Very-low-frequency station signal information extraction method and system based on phase-locked loop

Abstract

The invention belongs to the technical field of very low frequency fluctuation detection, and discloses a very low frequency station signal information extraction method and system based on a phase-locked loop. The method comprises the following steps of: amplifying and sampling a broadband signal containing a very-low-frequency station transmitting signal, setting a local MSK demodulation parameter, taking sampling data as input, and respectively carrying out carrier synchronization on upper and lower frequencies of an MSK modulation signal transmitted by a target monitoring very-low-frequency station by using a phase-locked loop technology; baseband synchronizing signals and carrier synchronizing signals are extracted from phase-locked loops of the upper frequency and the lower frequency respectively, amplitude information of the upper frequency and amplitude information of the lower frequency are calculated respectively through the baseband synchronizing signals, then amplitude information of the target monitoring very-low-frequency station is obtained, and amplitude information of the target monitoring very-low-frequency station is obtained through the carrier synchronizing signals of the upper frequency and the carrier synchronizing signals of the lower frequency. And extracting the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very-low frequency station, and further extracting the phase deviation information of the target monitoring very-low frequency station. According to the invention, the precision of very-low-frequency station signal information extraction can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of very low frequency (VLF) wave detection, and more specifically, relates to a method and system for extracting very low frequency station signal information based on a phase-locked loop (PLL). Background Art

[0002] Very low frequency (VLF) waves refer to electromagnetic waves with a frequency range between 3 kHz and 30 kHz. VLF waves can be divided into natural signals and artificial station signals. Among them, the VLF artificial station signals around the world cover different frequencies in this frequency band and are widely used in scientific research and communication fields. VLF waves play an important role in many fields such as geophysics, space environment monitoring, communication, and navigation, especially playing an indispensable role in solar activity monitoring, ionospheric sounding, marine communication, etc.

[0003] VLF signal detection equipment usually is equipped with a high-sensitivity receiving system and combines signal processing methods to demodulate signals and extract key features. However, in a complex electromagnetic environment, VLF signals are often affected by various noise sources, such as interference from man-made electromagnetic wave sources such as ground power lines and industrial facilities, which makes accurate signal extraction difficult. How to improve the extraction accuracy of very low frequency station signal information is a technical problem concerned and to be solved in this field. Summary of the Invention

[0004] The present invention solves the problem of low extraction accuracy of very low frequency station signal information in the prior art by providing a method and system for extracting very low frequency station signal information based on a phase-locked loop (PLL).

[0005] The present invention provides a method for extracting very low frequency station signal information based on a phase-locked loop, including the following steps:

[0006] S1. Amplify and sample a broadband signal containing a very low frequency station transmission signal to obtain sampling information;

[0007] S2. Set local MSK demodulation parameters according to the modulation information of the target monitored very low frequency station signal;

[0008] S3. Combine the local MSK demodulation parameters, use the sampling information as the input of the phase-locked loop, and use phase-locked loop technology to perform carrier synchronization on the upper frequency and the lower frequency of the MSK modulation signal transmitted by the target monitored very low frequency station respectively;

[0009] S4. After synchronization is achieved by using the phase-locked loop, extract baseband synchronization signals and carrier synchronization signals from the phase-locked loops of the upper and lower frequencies respectively;

[0010] S5. Using the baseband synchronization signal obtained in S4, calculate the amplitude information of the upper and lower frequencies respectively;

[0011] S6. Combine the amplitude information of the upper and lower frequencies obtained in S5 to obtain the amplitude information of the target monitored VLF station;

[0012] S7. Using the carrier synchronization signals of the upper and lower frequencies obtained in S4, extract the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station;

[0013] S8. Based on the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station obtained in S7, extract the phase offset information of the target monitored VLF station.

[0014] Preferably, in S2, the set local MSK demodulation parameters include the carrier frequency f c of the target monitored VLF station, the baud rate f b of the baseband signal modulated by MSK, and the data sampling rate and ADC bit number of the local receiving device.

[0015] Preferably, in S3, a Costas phase-locked loop is used for carrier synchronization; the Costas phase-locked loop includes a mixer, a low-pass filter, a phase detector, a loop filter, and a voltage-controlled oscillator.

[0016] Preferably, the initial frequency f h of the voltage-controlled oscillator at the upper frequency and the initial frequency f l of the voltage-controlled oscillator at the lower frequency are determined by the following formula:

[0017]

[0018] Wherein, f c is the carrier frequency of the signal of the target monitored VLF station, and f b is the baud rate of the baseband signal modulated by MSK.

[0019] Preferably, in S4, the extracted baseband synchronization signals include: the in-phase baseband synchronization signal at the upper frequency and the in-phase baseband synchronization signal at the lower frequency, and the quadrature baseband synchronization signal at the upper frequency and the quadrature baseband synchronization signal at the lower frequency.

[0020] Preferably, in S5, the amplitude information of the upper frequency and the amplitude information of the lower frequency are expressed as follows:

[0021]

[0022] In the formula, A h (t) and A l (t) are the amplitude information of the upper frequency and the amplitude information of the lower frequency respectively, and θ h(t) and θ l (t) is the phase of the baseband synchronization signal of the upper frequency and the phase of the baseband synchronization signal of the lower frequency respectively;

[0023] The baseband synchronization signal of the upper frequency includes the in-phase baseband synchronization signal of the upper frequency and the quadrature baseband synchronization signal of the upper frequency, and the baseband synchronization signal of the lower frequency includes the in-phase baseband synchronization signal of the lower frequency and the quadrature baseband synchronization signal of the lower frequency.

[0024] Preferably, in S6, the amplitude information of the target monitoring very low frequency station is expressed as follows:

[0025]

[0026] In the formula, A ′ (t) is the amplitude information of the target monitoring very low frequency station.

[0027] Preferably, specifically, the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station is expressed as follows:

[0028]

[0029] In the formula, f c ′ is the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station, f h ′ is the frequency of the carrier synchronization signal extracted from the upper frequency, f l ′ is the frequency of the carrier synchronization signal extracted from the lower frequency.

[0030] Preferably, in S8, the frequency f c ′ of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station is used as the frequency of the local mixing signal, and the local mixing signal is multiplied by the received MSK modulation signal transmitted by the target monitoring very low frequency station, and the baseband signal is obtained after passing through a low-pass filter:

[0031]

[0032] In the formula, S ′ baseband (t) is the baseband signal obtained after passing through the low-pass filter, LPF represents low-pass filtering, S ′ MSK (t) is the received MSK modulation signal transmitted by the target monitoring very low frequency station, f c ′ is the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station;

[0033] The amplitude information A of the target monitoring VLF station obtained by dividing the baseband signal obtained through the low-pass filter by S6 ′ (t), to obtain baseband phase information with a constant amplitude of unit amplitude; the baseband phase information is respectively combined with and multiply, and then integrate with respect to time respectively, with an integration step of 1 second, and the integration results are respectively denoted as S + (t) and S - (t):

[0034]

[0035] For S + (t) and S - (t), take the angles and denote them as θ + (t) and θ - (t), add them and divide by 2 to obtain phase offset information with a time resolution of 1 second It is expressed as follows:

[0036]

[0037] In the formula, is the phase offset information of the target monitoring VLF station, θ + (t) is the first angle, θ - (t) is the second angle, ∠S + (t) represents taking the angle of S + (t), ∠S - (t) represents taking the angle of S - (t).

[0038] On the other hand, the present invention provides a VLF station signal information extraction system based on a phase-locked loop, including:

[0039] A receiving and processing module for amplifying and sampling a broadband signal containing the VLF station transmission signal to obtain sampling information;

[0040] A demodulation setting module for setting local MSK demodulation parameters according to the modulation information of the target monitoring VLF station signal;

[0041] A carrier synchronization module for combining the local MSK demodulation parameters, using the sampling information as the input of the phase-locked loop, and respectively performing carrier synchronization on the upper frequency and the lower frequency of the MSK modulation signal transmitted by the target monitoring VLF station by using phase-locked loop technology;

[0042] A synchronization signal extraction module for respectively extracting a baseband synchronization signal and a carrier synchronization signal from the upper and lower frequency phase-locked loops after synchronization is achieved by using the phase-locked loop;

[0043] An amplitude information calculation module, which is used to calculate the amplitude information of the upper and lower frequencies respectively by using the obtained baseband synchronization signal, and to merge the amplitude information of the upper and lower frequencies to obtain the amplitude information of the target monitored VLF station;

[0044] A phase offset calculation module, which is used to obtain the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station by using the carrier synchronization signals of the upper and lower frequencies, and to extract the phase offset information of the target monitored VLF station based on the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station;

[0045] The VLF station signal information extraction system based on a phase-locked loop is used to execute the steps in the above-mentioned VLF station signal information extraction method based on a phase-locked loop.

[0046] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0047] The present invention first amplifies and samples a broadband signal containing the signal transmitted by a VLF station to obtain sampling information; sets local MSK demodulation parameters according to the modulation information of the target monitored VLF station signal; then, combines the local MSK demodulation parameters, takes the sampling information as the input of a phase-locked loop, and uses the phase-locked loop technology to perform carrier synchronization on the upper and lower frequencies of the MSK modulation signal transmitted by the target monitored VLF station respectively; after achieving synchronization by using the phase-locked loop, the baseband synchronization signal and the carrier synchronization signal are respectively extracted from the phase-locked loops of the upper and lower frequencies; then, the amplitude information of the upper and lower frequencies is calculated respectively by using the obtained baseband synchronization signal; the amplitude information of the upper and lower frequencies is merged to obtain the amplitude information of the target monitored VLF station; the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station is extracted by using the obtained carrier synchronization signals of the upper and lower frequencies; finally, the phase offset information of the target monitored VLF station is extracted based on the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station. The present invention aims at the VLF research field. In order to meet the requirements of VLF signal extraction and detection, by combining the phase-locked loop technology, it solves the problem of the influence of the carrier frequency offset of the received VLF station signal on information extraction, and can improve the accuracy of VLF station signal information extraction. The present invention can maintain high sensitivity and stability in a complex electromagnetic environment, and is especially suitable for conditions with a high noise background and weak signals, ensuring accurate amplitude and phase information is obtained. Description of the Drawings

[0048] Figure 1Flow chart of a method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention.

[0049] Figure 2 Schematic diagram of extracting amplitude information, baseband synchronization signal and carrier synchronization signal based on a Costas phase-locked loop in a method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention.

[0050] Figure 3 Schematic diagram of the loop filter used in the Costas loop in a method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention.

[0051] Figure 4 Schematic diagram of extracting phase offset information by using the frequency of the extracted carrier synchronization signal as the frequency of the local mixing signal in a method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention.

[0052] Figure 5 Comparison diagram of the input signal amplitude and the output signal amplitude in a method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention.

[0053] Figure 6 Comparison diagram of the input phase offset and the output phase offset (i.e., the output phase offset without frequency offset) in a method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention.

[0054] Figure 7 Comparison diagram of the input phase offset and the phase offset extracted without using the method for extracting very low frequency station signal information based on a phase-locked loop provided in Embodiment 1 of the present invention (i.e., the output phase offset with frequency offset). Detailed implementation manners

[0055] In order to better understand the above technical solutions, the above technical solutions will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners.

[0056] Embodiment 1:

[0057] Embodiment 1 provides a method for extracting very low frequency station signal information based on a phase-locked loop. Refer to Figure 1 , including the following steps:

[0058] S1, amplify and sample the broadband signal containing the very low frequency station transmission signal to obtain sampling information.

[0059] Specifically, two orthogonal magnetic loop antennas are used to receive very low frequency (VLF) fluctuation signals. According to Faraday's law of electromagnetic induction, the change in magnetic field strength is expressed in the form of current. After amplifying this weak current signal using an amplifier, the current signal is sampled by an ADC to obtain sampling information, which is then uploaded to a computer for storage via a data transmission line.

[0060] S2. Set the local MSK demodulation parameters according to the modulation information of the target monitored VLF station signal.

[0061] Among them, the set local MSK demodulation parameters include the carrier frequency f of the target monitored VLF station c , the baud rate f of the baseband signal modulated by MSK b , as well as the data sampling rate and ADC bit number of the local receiving device, etc.

[0062] Specifically, first, determine the modulation information of the target monitored VLF station signal. The present invention designs an information extraction method for MSK modulation signals. The MSK (Minimum Shift Keying) modulation signal S MSK (t) can be written as:

[0063]

[0064] In the formula, A(t) is the amplitude information, ω c is the carrier angular frequency, a k = ±1 is the symbol information, T b is the symbol duration, or π is the phase constant within one symbol period in MSK modulation.

[0065] In practice, the received MSK modulation signal S ′ MSK (t) transmitted by the target monitored VLF station often shows the following form:

[0066]

[0067] In the formula, A ′ (t) is the amplitude information of the received signal, Δω is the carrier frequency offset caused by the propagation path or hardware device, is the phase offset of the station signal.

[0068] The observer needs to extract A ′ (t) and from the received signal, and these two are of great significance for the study of the lower ionosphere. Currently, the methods for extracting A ′ (t) and rarely consider the influence of the carrier frequency offset Δω, but for For the extraction of [[ID=]], the existence of Δω will cause great interference. Based on the carrier synchronization technology of the phase-locked loop, the present invention eliminates the carrier frequency offset Δω, thereby being able to improve the accuracy.

[0069] After determining the carrier frequency and baud rate of the target monitoring very low frequency (VLF) station signal, it is also necessary to determine the sampling rate and the number of bits of the analog-to-digital converter (ADC) used by the observation device. Although the frequency range of VLF waves is 3 kHz to 30 kHz, the station signals based on MSK modulation around the world cover a wider frequency range. Therefore, to meet the Nyquist sampling theorem, the sampling rate is generally above 100 kHz. The number of bits of the ADC will affect the accuracy of the collected signal, but an excessively high number of bits sometimes cannot effectively improve the accuracy of information extraction, but instead increases the pressure of data processing and storage. Generally, a 16-bit ADC can meet the observation requirements.

[0070] S3. Combining the local MSK demodulation parameters set in S2, using the sampling information obtained in S1 as the input of the phase-locked loop, and using the phase-locked loop technology to perform carrier synchronization on the upper frequency and the lower frequency of the MSK modulation signal transmitted by the target monitoring VLF station respectively.

[0071] Specifically, referring to Figure 2 , a Costas phase-locked loop is used for carrier synchronization. The Costas phase-locked loop includes a mixer, a low-pass filter, a phase detector, a loop filter, and a voltage-controlled oscillator (VCO). Among them, the function of the mixer is to multiply the output signal of the voltage-controlled oscillator by the input signal, and move the frequency band range of the input signal to the vicinity of the baseband through spectral translation. The cut-off frequency of the low-pass filter is related to the MSK modulation parameters of the VLF station, and is generally half of the baud rate of the baseband signal. The phase detector uses the baseband signals of the in-phase branch and the quadrature branch to obtain the phase error. After passing through the phase detector, this phase error enters the loop filter to generate a control signal for the voltage-controlled oscillator.

[0072] Among them, referring to Figure 3 , the design of the loop filter adopts a first-order digital circuit architecture and is composed of two branches. The design parameters of the two branches are determined by the following formula:

[0073]

[0074] In the formula, C1 is the parameter of the 0th-order branch, C2 is the parameter of the 1st-order branch (containing an accumulator), K0 is the voltage-controlled oscillator gain, K d is the phase detector gain, ξ is the damping coefficient, ω n is the natural frequency, and T is the time period of the phase-locked loop.

[0075] After passing through the loop filter, the voltage-controlled oscillator changes the frequency of its output sine wave according to this control signal. After a period of time, the frequency of the output signal of the voltage-controlled oscillator of the phase-locked loop will be the same as the frequency of the input signal of the phase-locked loop, and the phase difference between the input signal and the output signal will remain stable, approximately equal to zero.

[0076] Among them, the initial frequency f h of the voltage-controlled oscillator of the upper frequency phase-locked loop and the initial frequency f l of the voltage-controlled oscillator of the lower frequency phase-locked loop are determined by the following formula:

[0077]

[0078] Among them, f c is the carrier frequency of the target monitoring very low frequency station signal, and f b is the baud rate of the baseband signal modulated by MSK.

[0079] For different very low frequency stations, f c is often different, and f b is mostly 200 Hz.

[0080] S4. After synchronization is achieved using the phase-locked loop, baseband synchronization signals and carrier synchronization signals are respectively extracted from the phase-locked loops of the upper and lower frequencies.

[0081] Among them, the extracted baseband synchronization signals include: the in-phase baseband synchronization signal and the quadrature baseband synchronization signal of the upper frequency, and the in-phase baseband synchronization signal and the quadrature baseband synchronization signal of the lower frequency.

[0082] That is, the baseband synchronization signals extracted by the Costas loop include: the baseband synchronization signal of the in-phase branch and the baseband synchronization signal of the quadrature branch. Also, due to the upper and lower frequencies, there are a total of four baseband synchronization signals: the in-phase baseband synchronization signal of the upper frequency, the quadrature baseband synchronization signal of the upper frequency, the in-phase baseband synchronization signal of the lower frequency, and the quadrature baseband synchronization signal of the lower frequency.

[0083] After the Costas loop is locked, the sine signal output by the voltage-controlled oscillator will be synchronized with the carriers of the upper and lower frequencies of the input MSK signal. Therefore, the carrier synchronization signals of the upper and lower frequencies can be extracted.

[0084] S5. Using the baseband synchronization signals obtained in S4, the amplitude information of the upper and lower frequencies is respectively calculated.

[0085] Specifically, referring to Figure 2 , the obtained in-phase baseband synchronization signal and quadrature baseband synchronization signal are respectively squared and then added to obtain the squared amplitude value. After taking the square root of this squared value, the amplitude information of the upper and lower frequencies is obtained.

[0086] The amplitude information of the upper frequency and the amplitude information of the lower frequency are expressed as follows:

[0087]

[0088] Wherein, A h (t) and A l (t) are the amplitude information of the upper frequency and the amplitude information of the lower frequency respectively, and θ h (t) and θ l (t) are the phase of the baseband synchronization signal of the upper frequency and the phase of the baseband synchronization signal of the lower frequency respectively.

[0089] The baseband synchronization signal of the upper frequency includes the baseband in-phase synchronization signal of the upper frequency and the baseband quadrature synchronization signal of the upper frequency, and the baseband synchronization signal of the lower frequency includes the baseband in-phase synchronization signal of the lower frequency and the baseband quadrature synchronization signal of the lower frequency.

[0090] S6. Combine the amplitude information of the upper and lower frequencies obtained in S5 to obtain the amplitude information of the target monitoring VLF station.

[0091] Specifically, after averaging the amplitude information of the upper frequency and the lower frequency, the amplitude information of the signal of this station is obtained. That is, the amplitude information of the target monitoring VLF station is expressed as follows:

[0092]

[0093] Wherein, A ′ (t) is the amplitude information of the target monitoring VLF station.

[0094] S7. Utilize the carrier synchronization signals of the upper and lower frequencies obtained in S4 to extract the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring VLF station.

[0095] Specifically, the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring VLF station is expressed as follows:

[0096]

[0097] Wherein, f c ′ is the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring VLF station, f h ′ is the frequency of the carrier synchronization signal extracted from the upper frequency, and f l ′ is the frequency of the carrier synchronization signal extracted from the lower frequency.

[0098] S8. Refer to Figure 4, based on the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station obtained from S7, the phase offset information of the target monitoring very low frequency station is extracted.

[0099] Specifically, the frequency f of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station c ′ is used as the frequency of the local mixing signal. The local mixing signal is multiplied by the received MSK modulation signal transmitted by the target monitoring very low frequency station, and after passing through a low-pass filter, a baseband signal is obtained:

[0100]

[0101] In the formula, S ′ baseband (t) is the baseband signal obtained after passing through the low-pass filter, LPF represents low-pass filtering, and S ′ MSK (t) is the received MSK modulation signal transmitted by the target monitoring very low frequency station, and f c ′ is the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring very low frequency station.

[0102] The amplitude information A of the target monitoring very low frequency station obtained by dividing the baseband signal obtained after passing through the low-pass filter by S6 ′ (t) is used to obtain the baseband phase information with a constant amplitude of unit amplitude; this baseband phase information is respectively multiplied by and and then integrated with respect to time respectively. The integration step is 1 second, and the integration results are respectively denoted as S + (t) and S - (t):

[0103]

[0104] Taking the angles of S + (t) and S - (t) respectively and denoting them as θ + (t) and θ - (t), adding them and then dividing by 2, the phase offset information with a time resolution of 1 second is obtained which is expressed as follows:

[0105]

[0106] In the formula, is the phase offset information of the target monitoring very low frequency station, θ + (t) is the first angle, θ - (t) is the second angle, ∠S +(t) represents S + (t) takes an angle, ∠S - (t) represents S - (t) takes an angle.

[0107] The present invention will be further illustrated by way of examples in combination with parameters.

[0108] In S1, two orthogonal magnetic loop antennas are used to receive very low frequency (VLF) fluctuation signals. According to Faraday's law of electromagnetic induction, the change in magnetic field strength is expressed in the form of current. After amplifying this weak current signal using an amplifier, the current signal is sampled by a 16-bit ADC to obtain sampling information, which is then uploaded to a computer for storage via a data transmission line.

[0109] In S2, the NWC VLF station is used as the target monitoring station. The carrier frequency of MSK demodulation is set to 19.8 kHz, and the baud rate is set to 200 Hz. In this embodiment, the sampling rate of the data acquisition system is 250 kHz, with a certain degree of oversampling; the ADC has 16 bits.

[0110] In S3, the cut-off frequency of the low-pass filter is related to the MSK modulation parameters of the VLF station, and is set here to 100 Hz. f c is 19.8 kHz, f b is 200 Hz, and calculating gives f h = 19.85 kHz, f l = 19.75 kHz.

[0111] In S4, referring to Figure 2 , for the in-phase branch and the quadrature branch of the Costas phase-locked loop, the difference lies in the phase of the output signal of the voltage-controlled oscillator. For the in-phase branch, the output signal of the voltage-controlled oscillator does not pass through a phase shifter; for the quadrature branch, the output signal of the voltage-controlled oscillator passes through a 90° phase shifter. It should be noted that after the input signal passes through the phase-locked loop, the output signal of the voltage-controlled oscillator will have a 90° phase difference from the input signal. Therefore, the output of the voltage-controlled oscillator passes through a phase shifter before being in phase with the input signal. Baseband in-phase synchronization signals and baseband quadrature synchronization signals are extracted at the output ends of the low-pass filters in the in-phase branch and the quadrature branch. Also, due to the upper frequency and the lower frequency, at this time, there are a total of four baseband synchronization signals output: the baseband in-phase synchronization signal of the upper frequency, the baseband quadrature synchronization signal of the upper frequency, the baseband in-phase synchronization signal of the lower frequency, and the baseband quadrature synchronization signal of the lower frequency. A carrier synchronization signal is extracted at the in-phase output end of the voltage-controlled oscillator.

[0112] In S5, using the baseband synchronization signals obtained in S4, the amplitude information of the upper and lower frequencies is calculated respectively.

[0113] S6. Combine the amplitude information of the upper and lower frequencies obtained in S5 to obtain the amplitude information of the target monitored VLF station.

[0114] S7. Use the carrier synchronization signals of the upper and lower frequencies obtained in S4 to extract the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station.

[0115] S8. Based on the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station obtained in S7, extract the phase offset information of the target monitored VLF station.

[0116] In this embodiment, an MSK modulation signal transmitted by the NWC VLF station is simulated and generated, with a carrier frequency of 19.8 kHz, a baud rate of 200 Hz, a frequency offset of 100 mHz, and a phase offset of 45°. This simulated signal is used as Figure 2 the input of the phase-locked loop in. After the phase-locked loop is locked, the baseband in-phase synchronization signal, baseband quadrature synchronization signal, and carrier synchronization signal of the upper and lower frequencies are extracted. By processing the baseband synchronization signals of the upper and lower frequencies, the amplitude information of the station signal can be obtained. As Figure 5 shown, the "-" line is the amplitude A(t) = 1 of the input MSK signal as the standard value, and the "-." line is the amplitude A ′ (t) of the station signal obtained by using the method of the present invention. Compared with the standard value, the error of the station amplitude information extracted by using the method of the present invention is within 0.01%. According to Figure 4 the steps shown, use the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitored VLF station as the frequency of the local mixing signal, mix the input MSK signal with the local mixing signal and perform low-pass filtering to obtain the baseband signal; multiply this baseband signal by and respectively, and then integrate at a step of 1 second; after combining the results of the two channels, finally obtain the phase offset information with a time resolution of 1 second. The phase offset information finally obtained in this embodiment is as Figure 6 shown, the "-" line is the phase offset of the input MSK signal, that is, the standard value; the "." line is the phase offset information obtained by using the method of the present invention, and the error is within 0.3°. As a comparison, Figure 7 is the phase offset information obtained without eliminating the frequency offset. The "-" line is the phase offset of the input MSK signal, that is, the standard value; the "-." line is the phase offset information obtained without using the method of the present invention, showing a linear relationship with time, that is, there is interference of the Δωt term, and the required

[0117] In addition, to ensure that the phase-locked loop can be precisely synchronized and to minimize the output frequency fluctuation range of the voltage-controlled oscillator, the design parameters of the loop filter can be further optimized. The optimization design can effectively reduce the frequency fluctuation of the system and improve the synchronization accuracy of the signal. However, changing the design parameters of the filter will increase the time required for the phase-locked loop to reach the synchronized state. Therefore, in practical applications, it is necessary to balance the design requirements: how to achieve synchronization in a short time while ensuring accuracy to meet the needs of real-time signal processing.

[0118] In summary, Embodiment 1 demonstrates a very low frequency station signal extraction method based on a phase-locked loop. Through precise synchronization and signal processing, it can effectively extract phase offset information from complex very low frequency signals. The method provided by the present invention has significant advantages in extracting weak signals, can significantly improve the signal-to-noise ratio of the signal, enhance the synchronization accuracy, and can provide reliable data support for fields such as geophysical monitoring, ionospheric sounding, and maritime communication.

[0119] Embodiment 2:

[0120] Embodiment 2 provides a very low frequency station signal information extraction system based on a phase-locked loop, including:

[0121] A reception processing module that amplifies and samples a broadband signal containing the very low frequency station transmission signal to obtain sampling information;

[0122] A demodulation setting module for setting local MSK demodulation parameters according to the modulation information of the target monitored very low frequency station signal;

[0123] A carrier synchronization module that, in combination with the local MSK demodulation parameters, uses the sampling information as the input of the phase-locked loop and uses phase-locked loop technology to perform carrier synchronization on the upper frequency and the lower frequency of the MSK modulated signal transmitted by the target monitored very low frequency station respectively;

[0124] A synchronization signal extraction module that, after achieving synchronization using the phase-locked loop, extracts baseband synchronization signals and carrier synchronization signals from the upper and lower frequency phase-locked loops respectively;

[0125] An amplitude information calculation module that uses the obtained baseband synchronization signals to calculate the amplitude information of the upper and lower frequencies respectively, and combines the amplitude information of the upper and lower frequencies to obtain the amplitude information of the target monitored very low frequency station;

[0126] A phase offset calculation module that extracts the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitored very low frequency station using the carrier synchronization signals of the upper and lower frequencies, and extracts the phase offset information of the target monitored very low frequency station based on the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitored very low frequency station;

[0127] The very low frequency station signal information extraction system based on a phase-locked loop is used to execute the steps in the very low frequency station signal information extraction method described in Embodiment 1.

[0128] Since the functions of the modules in the system provided in Embodiment 2 correspond to the steps in the method provided in Embodiment 1, therefore, reference can be made to the description in Embodiment 1 for understanding, and details will not be repeated here.

[0129] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and all of them should be covered by the scope of the claims of the present invention.

Claims

1. A method for extracting very low frequency station signal information based on a phase-locked loop, characterized in that: The following steps are involved: S1, amplifying and sampling the broadband signal including the very low frequency station transmission signal to obtain sampling information; S2, setting local MSK demodulation parameters according to the modulation information of the target monitoring VLF station signal; S3, combining the local MSK demodulation parameters, using the sampling information as an input of a phase-locked loop, and using a phase-locked loop technology to perform carrier synchronization on the upper frequency and the lower frequency of the MSK modulation signal transmitted by the target monitoring very low frequency station; S4, after synchronization is achieved using a phase-locked loop, a baseband synchronization signal and a carrier synchronization signal are extracted from the phase-locked loops of the upper and lower frequencies respectively; S5, using the baseband synchronization signal obtained in S4, respectively calculates the amplitude information of the upper and lower frequencies; S6, combining the amplitude information of the upper and lower frequencies obtained in S5 to obtain the amplitude information of the target monitored VLF station; S7, using the carrier synchronization signals of the upper and lower frequencies obtained in S4, extracts the frequency of the carrier synchronization signal of the MSK modulation signal transmitted by the target monitoring VLF station; S8, based on the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring very low frequency station obtained in S7, extract the phase offset information of the target monitoring very low frequency station.

2. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 1, characterized in that: In S2, the local MSK demodulation parameters set include the carrier frequency f of the target monitoring VLF station. c , MSK modulated baseband signal baud rate f b , as well as the data sampling rate and ADC bit number of the local receiving device.

3. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 2, characterized in that: In S3, a Costas phase-locked loop is used for carrier synchronization; the Costas phase-locked loop includes a mixer, a low-pass filter, a phase detector, a loop filter and a voltage-controlled oscillator.

4. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 3, characterized in that: The initial frequency f of the voltage controlled oscillator h , the initial frequency f of the voltage controlled oscillator at the lower frequency l Determined by the following formula: Among them, f c is the carrier frequency of the target monitoring VLF station signal, f b is the baud rate of the MSK modulated baseband signal.

5. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 1, characterized in that: In S4, the extracted baseband synchronization signal includes: an upper frequency baseband in-phase synchronization signal and a lower frequency baseband in-phase synchronization signal, and an upper frequency baseband orthogonal synchronization signal and a lower frequency baseband orthogonal synchronization signal.

6. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 5, characterized in that: In S5, the amplitude information of the upper frequency and the amplitude information of the lower frequency are expressed as follows: In the formula, A h (t) and A l (t) are the amplitude information of the upper frequency and the amplitude information of the lower frequency, θ h (t) and θ l (t) are the phase of the baseband synchronization signal of the upper frequency and the phase of the baseband synchronization signal of the lower frequency respectively; The upper frequency baseband synchronization signal includes the upper frequency baseband in-phase synchronization signal and the upper frequency baseband orthogonal synchronization signal, and the lower frequency baseband synchronization signal includes the lower frequency baseband in-phase synchronization signal and the lower frequency baseband orthogonal synchronization signal.

7. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 6, characterized in that: In S6, the amplitude information of the target monitoring VLF station is expressed as follows: In the formula, A ′ (t) is the amplitude information of the target monitoring VLF station.

8. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 1, characterized in that: In S7, the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring very low frequency station is expressed as follows: In the formula, f c ′ is the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring VLF station, f h ′ is the frequency of the carrier synchronization signal extracted from the upper frequency, f1 ′ is the frequency of the carrier synchronization signal extracted from the lower frequency.

9. The method for extracting very low frequency station signal information based on a phase-locked loop according to claim 3, characterized in that: In S8, the frequency f of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring VLF station is c ′ As the frequency of the local mixing signal, the local mixing signal is multiplied by the MSK modulation signal transmitted by the received target monitoring VLF station, and the baseband signal is obtained after passing through a low-pass filter: In the formula, S ′ baseband (t) is the baseband signal obtained after passing through a low-pass filter. LPF stands for low-pass filtering. S ′ MSK (t) is the received MSK modulated signal transmitted by the target monitoring VLF station, f c ′ It is the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring VLF station; The baseband signal obtained after passing through the low-pass filter is divided by S6 to obtain the amplitude information A of the target monitoring VLF station ′ (t), and obtain the baseband phase information with a constant amplitude of unit amplitude; the baseband phase information is respectively and Multiply them, and then integrate them over time, with an integration step of 1 second. The integration results are recorded as S + (t) and S - (t): For S + (t) and S - (t) respectively take the angle as θ + (t) and θ - (t), add them and divide by 2 to get the phase shift information with a time resolution of 1 second It is expressed as follows: In the formula, is the phase offset information of the target monitoring VLF station, θ + (t) is the first angle, θ - (t) is the second angle, ∠S + (t) represents the + (t) Take the angle, ∠S - (t) represents the - (t) Take the angle.

10. A very low frequency station signal information extraction system based on a phase-locked loop, characterized in that: include: A receiving and processing module is used to amplify and sample the broadband signal including the very low frequency station transmission signal to obtain sampling information; The demodulation setting module is used to set the local MSK demodulation parameters according to the modulation information of the target monitoring VLF station signal; A carrier synchronization module, for combining the local MSK demodulation parameters, using the sampling information as an input of a phase-locked loop, and using a phase-locked loop technology to perform carrier synchronization on the upper frequency and the lower frequency of the MSK modulation signal transmitted by the target monitoring very low frequency station; A synchronization signal extraction module is used to extract a baseband synchronization signal and a carrier synchronization signal from the upper and lower frequency phase-locked loops respectively after synchronization is achieved using a phase-locked loop; An amplitude information calculation module is used to use the obtained baseband synchronization signal to calculate the amplitude information of the upper and lower frequencies respectively, and to combine the amplitude information of the upper and lower frequencies to obtain the amplitude information of the target monitoring very low frequency station; a phase offset calculation module, for extracting the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring very low frequency station using the carrier synchronization signals of the upper and lower frequencies, and for extracting the phase offset information of the target monitoring very low frequency station based on the frequency of the carrier synchronization signal of the MSK modulated signal transmitted by the target monitoring very low frequency station; The phase-locked loop-based very low frequency station signal information extraction system is used to execute the steps in the phase-locked loop-based very low frequency station signal information extraction method according to any one of claims 1-9.

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