [3]. However, typical MHz-range XOs without fast startup techniques take milliseconds to enter the steady state. Such tardy startup time ( limits the ON–OFF latency of the duty-cycled devices for ultralow-power (ULP) Internet-of-Things (IoT) applications, such as environmental monitoring. Moreover, the recent on-demand high-performance temperature sensors in [4] and [5] achieve energy consumption below tens of nanojoule per conversion and require a clock for quantization/digitization. In this regard, reducing the XO’s startup energy ( to the nanojoule range can further improve the device’s efficiency. Hence, the quest to reduce both XO’s and has gained momentum in recent years [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].