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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.02\ \text{mm}$</tex> and achieves a startup time less than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$35 \ \mu\mathrm{s}$</tex>. The XO power consumption in the steady state is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$40\ \mu \mathrm{W}$</tex> from a 1.0-V supply, with a startup energy of 17.2 nJ.