A digitally temperature-compensated crystal oscillator

The base frequency of oscillators used in the Global System for Mobile Communication (GSM) network or Global Positioning System (GPS) receiver applications needs to be very stable with respect to temperature and supply-voltage variations. One approach to obtain extremely good frequency stability is the use of oven-stabilized crystal oscillators. With this kind of oscillator, a frequency stability versus temperature of a few ppb versus the standard temperature range can be achieved. In this paper, a digitally compensated crystal oscillator is described. The system provides a frequency stability of (Δf)/f<1.5 ppm for a temperature range of -40°C to 90°C compared to about ±20 ppm for a noncompensated crystal. The core of the system is an application-specified integrated circuit (ASIC) fabricated in a standard 0.8-μm CMOS process. The power consumption for the oscillator running at 13 MHz is 100 mW. The final device equipped with the ASIC, crystal blank, and a few external components fits into a 14×9×3 mm³ package