A method and apparatus for recovering the frequency of a down-converted signal based on modulation characteristics

Abstract

The application provides a frequency recovery method and device of a down-converted signal based on modulation characteristics, relates to the technical field of signal carrier frequency recovery, and adopts two different frequency femtosecond pulse light sources to perform multiple optical down-conversion and obtain one two-channel down-converted signal data. After a complex signal is obtained through Hilbert transform, autocorrelation and cross-correlation products are calculated, the autocorrelation and cross-correlation products are shifted in the frequency domain, and a cross-correlation coefficient is calculated to analyze correlation, so as to determine a frequency combination type. Based on the determined frequency combination type, a remainder matching method is used to recover original carrier frequency information. The method eliminates frequency ambiguity when a frequency is measured by using double femtosecond pulse light sources with different frequencies, improves the frequency measurement accuracy when a frequency is measured by using double light sources, effectively reduces the complexity of the system, and is more easily realized in engineering. Compared with a traditional method, the frequency measurement accuracy of the method is 100% for signals with modulation characteristics.

Description

Technical Field

[0001] This invention relates to the field of signal carrier frequency recovery technology, and more specifically to a method and apparatus for recovering the frequency of down-converted signals in an electromagnetic spectrum sensing system using optical down-conversion technology. Background Technology

[0002] Optical downconversion receiving technology using femtosecond pulse light sources can quickly acquire all electromagnetic signals within a wide frequency band. It has advantages such as wide instantaneous bandwidth, fast reception speed, and no signal loss, and has excellent application prospects in rapid detection of electromagnetic environments, and has received much attention in recent years.

[0003] As is well known, carrier frequency is a crucial characteristic parameter of electromagnetic signals. However, when receiving signals using optical down-conversion technology, the carrier frequency information is lost. For example, a linear frequency modulated signal with a carrier frequency of 3.6 GHz is down-converted by a femtosecond pulse laser with a repetition rate of 206.042 MHz. Although the down-converted signal contains all the modulation information of the original signal, its frequency becomes 97.286 MHz. To recover the carrier frequency information of the optical down-converted signal, researchers have proposed using multiple femtosecond pulse light sources with different repetition rates to simultaneously measure the electromagnetic signal and recover the carrier frequency through remainder matching.

[0004] Existing methods for recovering the carrier frequency of downconverted signals achieve 100% accuracy when using three femtosecond pulse light sources with different repetition rates. However, the system with three femtosecond pulse light sources is complex; using two sets can effectively reduce system complexity and save system costs. When using two femtosecond pulse light sources with different repetition rates, frequency ambiguity exists due to the inability to construct long and short baselines, resulting in a relatively low accuracy of 95% for carrier frequency recovery. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for frequency recovery of down-converted signals based on modulation characteristics. By utilizing the modulation characteristics of the down-converted signal, frequency ambiguity is eliminated when using dual light sources with different repetition rates for frequency measurement. Frequency ambiguity leads to multiple solutions and reduces accuracy. Eliminating frequency ambiguity can improve the accuracy of frequency measurement using dual light sources with different repetition rates.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for frequency recovery of down-conversion signals based on modulation characteristics includes the following steps:

[0008] Step S1: Use femtosecond pulse light sources with repetition rates of F1 and F2 to optically downconvert the electromagnetic signal to obtain one channel of downconverted signal data and two channels of downconverted signal data;

[0009] Step S2: Perform Hilbert transform on the down-converted signal data of one channel and the down-converted signal data of two channels to obtain the complex signal X1(t) of one channel and the complex signal X2(t) of two channels respectively;

[0010] Step S3: Calculate the self-product A1 and cross-product A2 of the dual-channel complex signals, where the self-product A1 is the product of one channel complex signal and itself:

[0011] A1 = X1(t)·X1(t)

[0012] The cross product A2 is the product of a one-channel complex signal and a two-channel complex signal:

[0013] A2 = X1(t)·X2(t);

[0014] Step S4: After shifting the self-product A1 and the cross-product A2 to the same frequency using the spectrum shifting method, calculate the correlation between A1 and A2 after shifting to the same frequency using cross-correlation calculation.

[0015] Step S5: Determine the correct frequency combination type based on the correlation between the self-product A1 and the cross-product A2;

[0016] Step S6: Based on the correct frequency combination type, use the remainder matching method to recover the original carrier frequency of the down-converted signal.

[0017] Furthermore: In step S2:

[0018]

[0019]

[0020] Where M1 and M2 represent the amplitude of the down-converted signal; f1 and f2 represent the carrier frequencies of the signals down-converted to the first half of the first repetition frequency interval by different repetition frequency light sources; and This represents the modulation information of the signal that has been downconverted to the first half of the first frequency range by different repetition frequency light sources.

[0021] Further: Step S4 specifically includes:

[0022] Step S401: Calculate the carrier frequencies of A1 and A2, and obtain the carrier frequency difference diff_f between them;

[0023] Step S402: Shift A2 according to the carrier frequency difference diff_f between A1 and A2 to obtain A2. 2搬移 :

[0024]

[0025] Step S403: Calculate the cross-correlation coefficient r between A1 and A2:

[0026] r = xcorr(A1, A2) 搬移 ).

[0027] Further: Step S5 specifically includes: if the self-product A1 and the cross-product A2 are strongly correlated, then the correct frequency combination type is [+,+] or [-,-]; if the self-product A1 and the cross-product A2 are weakly correlated, then the correct frequency combination type is [+,-] or [-,+].

[0028] Where [+,+] is: f1+n1·F1=f2+n2·F2;

[0029] [-,-] is: n3·F1-f1=n4·F2-f2;

[0030] [+,-] is: f1+n1·F1=n4·F2-f2;

[0031] [-,+] is: n3·F1-f1=f2+n2·F2;

[0032] n1, n2, n3, and n4 represent the number of downconversions, and f1 and f2 represent the carrier frequencies of signals downconverted to the first half of the first frequency range by different repetition frequency light sources.

[0033] Further: In step S5, if the correlation coefficient between the self-product A1 and the cross-product A2 is greater than a predetermined value, then the self-product A1 and the cross-product A2 are determined to be strongly correlated; otherwise, the self-product A1 and the cross-product A2 are determined to be weakly correlated.

[0034] Furthermore: the predetermined value is 0.6.

[0035] The present invention also provides a frequency recovery device for downconverted signals based on modulation characteristics, including an optical downconverter system for acquiring one channel of downconverted signal data and two channels of downconverted signal data;

[0036] Signal processing module: Used to process one-channel down-converted signal data and two-channel down-converted signal data, and output the correct frequency combination type;

[0037] Frequency recovery module: Used to recover the original carrier frequency of the downconverted signal using the remainder matching method based on the correct frequency combination type.

[0038] Furthermore, the optical downconversion module includes an optical electric field probe, a photodetector, and a photoelectric converter that are connected in sequence.

[0039] Compared with the original technology, the present invention has the following beneficial effects:

[0040] This invention utilizes the modulation characteristics of optical down-conversion signals and constructs a frequency combination screening method based on these modulation characteristics to eliminate frequency ambiguity when using dual femtosecond pulse light sources with different repetition rates for frequency measurement. Frequency ambiguity leads to multiple solutions, resulting in reduced accuracy. Therefore, eliminating frequency ambiguity can improve the accuracy of frequency measurement using dual light sources with different repetition rates. Compared with traditional methods, this method achieves 100% frequency measurement accuracy for signals with modulation characteristics. Attached Figure Description

[0041] Figure 1 This is a flowchart of a downconversion signal frequency recovery method based on modulation characteristics according to the present invention.

[0042] Figure 2 This is a structural diagram of the dual-channel optical down-conversion system described in this invention;

[0043] Figure 3 This refers to the frequency measurement error of the downconversion signal frequency recovery method and apparatus based on modulation characteristics described in this invention. Detailed Implementation

[0044] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0046] This invention provides a method and apparatus for frequency recovery of downconverted signals based on modulation characteristics. By using the modulation characteristics of the downconverted signal, frequency ambiguity that exists when using dual light sources with different repetition frequencies for frequency measurement is eliminated, thereby improving the accuracy of frequency measurement.

[0047] For carrier frequency f r The electromagnetic signal, down-converted by a femtosecond pulse laser with a repetition rate of F1 to the first half of the first repetition rate interval, can be expressed as:

[0048] f + =f r-m·F1 (1)

[0049] or f _ =m·F1-f r (2)

[0050] Where m is the number of down-conversion cycles; f + Indicates that the signal is from +f r This is due to downconversion; in this article, this situation is denoted as "+"; f - Indicates that the signal is generated by -f r This is due to downconversion; in this paper, this situation is denoted as "-", and f + with f - Satisfy: f + =F1-f - .

[0051] Since the number of down-conversion cycles is unknown, an optical down-conversion system using a single light source cannot recover the signal carrier frequency f. r Therefore, it is necessary to recover the signal frequency using multiple light sources with different repetition rates.

[0052] When using dual light sources to recover signal frequency, remainder matching is typically performed based on the frequency of the optical downconverted signal in the first half of the first repetition frequency interval. For the frequency combination of dual-channel downconverted signals, there are the following four cases:

[0053]

[0054] In equation (3) above, f1 and f2 are the frequencies of the down-converted signal in the first half of the first repetition frequency interval of the signal x(t); f r F1 and F2 are the repetition frequencies of the femtosecond pulse source; n1, n2, n3, and n4 are the down-conversion times.

[0055] To obtain the true frequency of the signal, only one of the four conditions in formula (3) must be true. However, if two or more of the above four conditions are true simultaneously, f r Multiple values ​​will appear, resulting in frequency ambiguity.

[0056] In the 1MHz-12GHz range, the main sources of frequency ambiguity are: There are four scenarios where there is no ambiguity between [+,+] and [-,-], or between [+,-] and [-,+].

[0057] For a carrier frequency of f in space r The signal x(t) can be expressed as:

[0058]

[0059] In equation (4) above, M and f r and These represent the amplitude, carrier frequency, and modulation information of the signal x(t), respectively.

[0060] After downconversion using femtosecond pulse light sources with repetition rates of F1 and F2, the dual-channel optical downconversion signals x1(t) and x2(t) can be expressed as:

[0061]

[0062]

[0063] In equations (5)-(6) above, M1 and M2 represent the amplitude of the down-converted signal; f1 and f2 represent the carrier frequencies of the signals down-converted to the first half of the first repetition frequency interval by different repetition frequency light sources. and This represents the modulation information of the signal that has been downconverted to the first half of the first frequency range by different repetition frequency light sources.

[0064] For the optical down-conversion signal x1(t) in the first half of the first repetition frequency interval, if the signal is converted from +f r From downconversion, then If the signal is -f r From downconversion, then

[0065] [+,+] indicates that both the down-conversion signals x1(t) and x2(t) originate from +f r And there are [+,-] indicates that the down-conversion signals x1(t) and x2(t) come from +f r with -f r And there are

[0066] For an optical downconversion signal with a frequency combination of [+,+], we have:

[0067]

[0068] For optical downconversion signals with [+,-] frequency combinations, we have:

[0069]

[0070] For the optical down-conversion signal x1(t), we have:

[0071]

[0072] According to formula (7-9), for dual-channel optical down-conversion signals x1(t) and x2(t), the following conclusions can be drawn:

[0073] (1) If x1(t)·x2(t) and x1(t)·x1(t) are strongly correlated, then their modulation information is considered to be the same, and [f1,f2] is the correct frequency combination as [+,+] or [-,-].

[0074] (2) If x1(t)·x2(t) and x1(t)·x1(t) are weakly correlated, then the modulation information is considered to be different, and [f1,f2] is the correct frequency combination as [+,-] or [-,+].

[0075] Therefore, the correct [f1,f2] frequency combination type can be selected based on the modulation characteristics of x1(t)·x2(t) to eliminate the first type of frequency ambiguity.

[0076] Example 1

[0077] like Figure 1 The flowchart shown is a method for frequency recovery of down-conversion signals based on modulation characteristics provided by the present invention, which includes the following steps:

[0078] Step S1: Use femtosecond pulse light sources with repetition rates of F1 and F2 to optically downconvert the electromagnetic signal. The computer acquires downconverted signal data from one channel and downconverted signal data from two channels.

[0079] Step S2: Perform Hilbert transform on the down-converted signal data of one channel and the down-converted signal data of two channels to obtain the complex signal X1(t) of one channel and the complex signal X2(t) of two channels, respectively.

[0080] Step S3: Calculate the self-product A1 and cross-product A2 of the dual-channel complex signal, where the self-product A1 is the product of one channel complex signal and itself.

[0081] A1 = X1(t)·X1(t)

[0082] The cross product A2 is the product of a one-channel complex signal and a two-channel complex signal:

[0083] A2 = X1(t)·X2(t);

[0084] Step S4: After shifting the self-product A1 and the cross-product A2 to the same frequency using the spectrum shifting method, the correlation between A1 and A2 after shifting to the same frequency is calculated by cross-correlation operation.

[0085] In some embodiments:

[0086] Step S4 specifically includes:

[0087] Step S401: Calculate the carrier frequencies of A1 and A2, and obtain the carrier frequency difference diff_f between them;

[0088] Step S402: Shift A2 according to the carrier frequency difference diff_f between A1 and A2 to obtain A2. 2搬移 :

[0089]

[0090] Step S403: Calculate the cross-correlation coefficient r between A1 and A2:

[0091] r = xcorr(A1, A2) 搬移 ).

[0092] Step S5: Determine the correct frequency combination type based on the correlation between the self-product A1 and the cross-product A2.

[0093] In some embodiments, in step S5, if the correlation coefficient between the self-product A1 and the cross-product A2 is greater than a predetermined value, then the self-product A1 and the cross-product A2 are determined to be strongly correlated; otherwise, the self-product A1 and the cross-product A2 are determined to be weakly correlated.

[0094] If the self-product A1 and the cross-product A2 are strongly correlated, the correct frequency combination type is [+,+] or [-,-]; if the self-product A1 and the cross-product A2 are weakly correlated, the correct frequency combination type is [+,-] or [-,+].

[0095] Where [+,+] is: f1+n1·F1=f2+n2·F2;

[0096] [-,-] is: n3·F1-f1=n4·F2-f2;

[0097] [+,-] is: f1+n1·F1=n4·F2-f2;

[0098] [-,+] is: n3·F1-f1=f2+n2·F2;

[0099] n1, n2, n3, and n4 represent the number of downconversions, and f1 and f2 represent the carrier frequencies of signals downconverted to the first half of the first frequency range by different repetition frequency light sources.

[0100] In this embodiment, the predetermined value is 0.6.

[0101] Step S6: Based on the correct frequency combination type, the original carrier frequency of the down-converted signal is recovered using the remainder matching method.

[0102] For details of step S6, please refer to the optical undersampling frequency recovery method based on remainder matching in patent number CN202110294565.5.

[0103] Example 2

[0104] The present invention also provides a frequency recovery device for downconverted signals based on modulation characteristics, comprising: an optical downconverted signal receiving module for acquiring one channel of downconverted signal data and two channels of downconverted signal data;

[0105] The signal processing module is used to process one-channel down-converted signal data and two-channel down-converted signal data, and output the correct frequency combination type.

[0106] The frequency recovery module is used to recover the original carrier frequency of the downconverted signal using the remainder matching method based on the correct frequency combination type.

[0107] The optical downconversion signal receiving module includes an optical electric field probe, a photodetector, and a photoelectric converter, which are connected in sequence. The optical downconversion system uses the optical electric field probe to modulate electromagnetic signals in space onto a femtosecond pulsed laser, then converts the optical signal into an electrical signal using the photodetector, and finally uses the photoelectric converter to receive the downconverted signal. The system connection is as follows: Figure 2 As shown.

[0108] Example 3

[0109] This invention also provides a simulation experiment on frequency recovery of down-conversion signals based on modulation characteristics:

[0110] Assume the detection bandwidth (BW) of the optical downconversion system is 1MHz to 18GHz. Within the 10GHz-18GHz range, 100,000 linear frequency modulated signals with different carrier frequencies are randomly generated, each with a bandwidth of 3MHz. Set the repetition rate of the femtosecond pulse source to [value missing].

[0111] Using the aforementioned light source repetition rate, 100,000 linear frequency modulated signals with different carrier frequencies were down-converted, generating 100,000 sets of optical down-converted signals. The method presented in this paper achieves 100% frequency recovery accuracy and effectively eliminates frequency ambiguity.

[0112] like Figure 3 The figure shows the statistical histogram of the frequency recovery error of the algorithm in this paper. The frequency recovery error of the algorithm conforms to a normal distribution.

[0113] Compared with traditional methods, the method in this paper eliminates frequency ambiguity by using modulation features, which can effectively eliminate frequency ambiguity in signals with modulation features and improve the accuracy of signal frequency recovery.

[0114] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent transformations or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for frequency recovery of down-conversion signals based on modulation characteristics, characterized in that: Includes the following steps: Step S1: Use repetition frequencies respectively and The femtosecond pulse light source performs optical down-conversion on the electromagnetic signal to acquire one channel of down-converted signal data and two channels of down-converted signal data. Step S2: Perform Hilbert transform on the down-converted signal data of channel one and channel two to obtain a complex signal of channel one. and two-channel complex signal ; Step S3: Calculate the self-product of the complex signals in the dual-channel configuration. and mutual product The self-product Multiplying a complex signal of one channel by itself: cross product Multiplying a one-channel complex signal and a two-channel complex signal: ； Step S4: Transform the self-product using a spectrum shifting method. and mutual product After relocating to the same frequency, the results are calculated using cross-correlation operations. and The correlation; Step S5: Based on the self-product and mutual product The correlation determines the correct frequency combination type; Step S6: Based on the correct frequency combination type, use the remainder matching method to recover the original carrier frequency of the down-converted signal; The frequency combination types include: for: ; for: - ; for: ; for: ; In the above formula, , , and This refers to the number of down-conversion cycles. and This indicates the carrier frequency of the signal that has been downconverted to the first half of the first frequency range by different repetition frequency light sources; Specifically, step S5 includes: If the product and mutual product If the correlation coefficient is greater than the set threshold P, then the correct frequency combination type is... ,or If the product is self-multiplied and mutual product If the correlation coefficient is less than or equal to the set threshold P, then the correct frequency combination type is: ,or .

2. The frequency recovery method for down-conversion signals based on modulation characteristics according to claim 1, characterized in that: In step S2: in, and Indicates the amplitude of the down-converted signal; and This indicates the carrier frequency of the signal that has been downconverted to the first half of the first frequency range by different repetition frequency light sources; and This represents the modulation information of the signal that has been downconverted to the first half of the first frequency range by different repetition frequency light sources.

3. The frequency recovery method for down-conversion signals based on modulation characteristics according to claim 1, characterized in that: Step S4 specifically includes: Step S401: Calculate the carrier frequencies of A1 and A2, and obtain the carrier frequency difference diff_f between them; Step S402: Shift A2 according to the carrier frequency difference diff_f between A1 and A2 to obtain A2. 2搬移 : A 2搬移= A2 * have been -j2πdiff_f*t Step S403: Calculate the cross-correlation coefficient r between A1 and A2: r=xcorr(A1,A2) 搬移 )。 4. The frequency recovery method for down-conversion signals based on modulation characteristics according to claim 1, characterized in that: The threshold P is 0.

6.

5. An apparatus for applying any one of the modulation feature-based down-conversion signal frequency recovery methods as described in claims 1-4, characterized in that, include: Optical downconversion signal receiving module: used to acquire one channel downconversion signal data and two channels downconversion signal data; Signal processing module: Used to process one-channel down-converted signal data and two-channel down-converted signal data, and output the correct frequency combination type; Frequency recovery module: Used to recover the original carrier frequency of the downconverted signal using the remainder matching method based on the correct frequency combination type.

6. The frequency recovery device for down-conversion signals based on modulation characteristics according to claim 5, characterized in that: The optical downconversion module includes an optical electric field probe, a photodetector, and a photoelectric converter that are connected in sequence.

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