A chirp-random code phase modulation method for a detection device

Abstract

A composite modulation method of linear frequency modulation (LFM) and random code phase modulation for detection devices is disclosed, relating to the fields of radar detection and millimeter-wave signal modulation technology. This method is implemented based on a suitable LFM detection device. A BPSK phase modulation circuit is added between the transceiver components and the transmitting antenna. The LFM signal is synchronously superimposed with random binary phase codes for BPSK phase modulation within each frequency sweep cycle. The complete sequence period of the random binary phase codes is completely consistent with a single frequency sweep cycle. For the sampled echo intermediate frequency signal, correlation processing of the local code and the echo code is performed first, followed by sequential FFT spectrum analysis in the range and velocity dimensions. Asynchronous clutter and interference are processed to exhibit noise characteristics. Target detection and parameter calculation are performed only on the synchronous echo. This invention fundamentally eliminates the range ambiguity problem of the LFM system, significantly improving the anti-interference capability and operational reliability of the detection device in low-altitude scenarios, while retaining the advantages of the LFM system, such as high ranging accuracy, low cost, and ease of engineering.

Description

Technical Field

[0001] This invention relates to the field of radar detection and millimeter-wave signal modulation technology, and in particular to a method for linear frequency modulation and random code phase modulation composite modulation for a detection device. Background Technology

[0002] Linear frequency modulation (LFM) systems have advantages such as simple structure and high ranging accuracy, and are widely used in detection devices.

[0003] The detection device consists of an antenna, transceiver components, and a digital signal processor, such as... Figure 2 As shown in the diagram, the transceiver unit generates a millimeter-wave carrier signal. This carrier signal is linearly frequency modulated by a modulation signal and radiated into space through a transmitting antenna. The receiving antenna receives the echo signal. The difference frequency signal of the echo signal is mixed, filtered, amplified, and a specific bandwidth difference frequency signal is extracted and sent to the signal digital processor as an intermediate frequency (IF) signal. The signal digital processor digitally samples the IF signal and performs FFT spectrum analysis. After FFT, the ratio of the main spectrum to the noise floor is used to determine whether a target has been detected; on the other hand, the target's distance and Doppler frequency are identified through the main spectrum frequency.

[0004] The basic principle of linear frequency modulation (LFM) ranging is to identify the distance based on the frequency difference signal after the echo signal and the transmitted signal are mixed. To simplify the analysis, the Doppler effect is ignored. Assume the transmitted signal waveform is a series of periodically sawtooth-patterned linearly frequency-modulated signals of constant amplitude, with the frequency modulation envelope as shown... Figure 3 The solid line in the diagram illustrates this. Due to the time delay τ caused by the signal propagating back and forth between detection distances, the envelope of the echo signal's frequency modulation also deviates from a value of τ, as shown in the diagram. Figure 3 As shown by the dashed line, the transmitted signal ft and the echo signal fr are mixed in a mixer to obtain the difference frequency signal fi(ft-fr).

[0005] Based on the relationship between similar triangles, we can obtain: Get ……………………… (1) Time delay: ; Modulation period: ; Substituting into equation (1), we get: ……………………(2) In the formula: R is the detection distance, fm is the modulation frequency of the sawtooth wave signal, B is the modulation bandwidth, C is the speed of light, and fi is the echo difference frequency. Once fm and B are determined, the echo difference frequency fi is uniquely determined at a certain distance. Therefore, the detection distance can be measured using the measured echo difference frequency.

[0006] As can be seen from the principle of linear frequency modulation ranging, the target distance can be known as long as the frequency of the difference frequency signal is calculated, and the presence or absence of a target can be known by judging the strength and signal-to-noise ratio of the difference frequency signal. This scheme uses the FFT method to identify the frequency of the difference frequency. After taking the modulus of the FFT result, the power spectrum of each frequency component is obtained. The spectral line with the largest power is taken as the main spectrum. The average power of all spectral lines after removing the low-end N1 lines, the high-end N2 lines, the main spectrum, and the left and right N3 lines can be regarded as the base. The ratio of the main spectrum to the base is the signal-to-noise ratio. The signal-to-noise ratio is compared with the detection threshold. If it is greater than the detection threshold, it means that the target has been found; otherwise, the target has not been found. After the target is found, the frequency of the main spectrum is substituted into equation (2) to calculate the target distance.

[0007] In the above method, because the output of the matched filter of the linear frequency modulated (LFM) signal has a periodic peak in time delay, with a period equal to the pulse width T, targets at different distances but with a time delay difference of mT will appear at the same range gate after pulse compression. Theoretically, LFM continuous wave has a periodic ambiguity in range. Therefore, LFM continuous wave detection devices are insufficient in resisting ground / sea clutter and active interference beyond the ambiguity range, resulting in a high false alarm rate in complex electromagnetic environments and at low altitudes. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned defects of the prior art and provide a composite modulation method of linear frequency modulation and random code phase modulation for a detection device. Under the premise of ensuring the ranging and velocity measurement performance of the detection device and without significantly increasing the hardware cost, this method eliminates the distance ambiguity of the linear frequency modulation system, improves the anti-interference capability of the detection device, and reduces the false alarm rate in complex electromagnetic environments and low-altitude scenarios.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for composite modulation of linear frequency modulation (LFM) and random code phase modulation (RCM) for a detection device is disclosed. The method is implemented based on a LFM detection device, which includes a transceiver assembly, a BPSK phase modulation circuit, a transmitting antenna, a receiving antenna, a video amplifier, and a digital signal processor. The LFM signal output of the transceiver assembly is electrically connected to the transmitting antenna via the BPSK phase modulation circuit. The signal output of the receiving antenna is electrically connected to the echo receiving end of the transceiver assembly. The intermediate frequency (IF) signal output of the transceiver assembly is electrically connected to the input of the video amplifier. The output of the video amplifier is electrically connected to the digital signal processor. The phase modulation control signal output of the digital signal processor is electrically connected to the control end of the BPSK phase modulation circuit. The method is characterized by comprising the following steps: S1. A millimeter-wave carrier frequency signal is generated through the transceiver component, and the millimeter-wave carrier frequency signal is linearly frequency modulated to obtain a linearly frequency modulated signal. The transmitted signal is radiated into space through the transmitting antenna. S2. The echo signal reflected by the target is received through the receiving antenna, and the echo signal is transmitted to the transceiver component for mixing with the local oscillator signal to obtain the difference frequency signal; the difference frequency signal is filtered and amplified by the video amplifier to obtain the intermediate frequency signal and transmitted to the digital signal processor. S3. The intermediate frequency signal is digitally sampled by the digital signal processor, and the sampled signal is subjected to FFT spectrum analysis. Based on the spectrum analysis results, the target detection and target distance and velocity parameters are calculated. In step S1, for the carrier signal that has completed the linear frequency modulation (LFM) processing, in each LFM sweep cycle, the digital signal processor outputs a random binary phase code (BPSK) phase modulation control signal synchronized with the sweep cycle to the BPSK phase modulation circuit. The LFM signal is synchronously superimposed with the random binary phase code for BPSK phase modulation processing to obtain a transmission signal that is composite modulated by LFM and random code phase modulation. The complete sequence period of the random binary phase code is completely consistent with a single sweep cycle of the LFM, and the cycle period of the random binary phase code is greater than the continuous working time of the detection device. In step S3, the digitally sampled intermediate frequency signal is first processed by a digital signal processor to perform code correlation processing between the sampled echo signal and the local synchronization random code. The local synchronization random code is a local random binary phase code that is homologous to the random binary phase code of the transmitting end and matches the time delay of the echo signal. Then, FFT spectrum analysis is performed on the signal after code correlation processing to obtain the frequency parameters corresponding to the target distance. Secondary FFT analysis is performed on the same-distance gate signals with multiple coherent accumulation periods to obtain the Doppler frequency parameters corresponding to the target. When the local synchronization random code is uncorrelated with the received echo code, the signal after correlation processing is a random sequence, which exhibits noise characteristics after FFT analysis. When the local synchronization random code is correlated with the received echo code, the signal after correlation processing is a regular sequence containing the Doppler information of the target, which forms the main peak of the spectrum corresponding to the target after FFT analysis. Based on the main peak of the spectrum, the target detection and parameter calculation of the target distance and velocity are completed.

[0010] Furthermore, the BPSK phase modulation circuit consists of a single-pole double-throw RF switch, a 0° phase modulation branch, and a 180° phase modulation branch. The moving end of the single-pole double-throw RF switch serves as the input terminal of the BPSK phase modulation circuit, and the two stationary ends are respectively connected to the input terminals of the 0° phase modulation branch and the 180° phase modulation branch. The output terminals of the two phase modulation branches are combined to serve as the output terminal of the BPSK phase modulation circuit. The control terminal of the single-pole double-throw RF switch is electrically connected to the phase modulation control signal output terminal of the digital signal processor.

[0011] Furthermore, the random binary code is an m-sequence pseudo-random code or a Gold code.

[0012] Furthermore, the BPSK phase modulation processing in step S1 is 0 / π phase modulation, specifically: when the random code BPSK phase modulation control signal is 0, the BPSK phase modulation circuit controls the phase of the linear frequency modulation signal to remain at 0°; when the random code BPSK phase modulation control signal is 1, the BPSK phase modulation circuit controls the phase of the linear frequency modulation signal to flip by 180°.

[0013] Furthermore, the specific steps of the code correlation processing in step S3 are as follows: the digitally sampled intermediate frequency signal is multiplied by the local synchronization random code using a conjugate multiplication process; when the local synchronization random code is synchronized with the received echo code symbol, a regular sine sequence containing target Doppler information is output after correlation processing; when the local synchronization random code is not synchronized with the received echo code symbol, an irregular random noise sequence is output after correlation processing.

[0014] Furthermore, the specific method for completing target detection and parameter calculation based on the main peak of the spectrum in step S3 is as follows: For the spectrum result output by FFT, calculate the signal-to-noise ratio of the main peak of the spectrum and the noise floor, compare the signal-to-noise ratio with the preset detection threshold, and if the signal-to-noise ratio is greater than the detection threshold, it is determined that a target has been detected; calculate the target distance based on the frequency corresponding to the main peak of the spectrum output by the first FFT, and calculate the target relative velocity based on the Doppler frequency output by the second FFT.

[0015] Furthermore, the transceiver module incorporates a millimeter-wave signal source, a linear frequency modulation module, a mixer, and a local oscillator module, which are respectively used to generate a millimeter-wave carrier signal, perform linear frequency modulation processing on the carrier signal, mix the echo signal and the local oscillator signal, and generate a local oscillator signal.

[0016] Furthermore, the digital signal processor employs a programmable logic device or a dedicated digital processing chip, and has a built-in analog-to-digital conversion unit; the input terminal of the analog-to-digital conversion unit is electrically connected to the output terminal of the video amplifier, and is used to digitally sample the intermediate frequency signal.

[0017] Furthermore, the video amplifier includes a bandpass filter unit and a low-noise operational amplifier unit connected in series. The bandpass filter unit is used to extract the effective component of a specific bandwidth in the difference frequency signal after mixing, and the low-noise operational amplifier unit is used to amplify the amplitude of the filtered signal.

[0018] The beneficial effects of this invention are: This invention performs phase modulation processing on a linear frequency modulated (LFM) signal using a composite random code. Leveraging the excellent autocorrelation, cross-correlation, and sufficiently long cycle period of the random code, the transmitted signal no longer exhibits periodicity during the detection device's operating time. This fundamentally eliminates the inherent distance ambiguity problem of the LFM system. Cross-cycle echoes are noise-enhanced due to mismatch with the local code, preventing false alarms. Furthermore, this solution combines the core advantages of both modulation systems. It retains the high ranging accuracy, simple circuit structure, and low hardware cost of the LFM continuous wave system while possessing the performance advantages of the random code phase modulation system, such as strong resistance to periodic ambiguity and interference. This significantly improves the reliability of the detection device in complex electromagnetic environments and low-altitude conditions. Attached Figure Description

[0019] Figure 1 This is a circuit block diagram of the linear frequency modulation detection device on which the method of the present invention is based; Figure 2 This is a block diagram illustrating the principle of a linear frequency modulation detection device in the prior art. Figure 3 This is the frequency modulation envelope diagram of a linear frequency-modulated signal that ignores the Doppler effect in the prior art. Figure 4 This is a schematic diagram of the timing of the composite modulation of linear frequency modulation and random code phase modulation described in this invention; Figure 5 This is the frequency modulation envelope diagram of the composite modulation signal that ignores the Doppler effect as described in this invention. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0021] All features, or steps in all methods or processes disclosed herein, may be combined in any manner, except for mutually exclusive features and / or steps. Unless specifically limited, any feature disclosed herein may be replaced by an equivalent or similarly purposed alternative feature as needed for design purposes. The specific embodiments described are for illustrative purposes only and are not intended to limit the scope of the invention.

[0022] This embodiment provides a linear frequency modulation and random code phase modulation composite modulation method for a detection device. The method is implemented based on a linear frequency modulation detection device, the core structure of which is as follows: Figure 1As shown, the device includes a transceiver assembly, a BPSK phase modulation circuit, a transmitting antenna, a receiving antenna, a video amplifier, and a digital signal processor. The transceiver assembly generates a millimeter-wave carrier signal, which is linearly frequency-modulated and phase-modulated by a modulation signal before being radiated into space through the transmitting antenna. The receiving antenna receives the echo signal reflected from the target. The difference frequency signal after mixing the echo signal is filtered and amplified, and a difference frequency signal with a specific bandwidth is extracted and sent to the digital signal processor as an intermediate frequency (IF) signal. The digital signal processor digitally samples the IF signal and performs spectral analysis. After FFT processing, it determines whether a target has been detected by using the ratio of the main spectrum to the noise floor, and identifies the target's distance and Doppler frequency by using the main spectrum frequency. The hardware implementation of this invention adds a BPSK phase modulation circuit to the transmitting link and a random code BPSK phase modulation control signal to the output of the digital signal processor, based on an existing linear frequency modulation detection device.

[0023] In this embodiment, the BPSK phase modulation circuit adopts a random binary phase-coded phase modulation scheme, consisting of a single-pole double-throw RF switch, a 0° microstrip transmission line branch (0° phase modulation branch), and a 180° microstrip delay line branch (180° phase modulation branch). The moving terminal of the single-pole double-throw RF switch serves as the input terminal of the BPSK phase modulation circuit, connected to the linear frequency modulation signal output terminal of the transceiver component; the two stationary terminals are respectively connected to the input terminals of the 0° phase modulation branch and the 180° phase modulation branch, and the output terminals of the two phase modulation branches are combined and connected to the transmitting antenna; the control terminal of the single-pole double-throw RF switch is electrically connected to the phase modulation control signal output terminal of the digital signal processor to receive the random code BPSK phase modulation control signal. The phase modulation logic of this scheme is 0 / π phase modulation: when the random code BPSK phase modulation control signal is 0, the single-pole double-throw RF switch switches to the 0° phase modulation branch, controlling the microwave transmission signal phase to remain at 0°; when the random code BPSK phase modulation control signal is 1, the single-pole double-throw RF switch switches to the 180° phase modulation branch, controlling the microwave transmission signal phase to flip 180°.

[0024] In this embodiment, the transceiver unit integrates a millimeter-wave signal source, a linear frequency modulation module, a mixer, and a local oscillator module. The millimeter-wave signal source generates a millimeter-wave carrier signal, and the linear frequency modulation module performs sawtooth wave linear frequency modulation processing on the carrier signal. The mixer mixes the echo signal and the local oscillator signal, and the local oscillator module generates the local oscillator signal. The local oscillator signal and the transmitted carrier signal originate from the same source and are not superimposed with random code modulation. The video amplifier includes a cascaded bandpass filter unit and a low-noise operational amplifier unit. The digital signal processor uses an FPGA chip and integrates an analog-to-digital converter (ADC) unit for digital sampling of the intermediate frequency signal.

[0025] The composite modulation timing in this embodiment is as follows: Figure 4As shown, the core logic of the composite modulation of linear frequency modulated continuous wave and random code is as follows: within one linear frequency modulation sweep cycle, a complete random binary phase code sequence is synchronously composite modulated, and the complete sequence period of the random binary phase code is completely consistent with a single sweep cycle of the linear frequency modulation. In this embodiment, the random binary phase code adopts an m-sequence pseudo-random code. As an alternative implementation, Gold code can also be used to obtain better cross-correlation characteristics; the cycle period of the m-sequence is greater than the maximum continuous working time of the detection device, ensuring that the transmitted signal does not repeat periodically during the device's working time.

[0026] In this embodiment, the synchronization triggering method between the random binary phase code and the linear frequency modulation sweep cycle is as follows: the global synchronization clock of the FPGA is used to simultaneously serve as the start trigger signal of the linear frequency modulation sweep cycle as the output start trigger signal of the random binary phase code sequence, ensuring that the phase modulation control signal is strictly synchronized with the sweep cycle and meeting the phase modulation timing accuracy requirements of the millimeter wave band.

[0027] After the echo signal and the local oscillator signal are mixed and filtered to obtain the intermediate frequency (IF) signal, this IF signal is sampled by an ADC and then input into an FPGA for processing. The core processing flow is as follows: the sampled echo signal is first subjected to code correlation processing with the local synchronization random code, and then two-stage FFT spectrum analysis is performed. Specifically, the code correlation processing is implemented by performing conjugate multiplication of the digitally sampled IF signal and the local synchronization random code; the local synchronization random code and the transmitter's random binary phase code are of the same origin, and time delay matching and synchronous acquisition with the echo signal are achieved through the sliding correlation method.

[0028] After code correlation processing, when the local synchronization random code and the received echo code are synchronized, the output is a regular sinusoidal sequence containing target Doppler information; when the local synchronization random code and the received echo code are out of sync, the output is an irregular random noise sequence. Utilizing the characteristics of phase modulation, transmitted signals of different periods no longer possess periodicity after phase modulation by the random code, thus eliminating range ambiguity. Cross-period echoes and repeater-type interference, due to their asynchrony with the local code, become noise after correlation processing, preventing false alarm signals.

[0029] In this embodiment, the specific process of the two-stage FFT spectrum analysis is as follows: First, the first FFT analysis is performed on the single-cycle intermediate frequency signal after code correlation processing to obtain the frequency parameters corresponding to the target range; then, the second FFT analysis is performed on the same-range gate signal with coherent accumulation period to obtain the Doppler frequency parameters corresponding to the target. The same-range gate signal mentioned in this embodiment refers to the time-domain signal sequence corresponding to the same frequency resolution unit after the first FFT analysis, which is a standard and commonly used term in the field of radar signal processing.

[0030] In this embodiment, the detection device is assumed to emit signal X. T (n) is shown in equation (3): .................(3) in: A T The amplitude of the transmitted signal; X （t） It is a linear frequency modulated continuous wave signal (sweep bandwidth is B, sweep period is T0); T0 is the modulation period; U （n） For the phase modulation code of the nth sweep cycle, when the phase modulation code is 0, U （n） =1, when the phase modulation code is π, U （n） =-1; N is the number of frames accumulated by coherence.

[0031] The echo signal is then shown in equation (4): .......(4) in: A R The amplitude of the echo signal; X （t） The echo signal is a linear frequency modulated continuous wave (sweep bandwidth is B, sweep period is T0). U （n） The phase modulation code for the nth sweep cycle; T is the sweep frequency period; τ is the transmit / receive delay, and τ = 2R / c (R is the target distance, and c is the speed of light). N is the number of frames for coherent accumulation; When the echo signal is mixed and filtered with the local oscillator signal and correlated with the local code, the intermediate frequency signal is obtained: .................(5) in: A IF Mid-frequency amplitude; U （n） This is the phase modulation code for the nth frequency sweep cycle, with a random value of +1 or -1. U （m） This is the local code for the m-th frequency sweep cycle, with a random value of +1 or -1. B represents the sweep bandwidth; T0 is the frequency sweep period; τ is the transmit / receive delay, and τ = 2R / c (R is the target distance, and c is the speed of light). v is the relative velocity; f0 is the center frequency of the sweep frequency; θ is the phase difference between the local oscillator and the transmitted signal, which does not change much within one processing cycle and can be assumed to be a fixed value; N is the number of frames accumulated by coherence.

[0032] Using local code U （m） Related to the received code, X only occurs when the local code and the received code are the same (within the same period). IF (N) is a regular sequence (N=0,1,2,....N-1); FFT is performed on the correlated intermediate frequencies to obtain the distance dimension frequencies: .................(6) Then, a second FFT is performed on the distance-dimensional signal (N) between multiple echoes to obtain the Doppler frequencies: .................(7) because Therefore, speed and distance can be calculated using the following formula: .................(8) When the local code and the received code are out of sync, X IF (N) is a set of random sequences. An FFT is performed on the correlated intermediate frequencies to obtain the distance-dimensional frequencies. .................(9) Where pn is a random code, it can be seen that although the position of the main peak remains unchanged after the first FFT processing, the positive and negative values ​​will be randomly shuffled. The signal input to the second FFT is no longer a regular Doppler envelope. After the second FFT analysis, the spectrum shows noise characteristics and cannot form an effective peak value that exceeds the detection threshold, thereby achieving distance ambiguity elimination and clutter and interference suppression.

[0033] The specific steps of the composite modulation method in this embodiment are as follows: S1. A millimeter-wave carrier frequency signal is generated by the millimeter-wave signal source of the transceiver component. The signal is then processed by the linear frequency modulation module to obtain a linear frequency modulated signal. In each linear frequency modulation sweep cycle, the FPGA outputs an m-sequence random binary phase code BPSK phase modulation control signal synchronized with the sweep cycle to the BPSK phase modulation circuit. The linear frequency modulation signal is synchronously superimposed with the random binary phase code and subjected to 0 / π BPSK phase modulation processing to obtain a transmission signal that is composite modulated by linear frequency modulation and random code phase modulation. The transmission signal is then radiated into space through the transmission antenna. S2. The echo signal reflected by the target is received by the receiving antenna and transmitted to the mixer of the transceiver component. The echo signal is mixed with the local oscillator signal output by the local oscillator module to obtain the difference frequency signal. The difference frequency signal is then processed by the bandpass filter and low noise amplification of the video amplifier to extract the effective intermediate frequency signal and transmit it to the analog-to-digital conversion unit of the FPGA. S3. The intermediate frequency signal is digitally sampled by the FPGA's analog-to-digital conversion unit to obtain a digital intermediate frequency signal. First, the sampled echo signal is correlated with the conjugate multiplication code of the local synchronization random code. Then, the single-cycle signal after code correlation processing is subjected to the first FFT spectrum analysis to obtain the frequency parameters corresponding to the target distance. The same-distance gate signal with coherent accumulation period is subjected to the second FFT analysis to obtain the Doppler frequency parameters corresponding to the target. When the local synchronization random code is uncorrelated with the received echo code, the signal after correlation processing is a random sequence, which exhibits noise characteristics after FFT analysis. When the local synchronization random code is correlated with the received echo code, the signal after correlation processing is a regular sequence containing the target's Doppler information, which forms the main peak of the spectrum corresponding to the target after FFT analysis. S4. For the spectrum result of the second FFT output, calculate the signal-to-noise ratio between the main peak of the spectrum and the noise floor, compare the signal-to-noise ratio with the preset detection threshold, and if the signal-to-noise ratio is greater than the detection threshold, it is determined that the target has been detected; substitute the frequency corresponding to the main peak of the spectrum output of the first FFT into equation (8) to calculate the target distance, and substitute the Doppler frequency of the second FFT output into equation (8) to calculate the target relative velocity.

Claims

1. A method for composite modulation of linear frequency modulation (LFM) and random code phase modulation (RCM) for a detection device, the method being implemented based on a LFM detection device, the device comprising a transceiver assembly, a BPSK phase modulation circuit, a transmitting antenna, a receiving antenna, a video amplifier, and a digital signal processor; the LFM signal output terminal of the transceiver assembly is electrically connected to the transmitting antenna via the BPSK phase modulation circuit, the signal output terminal of the receiving antenna is electrically connected to the echo receiving terminal of the transceiver assembly, the intermediate frequency (IF) signal output terminal of the transceiver assembly is electrically connected to the input terminal of the video amplifier, the output terminal of the video amplifier is electrically connected to the digital signal processor, and the phase modulation control signal output terminal of the digital signal processor is electrically connected to the control terminal of the BPSK phase modulation circuit; Its features are, The method includes the following steps: S1. A millimeter-wave carrier frequency signal is generated through the transceiver component, and the millimeter-wave carrier frequency signal is linearly frequency modulated to obtain a linearly frequency modulated signal. The transmitted signal is radiated into space through the transmitting antenna. S2. The echo signal reflected by the target is received through the receiving antenna, and the echo signal is transmitted to the transceiver component for mixing with the local oscillator signal to obtain the difference frequency signal; the difference frequency signal is filtered and amplified by the video amplifier to obtain the intermediate frequency signal and transmitted to the digital signal processor. S3. The intermediate frequency signal is digitally sampled by the digital signal processor, and the sampled signal is subjected to FFT spectrum analysis. Based on the spectrum analysis results, the target detection and target distance and velocity parameters are calculated. In step S1, for the carrier signal that has completed the linear frequency modulation (LFM) processing, in each LFM sweep cycle, the digital signal processor outputs a random binary phase code (BPSK) phase modulation control signal synchronized with the sweep cycle to the BPSK phase modulation circuit. The LFM signal is synchronously superimposed with the random binary phase code for BPSK phase modulation processing to obtain a transmission signal that is composite modulated by LFM and random code phase modulation. The complete sequence period of the random binary phase code is completely consistent with a single sweep cycle of the LFM, and the cycle period of the random binary phase code is greater than the continuous working time of the detection device. In step S3, the digitally sampled intermediate frequency signal is first processed by a digital signal processor to perform code correlation processing between the sampled echo signal and the local synchronization random code. The local synchronization random code is a local random binary phase code that is homologous to the random binary phase code of the transmitting end and matches the time delay of the echo signal. Then, FFT spectrum analysis is performed on the signal after code correlation processing to obtain the frequency parameters corresponding to the target distance. Secondary FFT analysis is performed on the same-distance gate signals with multiple coherent accumulation periods to obtain the Doppler frequency parameters corresponding to the target. When the local synchronization random code is uncorrelated with the received echo code, the signal after correlation processing is a random sequence, which exhibits noise characteristics after FFT analysis. When the local synchronization random code is correlated with the received echo code, the signal after correlation processing is a regular sequence containing the Doppler information of the target, which forms the main peak of the spectrum corresponding to the target after FFT analysis. Based on the main peak of the spectrum, the target detection and parameter calculation of the target distance and velocity are completed.

2. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The BPSK phase modulation circuit consists of a single-pole double-throw RF switch, a 0° phase modulation branch, and a 180° phase modulation branch. The moving end of the single-pole double-throw RF switch serves as the input terminal of the BPSK phase modulation circuit, and the two stationary ends are respectively connected to the input terminals of the 0° phase modulation branch and the 180° phase modulation branch. The output terminals of the two phase modulation branches are combined to serve as the output terminal of the BPSK phase modulation circuit. The control terminal of the single-pole double-throw RF switch is electrically connected to the phase modulation control signal output terminal of the digital signal processor.

3. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The random binary code is an m-sequence pseudo-random code or a Gold code.

4. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The BPSK phase modulation process described in step S1 is 0 / π phase modulation, specifically: when the random code BPSK phase modulation control signal is 0, the BPSK phase modulation circuit controls the phase of the linear frequency modulation signal to remain at 0°; when the random code BPSK phase modulation control signal is 1, the BPSK phase modulation circuit controls the phase of the linear frequency modulation signal to flip by 180°.

5. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The specific steps of the code correlation processing in step S3 are as follows: the digitally sampled intermediate frequency signal is multiplied by the local synchronization random code using a conjugate multiplication process; when the local synchronization random code is synchronized with the received echo code symbol, a regular sine sequence containing target Doppler information is output after correlation processing; when the local synchronization random code is not synchronized with the received echo code symbol, an irregular random noise sequence is output after correlation processing.

6. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The specific method for completing target detection and parameter calculation based on the main peak of the spectrum in step S3 is as follows: calculate the signal-to-noise ratio (SNR) between the main peak of the spectrum and the noise floor for the spectrum result output by FFT, compare the SNR with the preset detection threshold, and determine that the target has been detected if the SNR is greater than the detection threshold. The target distance is calculated based on the frequency corresponding to the main peak of the spectrum output of the first FFT, and the target relative velocity is calculated based on the Doppler frequency output of the second FFT.

7. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The transceiver module includes a millimeter-wave signal source, a linear frequency modulation module, a mixer, and a local oscillator module, which are used to generate millimeter-wave carrier signals, perform linear frequency modulation on the carrier signals, mix the echo signals and local oscillator signals, and generate local oscillator signals, respectively.

8. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The digital signal processor uses a programmable logic device or a dedicated digital processing chip and has a built-in analog-to-digital conversion unit; the input of the analog-to-digital conversion unit is electrically connected to the output of the video amplifier and is used to digitally sample the intermediate frequency signal.

9. The linear frequency modulation and random code phase modulation composite modulation method for a detection device according to claim 1, characterized in that: The video amplifier includes a bandpass filter unit and a low-noise operational amplifier unit connected in series. The bandpass filter unit is used to extract the effective component of a specific bandwidth in the difference frequency signal after mixing, and the low-noise operational amplifier unit is used to amplify the amplitude of the filtered signal.

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