Time frequency signal modulation transmission method and system

Abstract

The invention relates to a time frequency signal modulation transmission method and system, and the method comprises the steps: collecting time and frequency signals, and carrying out the modulation processing of the signals through an FPGA technology, and obtaining a time frequency modulation signal; transmitting the time frequency modulation signal; demodulating the return signal, and performing phase difference measurement on a frequency signal obtained after return demodulation and a local frequency signal by using an all-digital double-mixing phase discriminator to obtain a phase pre-compensation value; a digital adjustable delayer composed of a direct digital frequency synthesizer, a phase-locked loop, an FPGA and a resynchronization module is used for carrying out phase compensation on the time-frequency modulation signal. According to the invention, the FPGA technology is utilized to modulate the collected time signal and frequency signal, so that the simultaneous transmission of time and frequency is realized, and the stability of information transmission is improved; and a full-digital scheme is adopted, so that noise and nonlinear distortion in an analog transmission system can be avoided, and implementation and maintenance are easier.

Description

Technical Field

[0001] The present invention relates to the technical field of digital signal modulation transmission, and particularly to a time-frequency signal modulation transmission method and system. Background Art

[0002] Time-frequency transfer technology is widely used in many fields such as time-frequency metrology, radio astronomy, positioning and navigation, modern communication, power grid, basic physics research, deep space network, etc., and has become an important guarantee for the normal operation of human society. The traditional time-frequency transfer system based on satellite links cannot meet the transfer and comparison requirements of modern atomic clocks, and cannot provide short-term stability for applications with time-frequency synchronization requirements. Compared with the traditional satellite link, fiber optic time-frequency transfer is a new technology that transmits high-precision time-frequency signals over long distances through fiber optic networks. Optical fibers have excellent transmission characteristics, support high-bandwidth and high-speed transmission, are strongly resistant to electromagnetic interference, have high security, good confidentiality, and low loss.

[0003] The time-frequency transfer system consists of two parts: time transfer and frequency transfer. However, both the time transfer system and the frequency transfer system have certain limitations and limited application ranges. Time transfer is limited by equipment accuracy, environmental impact, and multipath interference, and there will be a certain delay in signal transmission; frequency transfer will reduce the frequency transfer stability due to problems such as phase noise accumulation, dispersion, and connection loss. In contrast, simultaneous time-frequency transfer can not only simplify system design, improve synchronization accuracy, but also enhance the robustness of the system.

[0004] In order to achieve high-precision time-frequency transfer, fiber optic time-frequency transfer needs to actively compensate for fiber optic link noise. Usually, the method of signal round-trip transmission in the fiber is used to measure the fiber optic link noise and then compensate it. The existing simultaneous fiber optic time-frequency transfer usually combines the time signal (1PPS of second pulse) and the frequency signal (square wave) together (such as through time-division multiplexing or using the falling edge of the time signal to trigger the frequency signal to generate movement), as Figure 1 shown, and then modulated onto the same laser carrier for transmission using an optical fiber. In this way, the transmission paths of the time and frequency signals are the same, so the same compensation mechanism can be used to achieve time-frequency signal transmission. However, in the phase noise compensation link, this method uses an analog adjustable delay line, which has problems such as nonlinear distortion and noise interference, resulting in poor noise compensation accuracy. In addition, there is also a method of modulating the time and frequency signals onto carrier lasers with different wavelengths respectively, and using the fiber optic wavelength division multiplexing method to simultaneously transmit in the same optical fiber, as Figure 2 shown, but the noises of the time and frequency signals in this method need to be measured and compensated separately, which are independent of each other, and the system is relatively complex.

[0005] Therefore, it is necessary to improve one or more problems existing in the above related technical solutions.

[0006] It should be noted that this section aims to provide background or context for the technical solutions of the present disclosure stated in the claims. The description herein is not admitted to be prior art merely because it is included in this section. Summary of the Invention

[0007] An object of the present invention is to provide a time-frequency signal modulation and transmission method and system, so as to overcome, at least to a certain extent, one or more problems caused by the limitations and defects of the related art.

[0008] The present invention first provides a time-frequency signal modulation and transmission method, including: Collect time signals and frequency signals, and use FPGA technology to modulate the collected time signals and frequency signals to obtain a time-frequency modulation signal including the time signals and the frequency signals; Convert the time-frequency modulation signal into an optical signal, and transmit the optical signal through an optical fiber; wherein, the optical signal can be converted into an electrical signal by a receiving end, and the electrical signal can obtain the time signal through demodulation processing and obtain the frequency signal through phase-locked processing; Receive a part of the optical signal returned by the receiving end and perform demodulation, measure the phase difference between the frequency signal obtained after the returned demodulation and the local frequency signal by using an all-digital double-mixer phase detector, and obtain a phase pre-compensation value; According to the phase pre-compensation value, use a digital adjustable delay line composed of a direct digital frequency synthesizer DDS, a phase-locked loop PLL, an FPGA, and a resynchronization module to perform phase compensation on the time-frequency modulation signal.

[0009] In the present invention, the step of using FPGA technology to modulate the collected time signals and frequency signals includes: Judge the relative positions of the rising edge of the time signal and the edges of the frequency signal; Process the pulse width of the frequency signal according to the relative position judgment result to obtain a time-frequency modulation signal.

[0010] In the present invention, the step of processing the pulse width of the frequency signal according to the relative position judgment result includes: When the rising edge of the time signal is at the low level of the frequency signal, widen the pulse width of the frequency signal; When the rising edge of the time signal is at the high level of the frequency signal, narrow the pulse width of the frequency signal.

[0011] In the present invention, after the step of judging the relative positions of the rising edge of the time signal and the edges of the frequency signal, the following steps are included: When the relative positions cannot be judged, delay processing is performed on the time signal; The upper edge of the time signal after the delay processing is detected; When the upper edge of the time signal is detected, second counting is performed on the frequency signal. Every time one second is counted up, pulse width broadening processing or narrowing processing is performed on the frequency signal to obtain a time-frequency modulation signal.

[0012] In the present invention, the step of performing phase compensation on the time-frequency modulation signal by using the digital adjustable delay device includes: Performing linear phase shift on the frequency signal by using the direct digital frequency synthesizer, and controlling the delay compensation amount less than 10 ns by adjusting the value of the phase control word of the direct digital frequency synthesizer; Locking the frequency and phase of the output signal of the direct digital frequency synthesizer by using the phase-locked loop; Controlling the delay compensation amount of an integer multiple of 10 ns by using the preset output data of the shift register in the FPGA; Performing resynchronization processing on the signal output by the FPGA to complete the phase compensation of the time-frequency modulation signal.

[0013] The present invention further provides a time-frequency signal modulation and transmission system, which includes: A modulation module, configured to collect a time signal and a frequency signal, and perform modulation processing on the collected time signal and frequency signal by using FPGA technology to obtain a time-frequency modulation signal including the time signal and the frequency signal; A first conversion module, configured to convert the time-frequency modulation signal into an optical signal and transmit the optical signal by using an optical fiber; A second conversion module, configured to receive the optical signal transmitted by the optical fiber and convert the optical signal into an electrical signal; A first demodulation module, configured to perform phase-locked processing on the electrical signal to obtain the frequency signal, and perform demodulation processing on the electrical signal to obtain the time signal; A phase difference measurement module, configured to measure the phase difference between the frequency signal obtained after backhaul demodulation and the local frequency signal by using an all-digital double-mixer phase discriminator, and obtain a phase pre-compensation value; A phase compensation module, configured to perform phase compensation on the time-frequency modulation signal according to the phase pre-compensation value by using a digital adjustable delay device composed of a direct digital frequency synthesizer, a phase-locked loop, an FPGA, and a resynchronization module.

[0014] In the present invention, the modulation module includes: A signal sampling unit, which includes a two-bit register and is used for real-time acquisition of time signals and frequency signals; A phase judgment unit, which is used for judging the relative positions of the rising edge of the time signal and the edges of the frequency signal; A pulse delay unit, which is used for delaying the time signal when the relative position cannot be judged; A second counter, which is used for counting seconds of the frequency signal when the time signal is at the upper edge; An edge processing unit, which is used for broadening or narrowing the pulse width of the frequency signal.

[0015] In the present invention, the digital adjustable delay line includes: A direct digital frequency synthesizer, which is used for linearly shifting the phase of the frequency signal and controlling the delay compensation amount less than 10 ns by adjusting the value of the phase control word; A phase-locked loop, which is used for locking the frequency and phase of the output signal of the direct digital frequency synthesizer; An FPGA, which is used for controlling the delay compensation amount in multiples of 10 ns; A resynchronization module, which is used for resynchronizing the signal output by the FPGA to complete the phase compensation of the time-frequency modulation signal.

[0016] In the present invention, the direct digital frequency synthesizer includes: a phase accumulator, a sine-cosine calculation module, and a multiplier.

[0017] In the present invention, the system includes: A transmitting end, which includes a modulation module, a second demodulation module, a first conversion module, a phase difference measurement module, and a phase compensation module. The second demodulation module is used for demodulating a part of the optical signal transmitted back; A receiving end, which includes a second conversion module, a first demodulation module, and a signal feedback module. The signal feedback module is used for feedbacking a part of the received time-frequency modulation signal as an optical signal.

[0018] The technical solution provided by the present invention may include the following beneficial effects: In the time-frequency signal modulation transmission method and system of the present invention, the collected time signal and frequency signal are modulated by using FPGA technology, so that the two are combined together to realize the simultaneous transmission of time and frequency signals, improving the stability of signal transmission; the phase difference between the demodulated frequency signal and the local frequency signal is measured by using an all-digital double-mixer phase detector, and a digital adjustable delay composed of a direct digital frequency synthesizer, a phase-locked loop, an FPGA and a resynchronization module is used to compensate the phase of the time-frequency modulation signal, which has a larger compensation range and improves the compensation accuracy, realizing the high-precision transmission of time and frequency signals; adopting an all-digital solution can avoid noise and nonlinear distortion in the analog transmission system and is easier to implement and maintain. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings herein are incorporated into the specification and constitute a part of this specification, showing embodiments consistent with the present disclosure, and are used together with the specification to explain the principles of the present disclosure. Obviously, the drawings in the following description are only some embodiments of the present disclosure, and those of ordinary skill in the art can obtain other drawings based on these drawings without creative efforts.

[0020] Figure 1 Showing a schematic diagram of simultaneously transmitting optical fiber time and frequency using a set of compensation devices in the prior art; Figure 2 Showing a schematic diagram of simultaneously transmitting optical fiber time and frequency with separate compensation for time and frequency signals in the prior art; Figure 3 Showing a schematic flowchart of the time-frequency signal modulation transmission method in an exemplary embodiment of the present disclosure; Figure 4 Showing a schematic flowchart of modulating and processing the collected time signal and frequency signal using FPGA technology in an exemplary embodiment of the present disclosure; Figure 5 Showing a schematic flowchart of applying the digital adjustable delay to compensate the phase of the time-frequency modulation signal in an exemplary embodiment of the present disclosure; Figure 6 Showing a schematic diagram of the module structure of the time-frequency signal modulation transmission system in an exemplary embodiment of the present disclosure; Figure 7 Showing a schematic diagram of the time-frequency modulation technology principle in an exemplary embodiment of the present disclosure; Figure 8 Showing a structural diagram of the digital adjustable delay in an exemplary embodiment of the present disclosure; Figure 9 Showing a schematic diagram of the principle of the all-digital double-mixer phase detector in an exemplary embodiment of the present disclosure; Figure 10Schematic diagram showing the structure of a system for modulating and transmitting time-frequency signals in an exemplary embodiment of the present disclosure; Figure 11 Diagram showing the formation process of a time-frequency modulation signal in an exemplary embodiment of the present disclosure; Figure 12 Technical principle diagram showing the demodulation of a time-frequency modulation signal in an exemplary embodiment of the present disclosure; Figure 13 Diagram showing the simulation results of a time-frequency modulation technique using an FPGA hardware development platform in an exemplary embodiment of the present disclosure. Detailed implementation manners

[0021] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, the exemplary embodiments can be implemented in various forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The features, structures, or characteristics described may be combined in any suitable manner in one or more embodiments.

[0022] In addition, the accompanying drawings are only schematic illustrations of the embodiments of the present disclosure and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and thus repeated descriptions thereof will be omitted. Some of the block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0023] In this exemplary embodiment, a method for modulating and transmitting time-frequency signals is first provided. Please refer to Figure 3 , and this transmission method includes: Step S101 - Step S104, specifically as follows: Step S101, collect a time signal and a frequency signal, and use FPGA technology to perform modulation processing on the collected time signal and frequency signal to obtain a time-frequency modulation signal including the time signal and the frequency signal. The time signal is a 1PPS time pulse signal, and the frequency signal is a 10MHz square wave frequency signal. The time-frequency modulation signal after being modulated by FPGA (Field Programmable Gate Array) has both the time information of the 1PPS signal and the frequency information of the 10MHz signal.

[0024] Step S102: Convert the time-frequency modulation signal into an optical signal and transmit the optical signal through an optical fiber. Among them, the optical signal can be converted into an electrical signal by a receiving end, and the electrical signal can obtain the time signal through demodulation processing and obtain the frequency signal through phase-locked processing. Transmitting the time-frequency modulation signal in an optical fiber link realizes the simultaneous transmission of time-frequency signals, improves the stability of the system, and simplifies the optical fiber time-frequency transmission system.

[0025] Step S103: Receive a part of the optical signal returned by the receiving end and perform demodulation, use an all-digital double-mixer phase discriminator to measure the phase difference between the frequency signal obtained after the returned demodulation and the local frequency signal, and obtain a phase pre-compensation value.

[0026] Measure the phase difference between the frequency signal after the returned demodulation and the local frequency signal. The measurement result is the link phase noise introduced during the signal transmission process. According to the measurement result, the phase pre-compensation value of the time-frequency modulation signal in the subsequent transmission process can be obtained.

[0027] Step S104: According to the phase pre-compensation value, use a digital adjustable delay composed of a Direct Digital Synthesizer (DDS), a Phase Locked Loop (PLL), an FPGA, and a resynchronization module to perform phase compensation on the time-frequency modulation signal. Specifically, the DDS, PLL, FPGA, and resynchronization module are connected in sequence, and the PLL is also connected to the resynchronization module.

[0028] In this embodiment, the FPGA technology is used to perform modulation processing on the collected time signal and frequency signal, so that the two are combined together to realize the simultaneous transmission of time and frequency, improving the stability of information transmission; use an all-digital double-mixer phase discriminator to measure the phase difference between the demodulated frequency signal and the local frequency signal, and use a digital adjustable delay composed of DDS, PLL, FPGA, and resynchronization module to perform phase compensation on the time-frequency modulation signal, which has a larger compensation range and improves the compensation accuracy, realizing the high-precision transmission of time and frequency signals; adopting an all-digital solution can avoid noise and non-linear distortion in the analog transmission system and is easier to implement and maintain.

[0029] Based on the above embodiments, the specific technical solutions of the present application are described.

[0030] Optionally, in some embodiments, please refer to Figure 4 , the steps of using the FPGA technology to perform modulation processing on the collected time signal and frequency signal include step S201 and step S202: Step S201, determine the relative positions of the rising edge of the time signal and the edges of the frequency signal. Specifically, determine the relative positions of the rising edge of 1PPS and the edges of 10MHz.

[0031] Step S202, process the pulse width of the frequency signal according to the relative position determination result to obtain a time-frequency modulation signal. The specific processing process includes the following three cases: (1) When the rising edge of the time signal is at the low level of the frequency signal, broaden the pulse width of the frequency signal.

[0032] After sampling the time signal and the frequency signal, represent them with logical values of 0 or 1 for edge detection. When the rising edge of 1PPS is detected, sample and observe the edge of 10MHz. If the sampling result is 00, it indicates that the rising edge of 1PPS is at the low level of 10MHz. At this time, broaden the pulse width of the 10MHz signal to obtain a time-frequency modulation signal.

[0033] (2) When the rising edge of the time signal is at the high level of the frequency signal, narrow the pulse width of the frequency signal.

[0034] When the rising edge of 1PPS is detected, sample and observe the edge of 10MHz. If the sampling result is 11, it indicates that the rising edge of 1PPS is at the high level of 10MHz. At this time, narrow the pulse width of the 10MHz signal to obtain a time-frequency modulation signal.

[0035] (3) When the relative position cannot be determined, delay the time signal and detect the upper edge of the time signal after the delay processing. When the upper edge of the time signal is detected, count seconds for the frequency signal. Every time one second is counted, broaden or narrow the pulse width of the frequency signal to obtain a time-frequency modulation signal.

[0036] Specifically, when the rising edge of 1PPS is detected, sample and observe the edge of 10MHz. If the sampling result is 01 or 10, it indicates that the relative position between the two cannot be determined at this time, and the time signal needs to be delayed.

[0037] Edge detection is performed on the 1PPS signal after delay processing. When the rising edge of the 1PPS is detected, the second counter is used to count the 10MHz signal for one second. When the count reaches one second, the pulse of the 10MHz signal at that moment is broadened or narrowed. In this way, the delayed 1PPS signal is embedded into the 10MHz signal to achieve the modulation of the time-frequency signal and obtain the time-frequency modulation signal. It should be noted that the process of broadening or narrowing the pulse width here also requires first judging the relative position of the rising edge of the 1PPS signal and the 10MHz signal. After judgment, if the rising edge of the 1PPS signal is at the low level of the 10MHz signal, the pulse width of the 10MHz signal is broadened; when the rising edge of the 1PPS signal is at the high level of the 10MHz signal, the pulse width of the 10MHz signal is narrowed.

[0038] The specific process of phase compensation is described below.

[0039] Please refer to Figure 5 , and the digital tunable delay line is used to perform phase compensation on the time-frequency modulation signal. The steps of this phase compensation include: Step S301: Use DDS to perform linear phase shift on the frequency signal, and control the delay compensation amount less than 10ns by adjusting the value of the DDS phase control word.

[0040] Step S302: Use PLL to lock the frequency and phase of the DDS output signal.

[0041] Step S303: Use the preset output data of the shift register in the FPGA to control the delay compensation amount of an integer multiple of 10ns.

[0042] Step S304: Resynchronize the signal output by the FPGA to complete the phase compensation of the time-frequency modulation signal. Specifically, resynchronize the signal output by the FPGA to ensure synchronous transmission of the signal.

[0043] In this embodiment, the digital tunable delay line completes the phase compensation of the time-frequency modulation signal by controlling the delay compensation amount less than 10ns and the delay compensation amount of an integer multiple of 10ns. The digital tunable delay line includes the functions of DDS, PLL, FPGA, and resynchronization. It is a wide-range and high-precision phase compensation method based on DDS. It uses the combination of DDS and FPGA for compensation. Compared with the analog compensation scheme, its compensation range is larger and the precision is higher.

[0044] Please refer to Figure 6 , and secondly, in this exemplary embodiment, a time-frequency signal modulation and transmission system is provided. The system includes: A modulation module is used to collect time signals and frequency signals, and use FPGA technology to perform modulation processing on the collected time signals and frequency signals to obtain a time-frequency modulation signal containing the time signals and the frequency signals; A first conversion module is used to convert the time-frequency modulation signal into an optical signal and transmit the optical signal using an optical fiber; A second conversion module is used to receive the optical signal transmitted by the optical fiber and convert the optical signal into an electrical signal; A first demodulation module is used to perform phase-locked processing on the electrical signal to obtain the frequency signal, and perform demodulation processing on the electrical signal to obtain the time signal; A phase difference measurement module is used to measure the phase difference between the frequency signal obtained after backhaul demodulation and the local frequency signal using an all-digital double-mixer phase detector, and obtain a phase pre-compensation value; A phase compensation module, please refer to Figure 8 This phase compensation module is used to perform phase compensation on the time-frequency modulation signal according to the phase pre-compensation value using a digital tunable delay composed of a direct digital frequency synthesizer (DDS), a phase-locked loop (PLL), an FPGA, and a resynchronization module.

[0045] Among them, this application has improved the design of the all-digital double-mixer phase detector. As Figure 9 shown, the all-digital double-mixer phase detector of this application uses multi-stage amplification to further eliminate the influence of signal metastability.

[0046] In this embodiment, FPGA technology is used to perform modulation processing on the collected time signals and frequency signals to achieve the simultaneous transmission of time and frequency, improving the stability of information transmission; an all-digital double-mixer phase detector is used to measure the phase difference of the demodulated frequency signal, and a digital tunable delay composed of DDS, PLL, FPGA, and a resynchronization module is used to perform phase compensation on the time-frequency modulation signal, which has a larger compensation range and improves the compensation accuracy, achieving high-precision transmission of time and frequency signals; an all-digital solution is adopted to avoid noise and non-linear distortion in the analog transmission system and is easier to implement and maintain.

[0047] Among them, please refer to Figure 7 Figure 7 is the schematic diagram of the time-frequency modulation technology. The modulation module includes: A signal sampling unit. The signal sampling unit includes two-bit registers and is used to collect time signals and frequency signals in real time; A phase judgment unit is used to judge the relative positions of the rising edge of the time signal and the edges of the frequency signal; A pulse delay unit for delaying the time signal when the relative position cannot be determined; A second counter for counting seconds of the frequency signal when the time signal is at the rising edge; An edge processing unit for broadening or narrowing the pulse width of the frequency signal.

[0048] Please refer to Figure 8 , the digital tunable delay includes: A DDS for linearly shifting the phase of the frequency signal, and controlling the delay compensation amount less than 10 ns by adjusting the value of the DDS phase control word; A PLL for locking the frequency and phase of the DDS output signal; An FPGA for controlling the delay compensation amount in whole 10 ns; A resynchronization module for resynchronizing the signal output by the FPGA, completing the phase compensation of the time-frequency modulation signal, and ensuring signal synchronous transmission.

[0049] The DDS includes a phase accumulator, a sine-cosine calculation module, and a multiplier.

[0050] Please refer to Figure 8 and 10 , the present invention designs a wide-range and high-precision phase compensation technology based on DDS, namely a digital tunable delay, for dynamically and real-time compensating the link phase noise, which is a scheme of combined compensation of DDS and FPGA. The DDS phase shift has good linearity and can achieve small-range delay compensation. In the present invention, the AD9954 module is used for DDS. This module can generate a target signal with a maximum frequency of 160 MHz, and the phase is controllable between 0 and 360 degrees. Taking the AD9954 output signal as the system clock of the phase fine compensation link, the phase noise can be finely compensated by adjusting its phase; the FPGA can achieve wide-range delay compensation. In the present invention, the system clock frequency used is 100 MHz and the period is 10 ns. By adjusting the preset value of the shift register in the FPGA, the phase noise delay compensation in whole 10 ns magnitude can be achieved.

[0051] The principle of the wide-range and high-precision delay compensation technology based on DDS is as Figure 8 shown. The FPGA is used to achieve the delay compensation in whole 10 ns, and the DDS is used to achieve the delay compensation less than 10 ns.

[0052] The DDS is used to generate precise frequency signals. It digitally controls the frequency, phase, and amplitude of the signals and can generate waveforms with high stability and high resolution. In this application, the DDS is used to perform linear phase shift on the 100 MHz system clock frequency signal. By adjusting the value of the DDS phase control word, a delay compensation amount less than 10 ns can be controlled; the PLL is used to lock the frequency and phase of the DDS output signal, reduce its phase noise, and ensure the stability and accuracy of the output signal; by controlling the preset output of the shift register in the FPGA, a delay compensation amount in whole 10 ns can be controlled; the signal processed by the FPGA is input to the resynchronization module, which eliminates the influence of the temperature drift of the FPGA transmission time delay on the output signal and ensures the synchronization of the signal during transmission between different modules, and finally outputs a stable processed signal.

[0053] This application is a method and system for simultaneous optical fiber transmission of time and frequency based on FPGA, which is a new solution different from the current simultaneous optical fiber transmission of time and frequency. On the one hand, the FPGA is used to implement the modulation and demodulation technology of time-frequency signals, so that the time-frequency signals are simultaneously transmitted in the optical fiber link. By using a set of phase noise detection and compensation systems, high-precision transmission of time and frequency signals can be achieved, improving the system stability and simplifying the optical fiber time-frequency transmission system; on the other hand, the entire optical fiber link is implemented in a fully digital manner, giving full play to the advantages of the FPGA hardware development platform, and achieving high-precision detection and compensation of phase noise. The fully digital solution can avoid the noise and nonlinear distortion in the analog transmission system and is easier to implement and maintain.

[0054] The signal transmission process in this system is described below.

[0055] Please refer to Figure 6 and Figure 10 This application's system for simultaneous optical fiber transmission of time and frequency based on FPGA includes a transmitting end and a receiving end. The transmitting end includes: a modulation module, a second demodulation module, a first conversion module, a phase difference measurement module, and a phase compensation module; the receiving end includes: a second conversion module, a first demodulation module, and a signal feedback module.

[0056] First, at the transmitting end, the FPGA is used to modulate the 10 MHz square wave frequency signal and the 1PPS pulse time signal to obtain a time-frequency modulation signal. Then, the laser is used to convert the time-frequency modulation signal into an optical signal, which is transmitted in the optical fiber and sent to the receiving end. At the receiving end, the optical signal is converted into an electrical signal by the laser, and then the FPGA is used to demodulate it to restore the 1PPS signal, while the 10 MHz signal is obtained by using a phase-locked loop.

[0057] Then, compensate for the phase noise introduced during the signal transmission process. Transmit a part of the time-frequency modulated signal received at the receiving end back to the transmitting end through an optical fiber. At the transmitting end, use an FPGA to demodulate the 10 MHz and 1PPS signals. For the modulation process, please refer to Figure 11 , at the rising edge of the 1PPS signal, the pulse width of the 10 MHz signal Figure 11 changes as shown. The changed signal is the time-frequency modulated signal. Transmit the time-frequency modulated signal in the experimental link and send it from the transmitting end to the receiving end through an optical fiber. Demodulate the time-frequency modulated signal at the receiving end to obtain the 10 MHz and 1PPS signals, realizing the simultaneous transmission of time-frequency signals. The time-frequency modulated signal contains both the time information of the 1PPS signal and the frequency information of the 10 MHz signal. When the time-frequency modulated signal is transmitted in the system loop, the simultaneous transmission of time-frequency signals is achieved.

[0058] Please refer to Figure 12 , Figure 12 is the technical schematic diagram for demodulating the time-frequency modulated signal in this application. The first demodulation module and the second demodulation module both include: a signal acquisition module, a high-level counting and comparison module, and a second counting module. The signal acquisition module samples the time-frequency modulated signal to obtain a logical value of 0 or 1 for high-level counting. The high-level counting and comparison module is used to count the high level of each pulse of the time-frequency modulated signal respectively and compare the high-level count values of two adjacent pulses. When the comparison result is unequal, it indicates that the pulse width has changed. Here, the time signal and the frequency signal are modulated, and at this time, an enable signal is generated to turn on the second counting module. When the second counting module counts up to one second, the 1PPS time signal is pulled high, and the time signal is demodulated from the time-frequency modulated signal, while the 10 MHz signal is obtained by a phase-locked loop.

[0059] After that, at the transmitting end, use an all-digital double-mixer phase detector to measure the phase difference between the demodulated square wave signal and the local square wave signal. The measurement result is the link phase noise introduced during the transmission process of the signal at the transmitting end and the receiving end. Based on this, the phase pre-compensation value of the time-frequency signal at the transmitting end can be obtained.

[0060] According to the phase pre-compensation value, use a digitally tunable delay line to perform phase compensation on the time-frequency modulated signal at the transmitting end to achieve high-precision transmission of time and frequency signals.

[0061] Please refer to Figure 13 , Figure 13This is the simulation result of the time-frequency modulation technology of this application using the FPGA hardware development platform. Among them, i_PPS and i_10MHz are the inputs of 1PPS and 10MHz signals, i_PPS_phase is the 1PPS pulse delay signal, and o_10MHz_PPS is the output modulation signal. It can be seen that at the rising edge of the pulse delay signal, the pulse width of the 10MHz signal changes as desired, generating the required time-frequency modulation signal.

[0062] Regarding the system in the above embodiments, the specific manners in which each module performs operations have been described in detail in the embodiments related to the method, and will not be elaborated herein.

[0063] It should be noted that although several modules of the system for action execution are mentioned in the above detailed description, this division is not mandatory. In fact, according to the embodiments of the present invention, the features and functions of two or more of the above-described modules can be embodied in one module. Conversely, the features and functions of one module described above can be further divided into multiple modules for embodiment. The components shown as modules may or may not be physical units, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of the present invention. Those of ordinary skill in the art can understand and implement it without creative efforts.

[0064] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit and scope protected by the present invention and the claims. All of these fall within the protection scope of the present invention.

Claims

1. A time-frequency signal modulation transmission method, characterized in that: include: Collecting a time signal and a frequency signal, and performing modulation processing on the collected time signal and the frequency signal using FPGA technology to obtain a time-frequency modulation signal including the time signal and the frequency signal; Convert the time-frequency modulation signal into an optical signal, and transmit the optical signal using an optical fiber; wherein the optical signal can be converted into an electrical signal by a receiving end, and the electrical signal can be demodulated to obtain the time signal and phase-locked to obtain the frequency signal; Receive and demodulate a portion of the optical signal returned by the receiving end, measure the phase difference between the frequency signal obtained after the return demodulation and the local frequency signal using a fully digital dual-mixer phase detector, and obtain a phase pre-compensation value; According to the phase pre-compensation value, the phase of the time-frequency modulation signal is compensated by using a digital adjustable delay device composed of a direct digital frequency synthesizer, a phase-locked loop, an FPGA and a resynchronization module.

2. The time-frequency signal modulation transmission method according to claim 1, characterized in that: The step of using FPGA technology to modulate the collected time signal and frequency signal includes: Determining the relative position of the rising edge of the time signal and the edge of the frequency signal; The pulse width of the frequency signal is processed according to the relative position determination result to obtain a time-frequency modulation signal.

3. The time-frequency signal modulation transmission method according to claim 2, characterized in that: The step of processing the pulse width of the frequency signal according to the relative position determination result comprises: When the rising edge of the time signal is at the low level of the frequency signal, the pulse width of the frequency signal is widened; When the rising edge of the time signal is at a high level of the frequency signal, the pulse width of the frequency signal is narrowed.

4. The time-frequency signal modulation transmission method according to claim 2, characterized in that: After the step of determining the relative position of the rising edge of the time signal and the edge of the frequency signal, the method further comprises: When the relative position cannot be determined, delaying the time signal; Detecting the upper edge of the time signal after delay processing; When the rising edge of the time signal is detected, the frequency signal is counted in seconds, and every time the count reaches one second, the pulse width of the frequency signal is widened or narrowed to obtain a time-frequency modulation signal.

5. The time-frequency signal modulation transmission method according to any one of claims 1 to 4, characterized in that: The step of using the digital adjustable delay device to perform phase compensation on the time-frequency modulation signal comprises: Using the direct digital frequency synthesizer to perform linear phase shift on the frequency signal, and controlling the delay compensation amount less than 10ns by adjusting the value of the phase control word of the direct digital frequency synthesizer; Using the phase-locked loop to lock the frequency and phase of the output signal of the direct digital frequency synthesizer; The preset output data of the shift register in the FPGA is used to control the delay compensation amount of the entire 10ns; The signal output by the FPGA is resynchronized to complete the phase compensation of the time-frequency modulation signal.

6. A system for time-frequency signal modulation transmission, characterized in that: The system comprises: A modulation module, used to collect the time signal and the frequency signal, and modulate the collected time signal and the frequency signal using FPGA technology to obtain a time-frequency modulation signal containing the time signal and the frequency signal; A first conversion module, used to convert the time-frequency modulated signal into an optical signal, and transmit the optical signal using an optical fiber; A second conversion module, used for receiving the optical signal transmitted by the optical fiber and converting the optical signal into an electrical signal; A first demodulation module, configured to perform phase-locking processing on the electrical signal to obtain the frequency signal, and to perform demodulation processing on the electrical signal to obtain the time signal; The phase difference measurement module is used to measure the phase difference between the frequency signal obtained after feedback demodulation and the local frequency signal using a fully digital dual-mixer phase detector, and obtain a phase pre-compensation value; The phase compensation module is used to perform phase compensation on the time-frequency modulation signal according to the phase pre-compensation value by using a digital adjustable delay device composed of a direct digital frequency synthesizer, a phase-locked loop, an FPGA and a resynchronization module.

7. The system for time-frequency signal modulation transmission according to claim 6, characterized in that: The modulation module comprises: A signal sampling unit, the signal sampling unit comprising a two-bit register, for real-time acquisition of a time signal and a frequency signal; A phase determination unit, used to determine the relative position of the rising edge of the time signal and the edge of the frequency signal; A pulse delay unit, used for delaying the time signal when the relative position cannot be determined; A second counter, used for counting seconds of the frequency signal when the time signal is at an upward edge; The edge processing unit is used to widen or narrow the pulse width of the frequency signal.

8. The system for time-frequency signal modulation transmission according to claim 7, characterized in that: The digital adjustable delay device comprises: A direct digital frequency synthesizer, used for linearly shifting the frequency signal, and controlling the delay compensation amount less than 10ns by adjusting the value of the phase control word; A phase-locked loop, used to lock the frequency and phase of the output signal of the direct digital frequency synthesizer; FPGA, used to control the delay compensation amount of 10ns; The resynchronization module is used to perform resynchronization processing on the signal output by the FPGA to complete the phase compensation of the time-frequency modulation signal.

9. The system for time-frequency signal modulation transmission according to claim 8, characterized in that: The direct digital frequency synthesizer comprises: a phase accumulator, a sine-cosine calculation module and a multiplier.

10. The system for time-frequency signal modulation transmission according to any one of claims 6 to 9, characterized in that: The system comprises: A transmitting end, the transmitting end comprising: a modulation module, a second demodulation module, a first conversion module, a phase difference measurement module and a phase compensation module, wherein the second demodulation module is used to demodulate part of the returned optical signal; The receiving end includes: a second conversion module, a first demodulation module and a signal feedback module, and the signal feedback module is used to feedback a part of the received time-frequency modulation signal in the form of an optical signal.

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