A low-phase-noise 0.004-ppm/step DCXO with guaranteed monotonicity in the 90-nm CMOS process

This paper presents an integrated 26-MHz digitally controlled crystal oscillator (DCXO) for GSM standard that achieves a phase noise of -140 dBc/Hz at 1 kHz and -152 dBc/Hz at 10 kHz offset. The measured supply pushing at 26 MHz is 0.5 ppm/V. Frequency tuning is done by a predistorted 14-bit MOS capacitor digital-to-analog converter (DAC) where the top 10 bits are thermometer coded and the bottom 4 bits are ΣΔ modulated. Monotonicity of ∼0.004 ppm/step across the entire ∼70 ppm tuning range is guaranteed by applying a sliding ΣΔ modulation to the last exercised thermometer element. This DCXO consumes 3 mW at 1.4 V supply and occupies 0.18 mm² in the 90-nm CMOS process.