Research On Digital Hold-over System Based On Adder Based Clock

Characteristics of GPS are world-wide, all-weather three-dimensional(3-D) coverage, continuity, real-time and high-precision. High-precision time can be obtained by using GPS signal to calibrate local time. When GPS satellites in the condition of closure or missing lock, the local equipment works in the way of hold-over, and system time depends solely on the crystal oscillator at this time.As accuracy error exists in crystal oscillator, and the accuracy is affected by factors of temperature, aging and so on, the capability of hold-over will gradually decline. Yet, the hold-over capability of local equipment could be greatly improved by giving appropriate amount of compensation. The compensation can be determined from the accuracy and aging characteristics of local crystal oscillator which relies on standard time signals from GPS.This thesis proposes the scheme of digital hold-over system based on ABC. The scheme is realized on the basis of relevant reference both at home and abroad and the requirement of hold-over system. The DTCXO of fixed frequency and wide temperature range is utilized. The accuracy of crystal oscillator is measured by the GPS signal. The advantages of utilizing variable step-counter are small volume and low power cost.Several key technologies in the scheme are researched, and the test platform is established. The measurement of small time interval is realized by TDC-GP2 whose resolution can achieve the order of ps. The error of the 1 pps from GPS receivers was measured and analyzed. The data with noise is processed by the Kalman filter. The scheme of using comparing phase to calculate frequency of crystal oscillator is designed. Moreover, the logic circuit of fine-tuning frequency with ABC is realized . According to the results and theoretical analysis, the system meets the design requirements, which laid a solid foundation for the next step of research.