18.5 A 54MHz Crystal Oscillator with 30× 18.5 Start-Up Time Reduction Using 2-Step Injection in 65nm CMOS

The start-up time of crystal oscillators (T_START) is a major bottleneck in reducing the average power of heavily duty-cycled wireless /wireline communication systems [1]. Among all the reported schemes to reduce T_START, techniques that increase initial noise amplitude by injecting a surge of energy into the crystal resonator are shown to be most effective [1-3]. These approaches are proven to be robust if the frequency of the injection signal is equal to the crystal oscillator (XO) frequency (F_INJ = F_X0), which is difficult to achieve across PVT with on-chip oscillators. Any mismatch (ΔF = F_INJ - F_X0) even as small as a few 100 ppm can greatly increase T_START. Sweeping the injection frequency using a chirp oscillator [2]or dithering the injection frequency between two values [1]can partially alleviate this issue but because δ text{F} neq 0, this only reduces T_START to about 14× the theoretical minimum in [2]. On the other hand, it was shown in [3] that the use of a precise injection period T_INJ,OPT can help reduce T_START even in the presence of large ΔF. T_INJ,OPT must be chosen such that current in the motional branch of the resonator, i_m(t), reaches its steady-state value, I_m,SS (i_m(T_{INJ, OPT}) = I_m,SS) as shown in Fig. 18.5.1 [3]. However, small T_START and large tolerance to ΔF can be achieved only when I_m,SS is very small, which translates to small XO output amplitude (V_XO < 200mV) and degraded phase noise. For example, as illustrated in Fig. 18.5.1, T_START ≈ T_INJ,OPT because i_m(T_{INJ, OPT}) = I_m,SS1 even when ΔF is as large as 1000ppm. However, for I_m,SS2 > I_m,SS1, no TINJ can ensure i_m(T_INJ) = I_m,SS2 if ΔF > 1000ppm, thus greatly increasing T_START. Therefore, ΔF must be small (<500ppm) to achieve a large V_XO even with precisely-timed injection, a condition that is difficult to meet in practice even with the best-reported temperature-compensated on-chip oscillators [4]. In view of these drawbacks, we present a robust 2-step injection technique that can tolerate large ΔF and achieves close to theoretical minimum T_START, large output swing and excellent phase noise. Fabricated in a 65nm CMOS process, the prototype XO achieves T_START of less than 20 μs across the -40°C to 85°C temperature range, which is within 1.5× of the theoretical minimum and represents an over-30× reduction in T_START compared to that of a normal/uninjected XO.