A computer controlled multi-reference clock system

This paper first describes an end-user attempt for a computerized, multi-reference clock system and its main components, then deals with the efforts of providing automatically one physical time and frequency output with the least possible historical delay and finally discusses the achieved accuracy. External references are provided through separate GPS clocks and a TV-line receiver. The primary internal clock is a rubidium oscillator assisted by an ovenized crystal unit and another rubidium, which are both controllable through a computer interface. Some of the microprocessor-based GPS receivers show unexpected behaviour and have frequently lost sync. TV-line frequency reception is scrambled by live transmissions extended to the mid day-time causing unpredictable phase jumps. The high quality rubidium, after several weeks of operation below the 10-12 level, may show abrupt frequency deviations. Main problems are associated with the low-drift, high resolution D/A-conversion and 10 MHz EMI from the rubidium unit. The needed automatic frequency correction is less than 2×10-10/day.