Research on Phase-Control Technology of Recovery Terminal by

Two-Way Optical Fiber Time Frequency Transfer Method

Cheng Ma

, Ming Ma

, Hang Gong, Jing Peng, Wenchi Zang and Ting Liu

College of Electronic Science, National University of Defense Technology, Changsha, China

Keywords: Time Frequency Transmission, Phase Control, Two-Way Time Comparison.

Abstract: Optical fiber time-frequency transmission technology is one of the key technologies of satellite navigation

system for global service. With the application background of the construction of time-frequency system in

satellite navigation field, the phase consistency control technology of two-way time-frequency transmission

of optical fiber is studied, and the experimental analysis is carried out. The results show that the phase

consistency control accuracy is better than 100ps when the optical fiber transmission distance is within 10km,

which provides a solution for engineering application of remote time-frequency technology with high

precision.

1 INTRODUCTION

Global Navigation Satellite System (GNSS) is new

generation of radio navigation system, which has the

advantages of global, all-day and all-weather

navigation, positioning, timing and speed

measurement to meet supply urgent demand for

space-time information in many fields in today's

information society

(Gang Xie, 2013). Due to the

complex structure and signal propagation

environment of the satellite navigation system and

scattered locations (Rong Qiang, 2011)

It is very

important to realize the high-fidelity time-frequency

signal transmission between the front-end equipment

and the terminal equipment during the time-

frequency signal transmission between the ground

stations of the large-scale satellite navigation system,

which requires the time-frequency signal transmitted

to the terminal to achieve impedance matching within

a long distance and provide a low amplitude Degree

loss, low noise insertion loss, maintain high fidelity

signal quality, especially the demand for signal phase

consistency and time signal synchronization

accuracy.

Optical fiber two-way time-frequency

transmission technology is an ideal transmission

means because of its small transmission attenuation,

Male, Master degree candidate，Engaged in GNSS

Time-frequency reference transmission and

measurement technology research.

less stability insertion loss, low implementation cost,

simple configuration, convenient and flexible (Zhu X,

2015). According to the relevant literature (Schnatz H

- Lopez O), it can be seen that the current optical fiber

frequency transmission method can reach or exceed

the stability of 1×10

-14

/s, 2×10

-16

/day, and the phase

noise of -120dBc/Hz@1Hz, and the accuracy of

frequency transmission can maintain the same order

of magnitude as the input. However, for the

requirement of phase consistency, this index (Schnatz

H - Lopez O), is not mentioned in the relevant

literature of optical fiber time-frequency transmission

method at present, and there is a lack of engineering

application guidance, so it is difficult to achieve and

needs to be analyzed and studied.

Optical fiber two-way time-frequency

transmission technology needs to solve the phase

synchronization problem of terminal recovery signal.

At the end of the signal recovery, phase noise

purification and distribution after amplification, the

phase between all signals is arbitrary, which is to

obtain the time difference between them and the

front-end signal through phase measurement

technology, and control them to maintain phase

consistency with the front-end signal through phase

adjustment.

Aiming at the difficulty of restoring terminal

phase consistency control by optical fiber time-

Corresponding author, Male, PhD, Engaged in satellite

navigation time frequency system technology research

520

Ma, C., Ma, M., Gong, H., Peng, J., Zang, W. and Liu, T.

Research on Phase-Control Technology of Recovery Terminal by Two-Way Optical Fiber Time Frequency Transfer Method.

DOI: 10.5220/0012286900003807

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Seminar on Artiﬁcial Intelligence, Networking and Information Technology (ANIT 2023), pages 520-525

ISBN: 978-989-758-677-4

frequency transmission method, this paper proposes

and discusses the design and implementation of a

phase consistency control method for optical fiber

time-frequency signal transmission, and verifies its

feasibility and performance through experimental

analysis.

2 BASIC PRINCIPLE OF FIBER

TWO-WAY TRANSMISSION

METHOD

Optical fiber time-frequency transmission technology

has been studied in foreign countries for more than 30

years. From the initial unidirectional time-frequency

transmission method to the widely used optical fiber

two-way time-frequency transmission method

(Xiaohui LI, 2010), the synchronization accuracy has

entered the sub-nanosecond order, which is an

important means to realize time synchronization

between stations.

The basic principle of fiber two-way time

frequency transmission technology is: station A and

station B send and receive time signals each other

(such as 1PPS), signals generally through

multiplexing technology coupled in A fiber

transmission, because station A and station B send

and receive signals at the same time, two-way

transmission path is basically the same, its

propagation delay can be effectively cancelled; In

addition, if station B can't independently generate the

time-frequency reference, it can also be combined

with one-way time-frequency transmission method to

pass the frequency reference of station A to station B.

Its implementation principle block diagram is shown

as Figure 1:

Figure 1: Principle of fiber optic two-way time frequency

transmission method.

The time of station A is denoted as T

, the time of

station B is denoted as T

, the comparison and

measurement result of station A is

△

, and the

comparison and measurement result of station B is

△

. It is assumed that the delay of the transmitting

device at station A is τ

, the delay of the receiving

device at station A is τ

, and the delay of the

transmitting device at station B is τ'

, the delay of the

receiving device at station B is τ'

, the optical fiber

path transmission delay is τ, then:

△

= T

–(T

+τ'

+τ+τ

)

(1)

△

= T

–(T

+τ'

+τ+τ

)

(2)

let station A and station B use the same

transmitting and receiving technology, it is

considered that τ

,= τ'

, τ

,= τ'

, then the time

difference between station A and station B is:

-T

= (

△

)

(3)

In formula (3), the circuit delay of station A and

station B at the receiving and transmitting ends is not

equal, and the delay of the device must be measured

and calibrated. At this time, the time difference

between station A and station B is:

221

ττττ

−+−+Δ−Δ

=−

(4)

In Formula (4), station A and station B use high-

precision phase measurement technology to measure

Δt

and Δt

, and adjust the time-frequency reference

of the recovery terminal of station B using high-

precision phase control technology according to the

comparison measurement results, so as to realize the

time-frequency transmission and time

synchronization of station A and station B.

3 PHASE CONTROL METHOD

OF OPTICAL FIBER

RECOVERY TERMINAL

When the time frequency signal of the optical fiber

time frequency transmission recovery terminal is

recovered, multiple types of local time frequency

signals will be generated, and the delay of multiple

PLL and frequency division circuit will be

inconsistent in the design, and each time the device is

started, reset or PLL relock will cause large delay

uncertainty, resulting in non-synchronization

between multiple types of time frequency signals.

Therefore, a local high-resolution phase regulation

method is needed to regulate the local signal

according to the time difference between stations

calculated by the two-way time and frequency

transmission method of optical fiber, so that the phase

consistency of the multi-class time and frequency

signals generated locally in real time is constrained

within a reasonable index range, and high-precision

Research on Phase-Control Technology of Recovery Terminal by Two-Way Optical Fiber Time Frequency Transfer Method

521

services are provided for the back-end time-used

frequency equipment.

3.1 Principle of High-Precision Phase

Adjustment Method

High-precision phase adjustment can be achieved by

controlling the output phase of the frequency

synthesizer. PLL (Phase Locked Loop) based on DDS

(Direct Digital Synthesis) The frequency synthesis

technology of Loop is a high-precision phase

adjustment method, which has the advantages of

simple structure, convenient control, high phase

control accuracy and resolution (<lps), unrestricted

phase control range, and low cost. The principle of

the specific design and implementation method is

shown as figure 2:

out

ref

Figure 2: A high-precision phase adjustment method for

PLL based on DDS.

Figure 2 PLL high-precision phase adjustment

method of DDS schematic block diagram. The above

is a closed-loop phase negative feedback control

system. In the basic PLL structure shown in the

schematic diagram, PLL is used to realize frequency

synthesis. In order to ensure the phase noise index of

the output frequency signal, OCXO is selected as

VCO; Loop phase is automatically controlled by DDS

negative feedback, PLL loop output phase is realized

by controlling 1N in the output phase of DDS, and at

the same time, the N frequency divider is matched

with the input reference R frequency divider for phase

identification: PLL output is used as the clock of the

post-stage time-frequency reference regeneration.

Considering the phase control relationship of PLL, in

the PLL loop, the phase discriminator thinks that the

output phase of the N divider is the output phase of

the VCO, that is, the final output phase of the PLL

loop. When the VCO output phase changes, the N

frequency divider phase changes, the phase

discriminator output adjustment voltage control VCO

to adjust the phase, and finally the phase

discriminator two input phase equal to reach a steady

state. R divider and N divider will introduce phase

delay when realizing frequency division, respectively

recorded as IR and IN, loop steady state phase divider

R divider and N divider output phase equal, then PLL

output phase 0out and reference input phase n has the

following relationship:

NRrefout 11

ϕϕϕϕ

−+=

(5)

According to formula (5), the PLL output phase is

determined by the phase delay of the R divider and

the N divider. Changing the phase delay of the N/R

divider will also change the output phase of the PLL,

and then realize the delay adjustment of the back-end

time-frequency reference signal. In this scheme,

phase adjustment is carried out by changing the N

frequency divider of DDS. The formula of res of DDS

phase control resolution can be obtained by referring

to (

Gong Hang, 2008):

DDSclk

DDSout

res

⋅==

(6)

In formula (6), where N is the phase control word

length and f

clk,DDS

is the DDS output frequency. As

can be seen, the resolution is not only related to the

phase control word length, but also directly related to

the output frequency of DDS, the greater N, the

higher f

clk,DDS

, the higher resolution.

3.2 Local Time-Frequency Signal

Phase Control Method

For the phase alignment of local time-frequency

signal, a phase control method of local time-

frequency signal based on optical fiber two-way

comparison measurement data, DDS phase lock

adjustment and ARM automatic control is proposed.

The implementation principle is shown in FIG3. Set

the reference input signal f, the measured signal f is

also the feedback signal of the local output time-

frequency signal, and fou is the local output time-

frequency signal.

If f

is completely synchronized with station A

through the method in Section 2, then when f

aligned with the phase of f

out

, the phase comparison

result measured by the external high-precision phase

comparator (PCO) is T

assuming that the time delay

between the two measuring channels of PCO is equal

to the cable time delay, then T

≈0,

It can be considered that the phase of the recovery

terminal time-frequency signal at station B has been

completely synchronized with that at station A. Set

the real-time phase difference of the measurement of

two-way comparison at this time to

△

T, then through

real-time monitoring of the difference between the

measurement value of two-way comparison and the

zero value

△

T-T

you can achieve real-time

ANIT 2023 - The International Seminar on Artiﬁcial Intelligence, Networking and Information Technology

522

control and alignment of the phase of the local time-

frequency signal, set the threshold of real-time phase

adjustment toΦ, then:







onturn

offturn

＞

，

Δτ=ΔΤ- Τ

Φ≤

，

Δτ=ΔΤ- Τ

(7)

Through the real-time monitoring of formula (7)

decision conditions by ARM microprocessor, the

phase of the local time-frequency signal can be

aligned in real-time self-closed loop phase to achieve

local phase consistency control.

tΔ

Figure 3: Principle of local time-frequency signal phase

alignment method.

The method principle of phase consistency

control of local time-frequency signal by using

optical fiber two-way comparison of real-time

measurement data and initial calibration zero value is

discussed above. The implementation process is

shown as figure 4:

Figure 4: Phase alignment method implementation flow

chart Specific.

Implementation steps are:

1) When measuring for the first time, calibrate

the zero value

△

T of the output time-

frequency signal by the local reference time-

frequency reference, and store the initial

calibration zero value in the phase control

2) the optical fiber time ratio is compared with

the phase comparison result T

of real-time

measurement and the zero value

△

T, and the

result is

△

3) When

△

t exceeds the threshold, turn on the

output signal Real-time phase adjustment of

out

;

4) Stop real-time phase alignment when

△

reaches within the threshold.

4 TEST ANALYSIS

4.1 The Design of Test

Test scheme combined with the phase control method

of the optical fiber recovery terminal in the upper

section, the two-way optical fiber two-way

comparison method and DDS phase modulation

technology were used to verify the method through

the test design, and the test design scheme was

connected with the block diagram is shown as figure

Figure 5: Block diagram of the test design scheme.

The time-frequency reference in the figure above

is a hydrogen atom clock. The homologous hydrogen

clock in the reference source of the test and

verification system has passed the metrological

inspection, proving that its indicators are accurate and

reliable; The time-frequency transmission terminal

and time-frequency recovery terminal are self-

developed prototypes; The test system is composed of

phase noise analyzer, time interval tester and other

measuring equipment, and the test fiber is 10km. The

temperature control box is used for temperature

regulation and constant temperature control of the test

optical fiber.

In the two-way time comparison method, the

positive pseudo-code ranging result is

△

,,the

positive pseudo-code ranging result is Ats, the time of

the main station is recorded as T

the terminal time

is recorded as T

, the link delay of optical fiber

transmission is τ, the time difference between the

central node IPPS and the distribution node 1PPS is

MS=

-T

,and according to formula (1) and formula

(2), then:

Research on Phase-Control Technology of Recovery Terminal by Two-Way Optical Fiber Time Frequency Transfer Method

523







−−=−−=+−=Δ

−=−=+−=Δ

τττ

-)()(

(8)

For the control of the phase consistency between

the central node 1PPS and the distributed node 1PPS,

the phase difference that the distributed node 1PPS

needs to compensate is T

, and the phase consistency

can be regarded as that the amplitude of T

, T

can

be controlled within the range of the phase

consistency index by compensating T

in real time.

4.2 Analysis of Results

During the test and verification, the output phase

consistency under two typical application scenarios is

analyzed.

1) When the central node is the same origin as the

distributed node, and the link delay t has no

fluctuation and is A constant, then:











−=−−=Δ

−=−=Δ

(9)

Formula (9) can be understood as that under ideal

conditions, the delay of one-way frequency

transmission is fixed, and the clock difference of two-

way time comparison is about zero. At this time, for

distributed nodes, it is necessary to compensate the

fixed zero value of the frequency transmission system

and align the internal phase to realize the phase

consistency control between stations A and B. The

test results in this scenario are shown as figure 6:.

Figure 6: 1PPS phase difference between Station A and

Station B(S1).

2) When the central node is the same origin as the

distributed node, the link delay rfluctuates and is a

variable value, then:











−=−−=Δ

−=−=Δ

ττ

T 0

(10)

Formula (10) can be understood as the real-time

phase alignment adjustment is enabled when the

optical fiber is subjected to the temperature regulation

delay fluctuation in the temperature control box, the

delay fluctuation of one-way frequency transmission

is effectively compensated, and the forward delay and

reverse delay change laws of two-way comparison are

consistent. At this time, the delay fluctuation of the

time frequency transmission of the recovery terminal

of station B is dynamically compensated, and the

fixed zero value of the system is also compensated to

realize the phase consistency control of station A and

station B. The test results in this scenario are shown

as figure 7:

Figure 7: 1PPS phase difference between Station A and

Station B(S2).

4.3 Test Summary

In the two test groups, due to the homology test, the

clock difference between station A and station B is

zero, and the 1PPS phase difference between station

A and station B is significantly affected by the

fluctuation of link delay. The T

values of the two

scenarios were statistically analyzed, and the

maximum value of each group of TMs in the table

was defined as the phase consistency measurement

results of the group of experiments. Details are

shown as Table 1:

Table 1: Statistics of phase consistency test results.

Experimental

Scenario

Phase difference consistency

control results

S1 0.05ns(50ps)

S2 0.09ns(90ps)

0 5 10 15 20 25

-0.08

-0.06

-0.04

-0.02

0.02

0.04

0.06

0.08

time (h)

(ns)

0 5 10 15 20 25

-0.08

-0.06

-0.04

-0.02

0.02

0.04

0.06

0.08

time (h)

(ns)

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524

Based on the above analysis, under the condition

of 10km indoor optical fiber, the phase-consistency

control accuracy of the time-frequency signal

transmission method designed in this paper is less

than 100ps.

5 CONCLUSION

In summary, we have come to the following

conclusion:

1) High precision time-synchronization within

and between stations can be achieved.

Using fiber two-way time-frequency transmission

can effectively eliminate the asymmetric error of

delay, but the accuracy of two-way comparison with

drastic change of delay is worse than that of stable

change of delay. Through the phase control of

terminal recovery signal, the phase synchronization

accuracy of recovery signal can reach sub-

nanosecond or even higher.

2) It is completely feasible to use the

measurement data of optical fiber two-way time

frequency method to carry out phase consistency

control on the recovery terminal.

The phase consistency test of two-way optical

fiber comparison under different scenarios is

analyzed. The results show that the phase consistency

control method of optical fiber time-frequency signal

transmission designed in this paper, by realizing

phase consistency control, recovers the phase

consistency control of the time-frequency signal

output by the terminal (station B) and the 1PPS signal

output by the main station (station A). At the

transmission distance of 10km optical fiber, the time

comparison measurement accuracy of the two

scenarios is better than 100ps, which is further

verified the proposed phase control method of optical

fiber recovery terminal is feasible in engineering

application.

ACKNOWLEDGMENTS

National Natural Science Foundation of Hunan

(2021RC3073).

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