Design And Implementation Of The Network Time Server Based On NTP

Network time synchronization is about keeping the time of the equipment on the Internet consistent with the international standard time. Network time synchronization technology has been proposed for more than thirty years. With the development of technology, network time synchronization has been widely used in many important fields. In the power system, time synchronization technology provides high time resolution and improves the failure analysis capability of the system so that operation and control of the system can be more stable. In large data applications, time synchronization technology is applied to unify the time of each node device from the distributed system in the international standard time. It guarantees the correct event record sequence and offers an important support for the whole system.At present, NTP is a network time protocol which is widely used and has outstanding function. The domestic and foreign NTP time server use satellite time as external time reference source and maintain the accuracy of time by using Rubidium clock or constant temperature crystal oscillator. Generally, deviation rate of rubidium clock is 6ms/day, the aging rate is-115*10 /month. Deviation rate of constant temperature crystal is 1ms/day and the aging rate is-910 /month. Visibly, rubidium clock is stable but with high price and constant temperature crystal oscillator has high accuracy just when it starts using with high rate of aging. Thus, this paper proposes a design of network time server based on embedded devices.The scheme in this paper can be divided into time server and punctual module design. On the one hand, Time server will use GPS receiver to acquire satellite time as an external time reference source. On the other hand, we need to design a mixed phase-locked loop for punctual module based on measuring time deviation between the second pulse produced by crystal oscillators and GPS by TDC module. We design a moving average filter on MCU to eliminate the interference and quantify the time deviation to frequency deviation. Then frequency deviation is converted into the voltage through DAC and fine-tunes the output frequency of constant temperature crystal oscillator so that we can reduce the influence brought by crystal aging. Before fine-tuning, first through the MCU to measure and coarse the crystal frequency. That can improve the measurement range of TDC sampling times and ensure the effectiveness of the moving average filter. Punctual module has a high precision time keeping ability so that it can consistently offer the accurate time when the GPS receiver is unable to receive satellite signals. The timing module platform adopts the embedded system, which reduces the cost. The server in the process of filling the NTP message using an external interrupt way of obtaining a timestamp, improve the timing precision of the server.Finally, this paper have designed and implemented the embedded NTP server and tested the time server and punctual module successfully. The test shows that the original second pulse of constant temperature crystal oscillators is 76 ns. We use GPS second pulse as a benchmark, second pulse of crystal oscillator has reduced to 1ns and frequency error has been 0.01 Hz after calibration of punctual module. Time accuracy now is 1ms/day that to meet the design requirements. Time servers carry out time service with server/client mode. The accuracy of time service can be up to milliseconds in LAN, we can observe the UTC information every second through the serial port.