A Fast Startup CMOS Crystal Oscillator Using Two-Step Injection

Fast startup crystal oscillators (XOs) are needed in heavily duty-cycled communication systems for implementing aggressive dynamic power management schemes. This article presents the ways to improve startup time of XOs. Using a two-step injection technique in a three-step process, the proposed technique reduces the XO startup time to within 1.5x the theoretical minimum. By solving the differential equation governing crystal resonator under injection for arbitrary injection frequency, the behavior of energy build-up inside a crystal resonator is analyzed and used to determine optimum injection time as a function of the desired XO steady-state amplitude and injection frequency error. Bounds on tolerable injection frequency error to guarantee the existence of optimal timing are provided. Fabricated in a 65-nm CMOS process, the proposed 54-MHz fast startup XO occupies an active area of 0.075 mm² and achieves a startup time of less than 20 μs across a temperature range of -40 °C to 85 °C while consuming a startup energy of 34.9 nJ and operating from a 1.0-V supply.