As the basic supporting technology of digital communication network, the evolution of clock synchronization technology has always been driven by the development of communication network technology. In terms of network, communication networks have evolved from analog to digital, from TDM networks to packet networks. In terms of services, from TDM voice services to packet services, multi-service models are fixed. The voice service is mainly developed to focus on both fixed and mobile voice services, from narrowband services to broadband services. In terms of transmission technology with very close correlation with synchronous networks, it has evolved from coaxial transmission to PDH, SDH, WDM and DWDM, as well as the latest OTN and PTN technologies. With the continuous development of new communication services and new technologies, the synchronization requirements are getting higher and higher. Basic clock technologies including clock source and phase-locked loop have undergone many updates, and synchronization technology is constantly being introduced. Time synchronization technology is more It is the focus of current industry attention. The most basic technologies involved in clock synchronization include clock source technology and phase-locked loop technology. With the continuous improvement of application requirements, continuous improvement of technology and technology, clock source technology and phase-locked loop technology have also been rapidly evolved and developed. (1) Zhongyuan Technology The clock oscillator is the basic component of all digital communication equipment. According to the application time, the clock source technology can be divided into ordinary crystal clock, high-stability crystal oscillator with constant temperature bath, atomic clock and chip-level atomic clock. (2) Phase-locked loop technology Phase-locked loop technology is a circuit technology that synchronizes the output signal with the input signal in frequency and phase. When the system enters the locked state or the synchronous state by using the phase-locked loop technology, the oscillator output signal and the input signal of the system are The phase difference is zero or remains constant. Phase-locked loop technology is the core technology of clock synchronization. It has experienced the era of analog phase-locked loop technology and digital phase-locked loop technology until it develops into today's intelligent phase-locked loop technology. The synchronization technology includes two aspects of frequency synchronization technology and time synchronization technology. The frequency synchronization technology is mature and will not be described. The following is an analysis of the time synchronization requirements in the communication field and the existing time synchronization technology applied in the communication field. Time synchronization has an increasing demand in the field of communication. The requirements for time synchronization of various communication systems can be divided into high-precision time requirements (microsecond and nanosecond) and normal precision time requirements (millisecond and second). ). About this item rx6900xt,amd rx 6900xt,radeon rx 6900xt,amd radeon rx 6900xt,rx6900xt price Easy Electronic Technology Co.,Ltd , https://www.pcelectronicgroup.com
Generally, the crystal oscillator has an accuracy between nE-5 and nE-7. Due to its advantages of low price, small size, low power consumption, etc., crystal oscillators are widely used in various industries and fields. However, the ordinary crystal clock is generally affected by the ambient temperature, so that a crystal clock with a constant temperature bath and even a high-stability crystal clock with a double thermostatic bath have been greatly improved. With the continuous development of communication technology, higher requirements are placed on clock accuracy and stability. The crystal clock source has been difficult to meet the requirements. The atomic clock technology has begun to be applied. The cuckoo clock and the cuckoo clock are the most representative atomic clocks. In general, the accuracy of the cuckoo clock can reach or exceed the magnitude of nE-10, while the cuckoo clock can reach or exceed the magnitude of 1E-12.
However, due to its large size, high power consumption and short life, it limits the application of atomic clocks in some fields. Chip-level atomic clocks are expected to solve this problem. At present, the chip-level atomic clock for civilian use is basically in the experimental stage. Its size is only on the order of cubic centimeters. The power consumption is only on the order of 100 milliwatts. It does not consume atoms and prolongs the service life. The clock accuracy is above the order of nE-10. Good stability. Chip-level atomic clocks will have broad application prospects in communications, transportation, power, finance, defense, aerospace and precision measurement.
Each component of the analog phase-locked loop is realized by an analog circuit, generally consisting of a phase detector, a loop filter, a voltage controlled oscillator, etc., wherein the phase detector is used to identify the phase difference between the input signal and the output signal. And output voltage error, the noise and interference components are filtered by the low-pass loop filter to form the control voltage of the voltage controlled oscillator. The result of acting on the voltage controlled oscillator is to pull its output oscillation frequency toward The frequency of the loop input signal, when the two are equal, the lock is completed.
Compared to analog phase-locked loops, the error control signal in a digital phase-locked loop is a discrete digital signal rather than an analog voltage, so the controlled output voltage changes are discrete rather than continuous. In addition, the loop components are also implemented by digital circuits, which improves the stability of the analog phase-locked loop. With the development of digital technology, the technology of intelligent phase-locked loop (DDS-Digital Direct Frequency Synthesis) has emerged. Intelligent all-digital phase-locked loops can be implemented in a single FPGA. With the phase-locked loop state monitoring circuit, the locking time of the phase-locked loop can be shortened by the CPU, and the jitter characteristics of the output frequency can be gradually improved to achieve the best phase-locking and frequency output effects.
(1) High-precision time requirement For CDMA base stations and cdma2000 base stations, the time synchronization requirement is 10 μs; for TD-SCDMA base stations, the time synchronization requirement is 3 μs; for WiMAX systems and LTE, the time synchronization requirement is 1 μs or even sub-microseconds. Magnitude, which requires the time synchronization service level to be on the order of 100 ns. If the time synchronization between the base station and the base station fails to meet the above requirements, an instruction mismatch may occur in the selector, resulting in the call connection not being established normally.
For location-based services in a 3G network, if the mobile phone is used to receive wireless signals transmitted by a plurality of nearby base stations for positioning, the base station must be time-synchronized. In general, a time synchronization error of 10 ns will cause a positional error of several meters, and the time precision required for a position service of different precisions is also different.
(2) Ordinary precision time requirement For the No.7 signaling monitoring system, in order to avoid false information due to the sequence error of signaling, the time information of all signaling flows must be required to be accurate, and the time synchronization requirement is 1ms. For the charging systems of various switching networks, in order to avoid large time deviations between switches, conflicting bills may occur, and the time synchronization requirement is 0.5 s. For the network management system of various services, in order to effectively analyze the source of the fault and the consequences, locate the fault and find the cause of the fault. The time synchronization requirement is 0.5s.
For RSTP in the IP network-based streaming media service, it provides a robust protocol for streaming media to achieve multicast and single-on-demand delivery. RTSP uses a timestamp method to guarantee the QoS of streaming media services. For the security of the SSL protocol based on the IP network, the "timestamp" method is adopted to solve the "retransmission of information" attack method, and the time synchronization requirement is at least about 0.1s. A large number of computer-based devices and application systems (such as mobile business systems, integrated query systems, customer service systems, etc.) in communication networks generally support NTP, and time synchronization requirements are in the order of seconds or minutes.
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1 Introduction