A Fast Startup Crystal Oscillator with Digital SAR-AFC Based Two-Step Injection

Crystal oscillators (XOs) provide a high-precision reference frequency but have a long startup time, which severely increases the average power consumption in duty-cycled systems. This paper proposes a fully-digital low-cost two-step injection technique, by using a successive approximation register (SAR) based auto frequency control (AFC) loop, to speed up the startup behavior of XOs. A theoretical analysis is carried out to determine the optimum injection time and design low-power XOs. Fabricated in a 65 nm CMOS process, the proposed 12 MHz fast startup XO occupies an active area of 0.02 mm ^2 and achieves a startup time less than 35 μs. The XO power consumption in the steady state is 40 μW from a 1.0-V supply, with a startup energy of 17.2 nJ.