CN102801469B - Optical fiber time frequency hybrid transmission method - Google Patents

Abstract

A mixed transmission method of optical fiber time and frequency, the master station normally transmits a 10MHz frequency signal to the slave station, and inserts a narrow negative pulse in the middle of the high level of a cycle signal before the whole second to complete the whole second calibration of the frequency signal. After the slave station recognizes the special calibration, it uses combinatorial logic to restore the frequency signal, and then uses the flip-flop to restore the timing signal. At the same time, the slave station returns the received time-frequency mixed transmission signal to the master station for loopback delay measurement of the master station. The master station restores the timing signal from the backhaul signal according to the method of the slave station, and dynamically measures the time delay between the restored timing signal and the local timing signal, and calculates the phase pre-compensation amount of the frequency standard signal within one second combined with the fixed time delay of the equipment. The master station uses the calculated phase compensation amount to achieve uniform phase compensation in the next second, so that the slave station recovers the frequency and timing signals and synchronizes with the master station. The invention has the advantages of high timing precision, low cost, convenient cascading, plug and play.

Description

A Fiber-optic Time-Frequency Hybrid Delivery Method

technical field

The method relates to a method of using optical fiber links for frequency and time mixed transmission to realize simultaneous time service and frequency transmission in different places (hereinafter referred to as the time-frequency mixed transmission method). The method has the advantages of high precision, low cost, convenient cascading, and plug-and-play.

Background technique

All major countries in the world are actively developing their own high-precision time and frequency unified systems. The more famous ones are mainly space-based systems, such as the GPS system in the United States, the GLONASS in Russia, the Galileo system in Europe, and the Beidou system under construction in my country. At the same time, the high-precision time-frequency network based on the optical network is also an important part of building a time-frequency system. It is not only capable of high-precision time-frequency transmission, but also complements and supports each other with the space-based time-frequency network to form an Integrated high-precision time-frequency network. Compared with space-based time-frequency systems, fiber optic channels have the advantages of stable and reliable transmission quality, relatively closed, anti-interference, slow change in transmission characteristics, and effective jurisdiction. Therefore, when higher-precision time-frequency transmission is required, it is an inevitable choice to use optical fibers with better channels for higher-precision time and frequency transmission.

At present, the methods of using optical fiber for time synchronization include the pre-compensation method and the two-way time comparison method. The existing methods are to measure the clock difference between the time-frequency central station and the terminal station and compensate it, and the terminal user can only obtain time synchronization. Signal, that is, the synchronous second pulse signal. If the end user wants to obtain a frequency signal that guarantees short-term stability and long-term stability at the same time, it needs to use the obtained second pulse or time difference signal to serve the local secondary oscillator (controlled crystal oscillator or rubidium atomic clock).

Contents of the invention

Technical problem: The purpose of this invention is to provide a new method for realizing high-precision time-frequency mixed transmission using optical fiber links. In this method, the time transfer is realized by special calibration of the transmitted frequency standard, the frequency phase difference is calculated by measuring the second time delay, and the time delay pre-compensation is realized by compensating the frequency standard phase, which ensures the high precision of the time delay transfer. In this method, the slave station simultaneously recovers the time and frequency signals from the received optical signal, without the need for a local secondary clock servo to regenerate the frequency signal, so it has the advantages of simple system structure, convenient cascade application, and plug-and-play.

Technical solution: The optical fiber time-frequency hybrid transfer method of the present invention includes the following steps:

a). Full-second calibration: the master station normally transmits a 10MHz frequency signal to the slave station, and performs special calibration in the 10MHz cycle where the whole second is located;

b) After the slave station recognizes the special calibration, remove the mark and restore the normal frequency signal, which is the required frequency standard signal, and use this frequency standard signal as the clock to get the timing signal at the second edge of the mark cycle output, and complete the timing; the slave station recovers At the same time as the frequency standard and time standard signals, the received time-frequency mixed transmission signal is returned to the main station as it is, for the loopback delay measurement of the main station;

c). Calculation of frequency standard phase compensation: the phase compensation of the frequency standard is used to offset the impact of the total forward delay, that is, the total amount of phase advance in one second is equivalent to the delay;

d). Frequency standard phase electric field compensation: the master station uses the delay unit to cooperate with the 10MHz clock before the electro-optical conversion, and realizes uniform phase compensation in the next second, so that the slave station recovers the frequency and timing signals and synchronizes with the master station.

The special calibration mentioned is that the special calibration is to encode the entire second period. During the encoding process, the position of the jump edge of the periodic signal remains unchanged, and only a narrow negative pulse is inserted in the middle of the high level of the 10MHz periodic signal to indicate the next The rising edge of the period is the rising edge of the second pulse. The receiving end uses combinational logic to remove the negative pulse mark to restore the normal frequency signal, and uses the restored frequency signal as the clock signal according to the position of the mark, and uses the timing logic to restore the timing signal.

Beneficial effects: the full-second calibration method proposed by the present invention for the first time realizes time-frequency mixed transmission in the optical fiber link, and the slave station can directly obtain the frequency and timing signal from the frequency signal calibrated in the optical fiber link, without the need for the secondary clock of the servo to regenerate the time frequency Signals can achieve high-precision time-frequency mixed transmission at a lower cost. The method has the advantages of unified benchmark, high stability, convenient cascading, and easy to ensure the long-term stability and short-term stability of time-frequency mixed transmission at the same time.

Adopt the method of the present invention to test the equipment prototype in the laboratory environment, its test block diagram as shown in Figure 2, the second pulse of the reference rubidium atomic clock output and 10MHZ frequency signal inject central station, central station utilizes the time-frequency mixing that the present invention proposes Through the method, the time and frequency signal is transmitted to the terminal station by using an optical fiber link of about 25Km, and the second pulse signal recovered by the terminal station and the second pulse signal of the clock source are compared and measured in real time by using the time interval measuring instrument SRS620 to obtain the timing error . Use SRS620 to compare and measure the 10MHZ frequency signal recovered by the terminal station with the 10MHZ frequency signal directly output from the rubidium atom, so as to measure the performance of frequency transfer.

Description of drawings

Fig. 1 is a working principle diagram of optical fiber frequency-time mixed transmission in the present invention.

Fig. 2 is an experimental test block diagram of the optical fiber frequency-time mixed transmission equipment of the present invention.

Fig. 3 is the timing error test structure of the experimental test.

Fig. 4 is the frequency transfer stability test result of the experimental test.

Detailed ways

(a). Full-second calibration: The master station normally transmits a 10MHz frequency signal to the slave station, and performs special calibration in the 10MHz period of the full second. The diagram of the calibration method is shown in Figure 1. After the slave station recognizes the special calibration, remove the mark and restore the normal frequency signal, which is the required frequency standard signal. And take the frequency standard as the clock, and get the timing signal at the second edge of the marking period to complete the time service. While recovering the frequency scale and time scale signals, the slave station returns the received time-frequency mixed transmission signal to the master station as it is, for the loopback delay measurement of the master station.

(b). Loopback delay measurement: the master station recovers the timing signal from the backhaul signal according to the above-mentioned method of the slave station, and uses the internal time interval measurement unit to dynamically measure the time delay T _m between the recovered timing signal and the local timing signal, The time delay T _m subtracts the two-way total fixed time delay τ _c of the equipment at both ends to obtain the two-way optical fiber link propagation time delay T _dp , T _dp = T _m -τ _c , and τ _c has nothing to do with the length of the optical fiber link.

(c). Calculation of one-way delay T _f from the master station’s local second timing to the slave station’s recovery second timing signal: the master station calculates the fiber length L, forward and reverse propagation delay difference δ, and then calculates the time from the master station to the slave station The forward propagation delay P _f of the optical fiber link of the station, plus the forward fixed delay τ _f to obtain the required one-way total delay T _f .

L=T _dp C/(2n), δ=D _λc (λ _f -λ _R )L

P _f =(T _dp +δ)/2, T _f =P _f +τ _f (Formula 1)

Among them, C is the speed of light in vacuum, n is the refractive index of the fiber, D _λc is the dispersion coefficient at the center wavelength, λ _f and λ _R are the forward and reverse laser carrier wavelengths from the master station to the slave station, respectively.

(d). Frequency standard phase compensation amount calculation: The phase compensation of the frequency standard is used to offset the impact of the total forward delay T _f , that is, the total amount of phase advance in one second is equivalent to the delay amount T _fo phase compensation through time delay compensation The method is completed, and the time delay is realized in a digital way with a resolution of 10 ps.

(e). Frequency standard phase electrical domain compensation: the master station uses an electrical signal delay unit with a resolution of 10 ps before the electro-optic conversion. The master station uses the delay unit to cooperate with the 10MHz clock to evenly select several cycles within the 10M cycles of the next second, advance each cycle by 10 ps in turn, and keep the calibration cycle unchanged. Therefore, it is ensured that the frequency signal recovered by the slave station is in phase with the master station and the second timing signal recovered is synchronized with the master station.

As shown in Figure 2, the optical fiber time-frequency mixed transmission method of the present invention comprises the following steps:

(a). The master station system is powered on, and the 10MHz frequency signal is calibrated according to the local frequency standard and the second timing signal and sent to the slave station every second.

(b). The slave station removes the frequency mark from the received signal to obtain the required frequency signal, and then generates the required second timing signal according to the frequency signal and the position of the mark, and at the same time, the received optical fiber line signal is looped back to the master station as it is.

(c). The master station extracts the reference second timing signal from the loopback signal in the same way as the slave station, and uses the internal time interval measurement unit to measure the time interval between the local timing and the reference timing.

(d). At the master station, combined with the fixed delay of the equipment, the total one-way delay from the master station to the slave station is accurately calculated according to formula 1.

(e). According to a certain filtering rule, the frequency standard phase compensation amount is obtained from the one-way time delay.

(f). The master station uses the fixed frequency standard reference and the electrical signal delay unit to realize uniform phase electric domain compensation in the next second, and complete the whole second calibration at the same time.

(g). Go back to step (b), and repeat.

Claims (2)

1. an optical fiber time frequency hybrid transmission method, is characterized in that the method comprises the following steps:

A). whole second is demarcated: main website normally transmits 10MHz frequency signal to slave station, carries out special demarcation in the place 10MHz cycle whole second;

B) after the special demarcation of slave station identification, remove mark and recover normal frequency signal, be required frequency standard signal, and with this frequency standard signal for clock, exporting second along obtaining timing signal in the mark cycle, completing time service; While standing in recovery frequency marking and timing signal, to the mixed number of delivering a letter of time-frequency that main website former state passback receives, measure for main website loopback delay; Described loopback delay is measured: main website recovers timing signal according to the method for above-mentioned slave station from return path signal, and utilize internal time interval measurement unit dynamic measurement recover the time delay T of timing signal and local timing signal _m, time delay T _mdeduct the two-way total fixed delay τ of terminal device _c, obtain bidirectional fiber link propagation delay T _dp, T _dp=T _m-τ _c, τ _chave nothing to do with optical fiber link length;

Main website is timed to slave station recovery timing signal One Way Delay second calculating T local second _f: main website calculates fiber lengths L, forward and backpropagation delay inequality δ thus the optical fiber link forward-propagating time delay P obtained from main website to slave station _f, add forward direction fixed delay τ _fobtain required unidirectional overall delay T _f;

$L=T_{\mathrm{dp}}\·C/\left(2n\right),\mathrm{\δ}=D_{{\mathrm{\λ}}_c}({\mathrm{\λ}}_f-{\mathrm{\λ}}_R)L$

P _f=(T _dp+ δ)/2, T _f=P _f+ τ _fformula 1

Wherein, C is vacuum light speed, and n represents optical fibre refractivity, represent central wavelength abbe number, λ _fand λ _rrepresent that main website is to slave station forward direction and reverse laser carrier wavelength respectively;

C). frequency marking phase compensation amount calculates: the phase compensation of frequency marking is for offsetting the impact of forward direction overall delay, and namely in one second time, phase place shifts to an earlier date total amount and is equivalent to delay volume;

D). frequency marking phase place electrical domain compensation: main website utilized delay unit to coordinate 10MHz clock before electro-optical conversion, realized uniform phase compensation within next second, make slave station recover frequency and timing signal synchronous with main website.

2. optical fiber time frequency hybrid transmission method according to claim 1, it is characterized in that described special demarcation is encoded to whole cycle second, the hopping edge invariant position of cataloged procedure hold period signal, only inserting the rising edge that a narrow negative pulse is used to refer to next cycle in 10MHz periodic signal high level centre position is pulse per second (PPS) rising edge; Receiving terminal utilizes combinational logic to remove negative pulse mark recovery normal frequency signal, and according to mark position, using the frequency signal recovered as clock signal, utilizes sequential logic to recover timing signal.