Stepping frequency modulation radar transmitter based on metasurface

Abstract

The invention discloses a step frequency radar transmitter based on a space-time coding metasurface, and belongs to the technical field of artificial electromagnetic metasurfaces. The transmitter comprises: 1) a space-time coding metasurface formed by sub-wavelength units, each unit integrating a variable capacitance diode and a chip capacitor, and applying a bias voltage through an FPGA to realize continuous regulation and control of a reflection phase 0-2pi; 2) a step frequency modulation signal generation method: generating a step frequency modulation reflection echo by designing a metasurface reflection coefficient coding vector and an incident signal of a time-varying frequency; (3) a waveform purity analysis model is used for quantifying the influence of amplitude errors and phase errors on the signal purity; and 4) a novel radar transmitter architecture. According to the invention, radar waveform generation and wave beam scanning functions can be realized synchronously, and functions of a waveform generator and a T / R assembly of a traditional transmitter are integrated in a single device. Compared with a conventional architecture, the hardware complexity and the integration cost are remarkably reduced, the broadband waveform generation capability is supported, and the target distance and speed information can be accurately obtained.

Description

TECHNICAL FIELD

[0001] The application relates to a stepped frequency modulation radar transmitter based on a space-time coding metasurface, which can synchronously realize the generation of a wideband stepped frequency modulation signal and a dynamic beamforming function, and belongs to the technical field of novel artificial electromagnetic metasurfaces. BACKGROUND

[0002] As a core equipment for all-weather target detection, the performance of a radar mainly depends on the design of a transmitter system. A conventional transmitter usually adopts a functional modular architecture, wherein waveform generation depends on a fixed hardware module, leading to limited signal reconstruction flexibility; beamforming depends on a phased array antenna, and high-precision T / R components need to be provided for each radiating unit, leading to exponential increase of system cost and volume with the number of radiating units; more importantly, in the existing scheme, waveform generation and beamforming adopt a physically separated architecture, which not only leads to low equipment integration and increased maintenance difficulty, but also becomes a core technical bottleneck restricting further development of radar technology.

[0003] As an artificial electromagnetic structure, a coding metasurface realizes dynamic regulation of an electromagnetic wave front through periodic arrangement of subwavelength units, and the spatial beam regulation capability of the coding metasurface can replace a phased array antenna, significantly reducing system complexity and cost. A space-time coding metasurface further introduces a time domain modulation dimension, realizes waveform generation through dynamic spectrum regulation, and experiments show that the space-time coding metasurface can generate frequency modulation / signals by using time sequence coding, meeting the diversified detection requirements of a radar. However, due to the limited modulation speed of a control module and the inherent defects of the metasurface, the conventional space-time coding metasurface is usually only suitable for processing narrowband signals, and the reduction of bandwidth means the reduction of radar detection precision. Therefore, it is urgent to develop a spectrum expansion method based on compensation of frequency response characteristics of the metasurface, and to construct a wideband space-time coding metasurface radar transmitter architecture to break through the technical boundaries of the existing metasurface radar transmitter. SUMMARY

[0004] TECHNICAL PROBLEM

[0005] The problem to be solved by the application is that the conventional space-time coding metasurface architecture does not support wideband signal processing, and the insufficient signal generation bandwidth of the conventional space-time coding metasurface architecture leads to system detection precision degradation, which substantially restricts large-scale engineering application of the technology in the field of radar transmitters. The specific implementation manner is as follows: a stepped frequency modulation signal is generated through frequency domain parameter configuration of an incident signal of a space-time coding metasurface and time sequence coding design of a control signal; based on a frequency step synthesis principle, phase alignment and bandwidth synthesis are performed on the signal generated by the space-time coding metasurface, to realize reconstruction of a frequency modulation signal with equivalent bandwidth expansion; and meanwhile, through space domain coding design, the system realizes directional radiation characteristics while generating the stepped frequency modulation signal.

[0006] TECHNICAL SCHEME

[0007] To solve the above technical problems, the application provides a time-space coding metasurface capable of generating step frequency modulation signals.

[0008] The super surface structure unit adopts F4B double-sided copper clad dielectric substrate (dielectric constant 2.65, loss tangent 0.001) as a dielectric substrate, and utilizes a PCB process to etch 8 groups of size-differentiated rectangular patch arrays on the top layer of the unit.

[0009] The super surface structure unit top layer integrates four SMV-2019 type varactor diodes and four 0.1 pF patch capacitors, and based on a field programmable gate array (FPGA), a voltage is applied across the varactor diodes, so as to realize dynamic regulation of the electromagnetic properties of the time-space coding metasurface.

[0010] When the control voltage is switched between any value in the range of 0V to 14V, the reflection phase of the metasurface can realize 0-2π full cycle coverage in the working frequency band of 4.0GHz to 4.26GHz.

[0011] The time-space coding metasurface generates a time sequence coding voltage signal through an FPGA controller to drive the reflection coefficient switching of the metasurface unit, and completes the phase and frequency modulation of the incident signal.

[0012] By synchronously regulating the frequency of the incident signal and the control coding time sequence, the time-space coding metasurface can generate step frequency modulation signals, synthesize frequency modulation signals with an equivalent bandwidth of 256MHz, and meet the technical requirements of radar systems for wideband signal generation and waveform agility.

[0013] The application further provides an analysis model of factors affecting the waveform purity of the time-space coding metasurface, which reveals the influence mechanism of the frequency modulation signal purity through a quantitative evaluation method.

[0014] The model constructs a reflection coefficient coding framework of the time-space coding metasurface, aiming to realize specific frequency waveform generation and energy concentration; in actual application, hardware non-ideal characteristics will introduce amplitude error and phase error: the former is caused by the nonlinear fluctuation of the reflection amplitude during dynamic phase modulation of the metasurface unit, and the latter is caused by the parasitic reactance transient response delay caused by the switching of the control signal.

[0015] The nonlinear accumulation of phase error is exacerbated by fast timing switching during high-frequency signal generation, resulting in a significant decrease in signal waveform purity; amplitude error is not sensitive to modulation rate changes, which manifests as a constant amplitude deviation. This model decouples the physical mechanisms of the two types of errors, providing a theoretical basis for super surface parameter optimization and signal purity improvement.

[0016] The application further proposes a stepped frequency radar transmitter architecture based on a space-time coded metasurface.

[0017] The architecture utilizes the time coding characteristics of the space-time coded metasurface to construct a dynamically coded mechanism for parameter-adaptive matching stepped frequency signals, thereby achieving efficient generation of high-purity stepped frequency signals.

[0018] At the same time, by integrating spatial domain coding technology, the waveform generation capability of the space-time coded metasurface is maintained, and a full spatial domain coverage beam scanning is formed.

[0019] At the same time, through the cooperative work of the high-speed data acquisition module and the backend processing unit, and the integration of signal feature extraction algorithms, the target "range-velocity-angle" three-dimensional parameters can be inverted in real time.

[0020] Advantages:

[0021] 1. Evaluation model of factors affecting the generation of metasurface signals: a dynamic evolution model of the reflection coefficient of space-time coded metasurface is first established, breaking through the limitations of traditional qualitative analysis, and by separating the coupling effects of amplitude error and phase error, quantitative evaluation of signal purity degradation mechanism is realized, providing a mathematical modeling benchmark for metasurface parameter optimization.

[0022] 2. High-precision, high-freedom, and low-complexity radar transmitter: the application constructs an integrated "signal modulation-beamforming" architecture, which realizes directional radiation while generating stepped frequency signals, and all processes are completed directly on the metasurface, providing module integration and eliminating the complex RF links in traditional radar systems. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The working principle diagram of the stepped frequency radar transmitter based on the space-time coded metasurface in the application;

[0024] Figure 2 The unit structure diagram of the space-time coded metasurface in the application;

[0025] Figure 3 The transformation relationship between the reflection amplitude and reflection phase of the space-time coded metasurface in the application at 4GHz to 4.26GHz with the change of bias voltage;

[0026] Figure 4The experimental setup for this invention includes: (a) a physical image of the designed spatiotemporally encoded metasurface sample; (b) a schematic diagram of the laboratory ranging test scenario layout; and (c) a schematic diagram of the ranging test scenario layout based on a radar echo simulator.

[0027] Figure 5 This paper compares the signal quality of broadband FM signals generated directly using a spatiotemporally coded metasurface and those generated using the method described in this invention. In this paper, (ab) represents the time-frequency analysis results of the signals generated by the two methods, and (cd) shows the purity of the signals generated by the two methods, as well as the relationship between amplitude error and phase error as a function of frequency.

[0028] Figure 6 The results of target detection using the step-frequency modulated radar transmitter based on a spatiotemporally coded metasurface designed in this invention are shown in the following diagrams: (ab) Ranging results of single and multiple targets in laboratory tests; (cd) Ranging results of single and multiple targets based on a radar echo simulator; (e) Range resolution test results; (f) Target velocity measurement results.

[0029] Figure 7 The results of beam scanning of the spatiotemporally encoded metasurface in this invention. Detailed Implementation

[0030] This invention provides a method for generating stepped frequency-modulated signals based on a spatiotemporally coded metasurface. This method combines the beamforming properties of the metasurface to construct a novel radar transmitter architecture. Its conceptual diagram is shown below. Figure 1 As shown in the figure. This embodiment designs a 12×16 all-phase modulation reflective metasurface sample, the unit structure of which is as follows. Figure 2 As shown. This metasurface sample adopts a three-layer structure design: a top metal patterned layer, a middle dielectric base layer (F4B, ε r =2.65, tanδ=0.0015) and a metal ground. A varactor diode (SMV-2019) and a surface-mount capacitor (0.1pF) are integrated on the surface of the metasurface unit. By adjusting the voltage across the varactor diode, the phase of electromagnetic wave reflection can be precisely controlled.

[0031] Figure 3 (ab) shows the test results of the designed spatiotemporally coded metasurface in the 4 GHz-4.26 GHz frequency band, including the reflection amplitude as a function of the control voltage. Figure 3 a) and the trend of reflection phase with control voltage ( Figure 3 b). Experimental results show that the metasurface unit can achieve continuous adjustment of the reflection phase in the range of 0-2π while maintaining a strong reflection amplitude, and has the ability to generate frequency-modulated signals.

[0032] Build asFigure 4 The frequency-modulated radar transmitter system is shown. The system uses a voltage waveform generator (PXIe-5433) with high-speed DAC conversion capability to accurately control the space-time coded metasurface, and completes the transmission and reception of signals through a vector signal transceiver (PXIe-5841). In the experiment, the corner horn antenna transmits a single frequency signal corresponding to the frequency to the metasurface in the normal direction. The waveform generator generates a periodic voltage control signal, which is converted into a continuous voltage waveform to drive the metasurface to realize the periodic switching of the transmission phase. Under this configuration, the frequency spectrum of the reflected echo will completely carry the frequency spectrum information of the reflection coefficient of the metasurface. By inputting the control voltage code corresponding to the specific radar signal, the metasurface reflected echo can accurately output the required radar waveform, thereby fully meeting the performance requirements of the radar transmitter.

[0033] Further, the present application proposes an analysis model of the influence factors of metasurface waveform purity. Since the reflection coefficient of the metasurface modulation is composed of discrete digital codes, the modulation period of the space-time coded metasurface is divided into M time slots, and the reflection coefficient of each time slot is Γ m , then the time-varying reflection coefficient function Γ(t) can be discretely represented by this set of time slot codes Γ m , and expressed by Fourier series expansion as follows:

[0034]

[0035] In the formula, f n represents the nth harmonic frequency, A n represents the harmonic coefficient. When a single frequency signal with a frequency of f c is incident to the metasurface, the reflected electric field after the modulation of the space-time coded metasurface in the frequency domain is:

[0036]

[0037] Assuming that the +1 order harmonic of the space-time coded metasurface is selected for signal generation, in order to ensure the purity and efficiency of the generated signal, it is required that A n satisfies: |A +1 |>0 and A n =0, n≠1, to ensure the concentration of effective energy and the purity of the signal. The reflection coefficient code is obtained by solving:

[0038]

[0039] Formula (2) can be simplified as:

[0040]

[0041] Where f +1 is f n(n = +1). It can be seen that the interaction of the incident electromagnetic wave with the metasurface effectively realizes the frequency modulation and harmonic synthesis of the electromagnetic wave, and the reflected wave only has a frequency component at frequency f c +f +1 , which can be used for signal generation.

[0042] Although theoretical analysis shows that the metasurface has the ability to generate a pure signal, in actual operation, errors are inevitable. According to the source of the error, it can be classified as amplitude error ε A and phase error ε Φ . Regarding the amplitude error, when analyzing the phase-modulated spatiotemporal encoding metasurface, it is usually assumed that the reflection amplitude A = 1 to simplify the derivation. However, in practice, the reflection amplitude corresponding to different reflection phases is not constant, but changes with the phase. Specifically, there is a dependence between the reflection amplitude and the phase, so the reflection amplitude should be considered as a function of the phase, and the error caused by the fluctuation of the amplitude should also be evaluated as a function of the phase, denoted as where represents the reflection phase of the spatiotemporal encoding metasurface. As the fluctuation of the reflection amplitude intensifies, the influence of the amplitude error also becomes more significant.

[0043] Regarding the phase error, the regulation of the spatiotemporal encoding metasurface usually depends on the introduction of adjustable active devices in the structure design, by embedding adjustable factors such as variable resistance and reactance elements in the unit structure, to accurately adjust its electromagnetic properties. However, the additional introduction of adjustable factors often causes deviations between the theoretical control signal and the actual response. Taking the varactor diode as an example, due to the charging and discharging effect of the tuning capacitor, when the control voltage is switched at a high speed, there is a lag phenomenon between the actual control voltage across the diode and the theoretical expectation, and the higher the switching rate, the more significant the phase delay and distortion caused. In the phase-modulated spatiotemporal encoding metasurface system, there is a strict functional correspondence between the control signal and the reflection phase, therefore, the difference between the theoretical reflection phases of adjacent time slots can be used to quantify the degree of switching of the control signal. Based on this, the phase error can be considered as a function of , denoted as

[0044] Considering and , the actual reflection coefficient of the spatiotemporal encoding metasurface is represented as:

[0045]

[0046] Since two error terms are introduced, A nNot meeting the above requirements, this not only inevitably reduces the +1 order harmonic efficiency, but also introduces other high-order harmonic components, which has a negative impact on the efficiency of the spatiotemporal encoding metasurface generated signal. Therefore, it is essential to explore the actual impact of error factors on signal generation and evaluate the spatiotemporal encoding metasurface signal generation capability at different frequencies. The amplitude error is a function, and its impact on harmonic generation is only determined by the degree of fluctuation of the reflection amplitude with the change of phase, and has nothing to do with the harmonic frequency f +1 size. In contrast, although the phase error is a function, there is a correlation between and f +1 :

[0047]

[0048] where f s represents the sampling rate. When f s remains unchanged, f +1 is proportional to , which means is also a function of f +1 . The greater the harmonic frequency, the more significant the introduced phase error.

[0049] In order to quantify the waveform purity of the generated waveform, the harmonic efficiency η is defined:

[0050]

[0051] where P main represents the power of the main frequency component, and P total represents the total power. According to the performance standards for waveform generators in IEEE Std, the device needs to ensure that at least 90% of the power is concentrated in the main frequency, that is, the energy is concentrated in the spatiotemporal encoding metasurface generated waveform f +1 . Based on the above analysis, and the existence of will lead to the decrease of η of the spatiotemporal encoding metasurface generated waveform. And because the increase of f +1 leads to the rapid increase of , which directly limits the current spatiotemporal encoding metasurface technology to generate high f +1 ability. And the key feature of wideband signal is that the waveform frequency needs to cover a large enough frequency range, so the bandwidth of the generated signal is directly related to f +1There is a direct positive correlation between the maximum tunable range of the metasurface and the value of η. The frequency dynamic range of the spatiotemporal coding metasurface can only cover the narrowband demand, which leads to its inability to achieve the high-resolution target detection function required by modern radar systems. Therefore, it is crucial to explore a method that can improve the bandwidth of the signal generated by the spatiotemporal coding metasurface while ensuring that η meets the performance standard requirements.

[0052] Based on the analysis model of the influencing factors of the waveform purity of the metasurface, the present application proposes a step frequency modulation signal generation theory based on the spatiotemporal coding metasurface: for a signal with a frequency that changes over time, the constant f in formula (4) +1 needs to be modified into a function of time f +1 (t). Taking a linear frequency modulation signal with a pulse width of T p and a frequency modulation rate of K r as an example, the corresponding harmonic modulation frequency is represented as:

[0053] f +1 (t)=K r t,0≤t<T p (9)

[0054] The corresponding reflection coefficient coding is modified as wherein M(t)=f s / f +1 (t), represents the floor function. When the generated waveform frequency steps, the carrier frequency is modified as:

[0055]

[0056] wherein Δf represents the single frequency step amount, T r represents the pulse repetition time (PRT) of the frequency modulation signal. In combination with formula (4), the frequency domain representation of the reflection echo of the spatiotemporal coding metasurface at this time is:

[0057]

[0058] The instantaneous frequency of the generated signal is represented as f c ′(t)+f +1 (t). In summary, the instantaneous frequency expression of the linear frequency modulation signal emitted by the spatiotemporal coding metasurface is:

[0059]

[0060] wherein rect{·} is a rectangular window used to limit the pulse width T p of the transmitted waveform.

[0061] Meanwhile, the beam scanning ability of the metasurface is combined to further construct a stepped frequency radar transmitter system: assuming the width of each column of the metasurface is d, the metasurface units in the same column share the same control signal. In the experiment, only the initial phase between different column units is changed, and in addition, each column control signal has the same modulation mode and modulation period. Let the initial reflection phase of the nth column unit be φ n = φ1+ Δφ n (i > 1), φ1 represents the initial reflection phase of the first column unit, and the phase difference between the nth column unit and the first column unit of the +1 harmonic coefficient is

[0062]

[0063] That is, the phase difference between the signals generated by the two column switching mode and the modulation period of the spatiotemporal encoding metasurface is only determined by their initial phase. Therefore, according to antenna theory, when the outgoing beam azimuth angle is θ, the phase of the nth column on the corresponding metasurface can be represented as: n

[0064]

[0065] In the experiment, since f c ′(t) >> f +1 (t), in order to simplify the problem analysis, f r (t) ≈ f c ′(t) can be approximated. Therefore, the initial phase distribution of each column unit of the metasurface is simplified as:

[0066]

[0067] In order to enhance the rigor of the theoretical derivation, three groups of test cases are provided.

[0068] Example 1: Generation signal quality evaluation

[0069] Figure 5 (a) shows the time-frequency analysis results of the linear frequency modulation signal with a frequency range of 0-30MHz directly generated by the spatiotemporal encoding metasurface sample. Since the frequency of the linear frequency modulation signal increases linearly with time, the time-frequency analysis can effectively present the purity of the generated waveform at different frequencies. As can be seen from the results, as the frequency of the generated waveform gradually increases, the energy of the fundamental wave and the higher harmonic wave also increases, and the +1 order harmonic efficiency is significantly reduced. Figure 5 ​​(c) shows the harmonic efficiency of the spatiotemporal encoding metasurface generating signals and the change relationship between the amplitude error and the phase error in the theoretical analysis and the generation efficiency. The results are consistent with the theoretical analysis, and the amplitude error remains relatively stable during the increase of the generation frequency, while the phase error increases rapidly with the increase of the generation frequency. Therefore, there is an obvious upper limit of the frequency when using the spatiotemporal encoding metasurface sample to generate waveforms. On this basis, the equivalent bandwidth of the spatiotemporal encoding metasurface generating waveforms is expanded by using the step frequency technology proposed in the application. In the experimental process, the single pulse signal bandwidth generated by the spatiotemporal encoding metasurface is B = 4 MHz, and the linear frequency modulation signal with a step size of 4 MHz. The time-frequency analysis results of the synthesized signal are shown in Figure 5 (b). Compared with Figure 5 (a), the higher harmonic components of the spatiotemporal encoding metasurface generating waveforms are significantly weakened. The harmonic efficiency and error influence of the signal are shown in Figure 5 (d), and the results show that the harmonic efficiency is maintained above 95%, and a relatively pure frequency modulation signal is generated. Therefore, it can be considered that the scheme of the spatiotemporal encoding metasurface generating step frequency modulation signals is feasible.

[0070] Example 2: Target distance / speed detection

[0071] A set of classical step frequency radar parameters are selected for experimental design, and the transmitter single pulse signal bandwidth is set to B = 4 MHz, the step size is 4 MHz, the pulse number N = 64, and the synthesized bandwidth of the step linear frequency modulation signal is 256 MHz, and the frequency range is from 4 GHz to 4.256 GHz. Based on the set radar waveform parameters, the corresponding distance resolution can be represented as:

[0072]

[0073] Wherein, c represents the speed of light c = 3 × 10 8 m / s. The experimental scene is shown in Figure 4 (b), and an angle reflector is selected to represent the target. Figure 6 (a) shows the distance measurement results of the radar transmitter when only the Target I angle reflector exists, and the target echo signal is synthesized and pulse compressed. The actual angle reflector distance is 6.5 m, and the measurement error is within the allowable range of the distance resolution error. The Sinc function image obtained by interpolating the numerical values at nearby points is shown in Figure 6(e) shows that the 3dB width of the function corresponds to a distance Ar=0.545m, which is basically consistent with the theoretical distance resolution AR. This shows that the display resolution after pulse compression meets the expected standard, verifying the accuracy of the ranging experiment results and the effectiveness of the proposed scheme. In order to better illustrate the wideband working characteristics of the designed radar transmitter based on the spatiotemporal coding metasurface, a second target Target II is placed in front of Target I at d=1.5m, and the distance measurement results are as shown in Figure 6 (b) shows that the proposed radar transmitter can distinguish the distance difference between the two targets at the same time as the reaction target position, verifying the effectiveness and feasibility of the designed transmitter system.

[0074] In order to enhance the persuasiveness of the experimental setup, further experiments simulating the radar environment are designed and implemented, and the experimental scene is as shown in Figure 4 (c). In this setup, the signal generated by the metasurface is input into the radar echo simulator through the receiving antenna, and after signal processing, it is returned to the radar receiving antenna. The radar echo simulator is used to simulate the radar measurement environment, and by setting the virtual target information as well as the type of noise and interference, it can generate echo signals similar to actual radar targets, thereby effectively reproducing the radar environment. Figure 6 (c) and (d) respectively show the processing results of the radar echo simulator output signal in the single target distance of 1025m and the multi-target distance of 1200m and 1775m measurement scene. In order to verify the feasibility of the proposed scheme, real radar measurement data is introduced for comparison and verification. Although the radar measurement results based on the spatiotemporal coding metasurface have a slight decrease in signal-to-noise ratio compared to the real radar, they still maintain a relatively high signal-to-noise ratio, and the measurement results of the two are consistent, which can correctly reflect the target information. In addition, on the basis of the ranging experiment, the target velocity is measured, Figure 6 (e) shows the velocity measurement results of a target with a distance of 1.3km and a speed of 23m / s, which are accurate, proving that the transmitter architecture proposed in this paper has the ability to replace the real radar transmitter.

[0075] Embodiment 3: Beam scanning

[0076] According to formula (14), the spatiotemporal coding strategy of the spatiotemporal coding metasurface is designed, Figure 7 shows the measured results of different frequency harmonics and different exit angles under the condition of vertical incidence. The measured results only have slight fluctuations compared with the theoretical values, which shows that the proposed radar system has good beam scanning capability within the scanning angle range of ±40°. The test results prove the beam scanning capability of the new radar system, verifying its feasibility and reliability in practical applications.

[0077] In summary, the application realizes the generation of step frequency modulation signal by designing a simple periodic control voltage signal using a space-time coding metasurface, and further constructs a step frequency modulation radar transmitter using the space-time coding metasurface. The radar transmitter has high integration, simple reception, low cost, wide application prospect and important research value, and is suitable for multiple fields such as radar, communication, sensing and the like.

[0078] The core technical concept, key features and outstanding advantages of the application have been fully described in the above content. According to the general understanding of the technical field, the application should not be limited to the implementation modes listed here, and the related descriptions in these exemplary examples and technical documents are only specific examples to illustrate the core design idea of the application. Under the premise of maintaining the basic essence and coverage of the application, any equivalent alternative, adaptive adjustment or optimized improvement based on the technical essence of the application belongs to the effective protection scope of the patent right of the application. Finally, the protection boundary of the application should be based on the specific content of the claims attached to the patent document and its legal equivalent range.

Claims

1. A method for generating a stepped frequency signal based on a spatiotemporal coded metasurface, characterized in that, The method comprises the following steps: The reflection coefficient coding vector of the spatio-temporal coding metasurface is designed as: where K r represents the frequency modulation of the frequency modulated signal generated by the metasurface, T p represents the signal pulse width, represents the floor function, f s represents the sampling rate; The incident signal frequency is designed as: where f c represents the initial frequency of the frequency-modulated signal, Δf represents the single frequency step size, T r represents the pulse repetition time of the frequency-modulated signal; After the incident signal acts on the spatio-temporal coding metasurface, the echo reflected by the metasurface is a stepped frequency signal, and the mathematical model is as follows: Wherein, N is the number of signal periods participating in bandwidth synthesis.

2. The method of claim 1, wherein, The time coding metasurface adopts a three-dimensional reflective architecture constructed by M rows and L columns of basic units, and each column of units realizes synchronous control of excitation signals through a column selection bus; each basic unit is composed of a stacked structure of a dielectric substrate, a metal radiation layer and a ground plate, wherein the metal radiation layer integrates a voltage-controlled diode device, and the capacitance parameter of the voltage-controlled diode device is controlled by applying a bias voltage to realize the continuous adjustment characteristic of the reflection phase, and the phase adjustment range covers 0 to 2π full period; The bias voltage signal required by the time coding metasurface is generated by FPGA, and FPGA generates control signals in real time to ensure that the time coding metasurface can generate frequency-varying signals.

3. A stepped frequency radar transmitter based on a spatio-temporal coding metasurface based on the stepped frequency signal generation method of claim 1 or 2, characterized in that, The integrated FPGA and DSP collaborative control architecture realizes the generation of stepped frequency signals and beam scanning through spatio-temporal coding synchronization of the metasurface; The discrete T / R components and waveform generator are omitted, and the frequency modulation signal with an equivalent bandwidth of 256 MHz is generated through the integration of spatial domain phase modulation and time domain signal modulation.

4. A stepped-frequency radar transmitter according to claim 3, characterised in that, The beam scanning is realized by the following way: (a) Set the target beam azimuth angle θ and the operating wavelength c; (b) calculating the initial reflection phase of the nth column of cells according to the relationship n > 2, where d is the cell width and φ1 represents the initial reflection phase of the first column of cells; all column cells use the same control signal modulation scheme and modulation period, except for the initial phase difference. (c) Generate the bias voltage coding corresponding to the initial phase of each column through FPGA to drive the metasurface to realize directional beam scattering.

5. A spatiotemporal encoding metasurface waveform purity analysis method based on the stepped frequency signal generation method of claim 1, characterized in that, The method comprises the following steps: 1) determining reflection phase based on measured electromagnetic response of a metasurface unit reflection amplitude non-linear fluctuation function caused by variation 2) Switching the rate according to the control signal, establishing the theoretical reflection phase difference of adjacent time slots Mapping function with parasitic reactance delay 3) Construct the actual reflection coefficient model: Wherein, M is the number of discrete time slots in the spatio-temporal coding period division of the metasurface unit, and m represents the time slot sequence index; 4) The measured function and is substituted into the model to calculate the harmonic efficiency where P main represents the power of the main frequency component, P total represents the total power; 5) Quantify the signal purity degradation degree through η value, and optimize the metasurface structure parameters and control timing.

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