Abstract: This paper presents a 25-MHz fully-integrated digitally controlled crystal oscillator (DCXO) with automatic amplitude control (AAC). The DCXO is based on Colpitts topology for one-pin solution. The AAC circuit is introduced to optimize the phase noise performance. The automatic frequency control is realized by a 10-bit thermometer-code segmental tapered MOS capacitor array, ensuring a ～35 ppm tuning range and ～0.04 ppm frequency step. The measured phase noise results are -139 dBc/Hz at 1 kHz and -151 dBc/Hz at 10 kHz frequency offset, respectively. The chip consumes 1 mA at 1.8V supply and occupies 0.4 mm² in a 0.18-μm CMOS process.

Key words: crystal oscillator, DCXO, phase noise, supply pushing, VCTCXO, VCXO