Microcomputer-Compensated Crystal Oscillator for Low-Power Clocks

Temperature-compensated crystal oscillators TCXO are widely used in applications requiring precision frequency control or timing, especially when power availability is limited and the temperature range is wide. The best overall accuracy in wide temperature range e.g., -55 deg C to 85 deg C TCXO has remained at about 1 ppm for the past 30 years because of thermal hysteresis, the trim effect, and thermometry inaccuracies. In this paper a microcomputer-compensated crystal oscillator MCXO is described. This MCXO employs external compensation, i.e., there is no frequency pulling. It thus permits the use of a low-hysteresis, low deviation-sensitivity SC-cut crystal in a non-trimmed oscillator. Resonator self-temperature-sensing, using a dual-mode oscillator, virtually eliminates thermometry-related errors. As a consequence, all basic TCXO limitations are overcome or significantly reduced. The MCXO incorporating this technology is capable of providing at least 10- to 100-times improvement in overall frequency accuracy when compared to conventional TCXO. Milliseconds-per-day timekeeping accuracy, with an input power of less than 40 milliwatts, is now available.