Research On GNSS Disciplined Crystal Oscillator And Holdover Technologies

With the continuous deepening of social informatization,frequency sources with high accuracy and high stability play an increasingly important role.Generally speaking,the frequency sources with high accuracy and high stability mainly refer to atomic clocks and crystal oscillators.Atomic clocks have good frequency accuracy and long-term stability but are expensive and large in size.Crystal oscillators are relatively cheap and have good short-term stability.But due to its own aging,its frequency accuracy and long-term frequency stability are poor.The GNSS system can provide users with global,all-weather,and all-weather navigation timing services.Its ground time-frequency system is composed of multiple atomic clocks,and the generated system time has good frequency accuracy and frequency stability.This paper studies the controlled and maintenance technology of crystal oscillator based on GNSS second pulse signal,so that the high-stability crystal oscillator maintains its own short-term frequency stability,while having the frequency accuracy and long-term stability of the GNSS system time,so as to achieve short-term stability.The main research contents include:(1)The main technical indicators of the crystal oscillator and the aging and temperature factors that affect the output frequency of the crystal oscillator are analyzed,and the reasons for the low accuracy and long-term stability of the crystal oscillator are revealed.(2)In lock mode,use Kalman filtering algorithm to remove singular values and smooth filter the clock error data to weaken the influence of GNSS 1PPS signal phase jitter or other factors on controlled oscillator.Through the DAC voltage control word and crystal output frequency The relationship between the frequency correction amount is calculated,and the correction amount is compensated to the crystal oscillator to achieve locked,so that the long-term stability of the frequency of the crystal oscillator output after controlled is maintained at the same level as the GNSS system time.(3)In the loss-of-lock mode,the crystal oscillator will cause frequency drift due to external environmental temperature changes and its own aging,which will affect the output frequency accuracy.The crystal oscillator adaptive retention model is established to model the temperature and aging frequency drift characteristics of the crystal oscillator.The recursive least squares method with genetic factor trains the clock error data when locking,completes the effective compensation of the crystal frequency,and better improves the problem of crystal frequency accuracy and long-term frequency stability deterioration over time.A test platform was built to test the GNSS disciplined oscillator and holding modes,and the results showed that after rapid controlled,the frequency accuracy of the crystal oscillator was improved from 2.2×10^-8 to 5.5×10^-11.After disconnecting the GNSS signal,the phase difference drift of the crystal oscillator within 24 hours is about 4?s.The high-stability oscillator controlling and maintaining technology in this article can be applied to scenarios that require high-precision time-frequency signals and are sensitive to volume and cost.