Precision crystal frequency reference disciplined using mains power

Abstract

We describe a clock that keeps time and provides a reference frequency with better than 1PPM accuracy, through a quartz crystal automatically calibrated via the power line frequency. The long-term precision of line-frequency-based time is used to correct the crystal-based timekeeping system, a process referred to as disciplining. A microcontroller calibrates the internal reference dynamically with no user assistance. Precision is improved to better than 1ppm as measured against a rubidium standard. The precise signal is useful for calibrating frequencybased instrumentation or keeping time without periodic correction and costs much less than comparable alternatives. The algorithm can be implemented in any mains-powered device using a microcontroller, such as thermostats and lighting timers. The prototype clock uses a Nixie-tube display, requires no transformer, is housed a glass tube, and is offered as an opensource design.