The Locked Technique Of Second-Class Frequency Standards By The 1PPS Derived From GPS

1PPS (One Pulse Per Second) derived from GPS (Global Positioning System) has excellent long-term average precision and stability, and second-class frequency standards are of excellent short-term stability and low-cost. To combine the advantages of both together, this paper conducts a study on the locked technique of second-class frequency standards with the 1PPS from GPS as the frequency reference.In the technique mentioned above, the direct counting method and the time-amplitude convertor are combined to measure the short time interval between the 1PPS from GPS and the frequency-divided 1PPS from second-class frequency standards. The measurement resolution achieves 100ps with a range of several seconds. And then, a Kalman filter is used to process the measurement data to eliminate the effects of the phase jitter contained in the 1PPS from GPS on the lock process. After the filtering, the frequency correction is calculated by using a PID controller, and then it is output through a high-resolution DAC convertor and proper signal conditioning circuits to carry out the accurate lock of second-class frequency standards. At the same time, a simple measure for time synchronization is taken between the frequency-divided 1PPS from second-class frequency standards and the 1PPS from GPS, which makes the time error between both within certain range ( ). At last, this paper explores the technique which can correct the frequency errors of second-class frequency standards and maintain its lock precision when the 1PPS from GPS becomes invalid.Based on the technique above, this paper designs a prototype of the lock system, presents the design scheme of hardware and software of the system, and does some tests on the lock performance of the system. The test results show that the VCOCXO locked by using this technique maintains its original short-term stability, and its long-term stability and average precision are improved to some extent.