A non-periodic time-modulated corner reflector with wide-angle control and its harmonic spectrum conversion method

Abstract

The present application provides a non-periodic time modulation corner reflector with wide-angle regulation and a harmonic spectrum conversion method thereof, the reflector comprising a time modulation metasurface and a three-face corner reflector structure; wherein the time modulation metasurface is a single plane structure composed of an active adjustable metasurface and a field programmable gate array (FPGA) time modulation control system. The conversion method comprises the following steps: step one, wide-angle time modulation corner reflector design; step two, non-periodic time modulation model establishment based on the wide-angle time modulation corner reflector; and step three, continuous harmonic spectrum conversion method based on the wide-angle time modulation corner reflector. The present application effectively improves the wide-angle stability of the time modulation metasurface by combining the time modulation metasurface with the corner reflector, and can control the harmonic spectrum distribution of the backward reflected electromagnetic wave in a wide-angle range. At the same time, a non-periodic time modulation model is established from the signal level, and the effect of reflective continuous harmonic spectrum conversion is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electromagnetic regulation of metamaterials, and particularly to a non-periodic time-modulated corner reflector with wide-angle regulation and a harmonic spectrum conversion method thereof. BACKGROUND

[0002] A corner reflector is usually composed of two or three mutually perpendicular metal plates, which can form a strong backward reflection characteristic for incident electromagnetic waves in a wide-angle domain, and is widely used in target identification, electronic countermeasures and other fields. However, once the conventional corner reflector is processed, its electromagnetic characteristics are difficult to change, which cannot meet the needs of future special occasions and multi-functional multi-standard radar applications, and seriously limits its application in the future information regulation field.

[0003] Electromagnetic metamaterials are composed of a series of sub-wavelength units arranged in a periodic manner, which can produce electromagnetic properties that natural materials cannot form. Due to its unique electromagnetic wave modulation ability, it has been widely concerned and studied by many scholars. Therefore, electromagnetic metamaterials are widely designed and applied in many aspects, such as target stealth, spatial filtering, electromagnetic shielding, beam control, etc. Based on this, research work on the combination of electromagnetic metamaterials and corner reflectors has also been proposed. By replacing the metal surface structure of the corner reflector with a metamaterial structure, the reflection direction of the reflected wave can be controlled to achieve the effect of RCS enhancement. Moreover, by combining polarization conversion metamaterials with corner reflectors, a new type of variable polarization corner reflector structure for polarization SAR calibration can be designed. Although the application of electromagnetic metamaterials to corner reflectors has achieved different electromagnetic functions, the electromagnetic metamaterials applied are still mainly in the passive form, and once processed, the frequency response cannot be changed, lacking the real-time and flexibility of regulating electromagnetic waves.

[0004] As a hot spot of electromagnetic metamaterial research, active metasurface technology has the ability to reconfigure and can realize real-time manipulation of electromagnetic waves under external excitation, thus attracting extensive research. However, the research on active metasurfaces mainly focuses on quasi-static conditions with only one control function. The influence of time-varying modulation on the interaction mechanism between active metasurfaces and electromagnetic waves is rarely studied. In recent years, time-modulated metasurfaces have been proposed, which reveal that time modulation can generate a series of frequency harmonics and achieve harmonic spectrum transformation of scattered signals by taking time modulation as a modulation parameter of active metasurfaces. At the same time, digital coding and programmable metasurfaces have also been extensively studied. By using different digital coding waveforms, different electromagnetic functions such as beam control and scattering manipulation can be achieved. The application of time-domain digital coding metasurfaces in wireless communication systems has also been deeply explored. However, the application of time-modulated metasurfaces in radar target feature manipulation is rarely studied, and the modulation methods of the above researches mainly focus on periodic modulation, and the research on other waveform modulation is still lacking. Especially in the current design of time-modulated metasurfaces, the polarization information is often ignored, and most time-modulated metasurfaces can only work in a specific single polarization state, which significantly reduces the ability to realize complex electromagnetic functions and multi-information manipulation. Moreover, in the aspect of single-station RCS regulation, the current time-modulated metasurfaces are in a planar state, which only has a regulation effect in the vertical normal incidence direction, severely limiting its application. Therefore, in practical applications, many influencing factors have not been considered, such as wide-angle stability and polarization insensitivity, which significantly reduce the applicability of time-modulated metasurfaces and hinder their development.

[0005] In view of the problems existing in the current corner reflector and time-modulated metasurface, inspired by the wide-angle stability of the corner reflector and the flexibility of the time-modulated metasurface regulation, by combining the corner reflector structure and the time-modulated metasurface, the incident electromagnetic wave can be quickly and real-time regulated, solving the problem of insufficient flexibility and diversity of traditional corner reflectors in electromagnetic regulation. Moreover, by combining the time-modulated metasurface with the corner reflector, the wide-angle stability of the time-modulated metasurface can also be effectively improved, which can manipulate the harmonic spectrum distribution of the back-reflected electromagnetic wave in a wide angle range, further expanding the practical application ability of the time-modulated metasurface. SUMMARY

[0006] An object of the present application is to provide a time-modulated corner reflector based on a non-periodic time-modulated metasurface, which can quickly and real-time regulate the incident electromagnetic wave, solve the problem of insufficient flexibility and diversity of traditional corner reflectors in electromagnetic regulation. By combining the time-modulated metasurface with the corner reflector, the wide-angle stability of the time-modulated metasurface can also be effectively improved, which can manipulate the harmonic spectrum distribution of the back-reflected electromagnetic wave in a wide angle range.

[0007] Another object of the present application is to provide a method capable of realizing continuous harmonic spectrum transformation of incident electromagnetic waves in a wide angular domain range. Based on the design of time-modulated angular reflector, the effect of non-periodic time-modulated angular reflector on the modulation of incident electromagnetic waves is explored, and the time-modulated angular reflector with rapid dynamic tuning over time can manipulate the echo signal in a wide angular domain range. The present application establishes a non-periodic time-modulation model from the signal level, and realizes the effect of reflective continuous harmonic spectrum transformation.

[0008] The technical scheme adopted by the present application to solve the above technical problems is:

[0009] A time-modulated angular reflector with wide-angle electromagnetic control, which can realize real-time fast dynamic control of incident electromagnetic waves by replacing one or more metal plate structures of a traditional angular reflector with an active tunable metasurface structure. It mainly includes an active tunable metasurface, a three-sided angular reflector structure, and a field programmable gate array (FPGA) time-modulation control system matched therewith.

[0010] The time-modulated metasurface is a single plane structure, mainly composed of an active tunable metasurface and a field programmable gate array (FPGA) time-modulation control system.

[0011] The overall structure of the active tunable metasurface includes an upper surface metal layer, a dielectric substrate, a lower surface metal layer, and a metallized via connecting the upper and lower surface metal layers, and two PIN diodes welded on the upper and lower surfaces. The upper and lower surface metal layers are each composed of a plurality of two-dimensional periodic array distributed metasurface metal units, and are printed on the upper and lower surfaces of the dielectric substrate.

[0012] The time-modulated metasurface contains two pairs of control interfaces, which can be connected to the FPGA time-modulation control system and can be real-time encoded and driven by the FPGA time-modulation control system. When the two pairs of control interfaces are driven simultaneously, the metasurface can realize polarization-insensitive electromagnetic switching function, and can simultaneously manipulate all polarized incident electromagnetic waves, which has excellent applicability in practical applications.

[0013] The three-sided angular reflector structure is composed of two or three mutually perpendicular metal plates or metal frames, and by mounting the active tunable metasurface on the metal plate or metal frame, it can realize strong back reflection control function of radar incident signals in a wide angle range. For electromagnetic waves incident on the angular reflector in any direction in space, after two or three reflections, they can be reflected back along the original incident direction.

[0014] The active tunable metasurface loaded on the corner reflector must have good angle stability and polarization insensitivity to play a good electromagnetic regulation effect in the corner reflector.

[0015] The upper surface metal layer and the lower surface metal layer in the active tunable metasurface adopt a shape structure of a metal meander line. r The metal meander line structure can increase the equivalent resonance length of the metal strip, make the unit structure more compact, realize the miniaturization of the unit, and thus improve the angle stability of the metasurface. The metal meander line structure is composed of four identical meander lines, which are respectively printed on the two sides of the dielectric substrate and are connected through four metallized vias. The four meander lines can be rotated by 90°, 180° and 270° respectively through one of the meander lines. Therefore, the designed unit structure has good symmetry, and the metasurface structure can exhibit excellent polarization insensitivity.

[0016] The FPGA time modulation control system mainly includes a host computer and a FPGA controller. The FPGA controller is physically and signal connected with the time modulation corner reflector through an expansion interface. In order to generate a time modulation code sequence for controlling the time modulation corner reflector, the host computer first generates a binary code instruction, which is then transmitted to the FPGA controller through Ethernet to generate a time-varying direct current bias voltage required for controlling the reflection coefficient change of the time modulation corner reflector.

[0017] The two PIN diodes in the active tunable metasurface are respectively integrated on the upper and lower sides of the dielectric substrate, and are used to connect the horizontal and vertical metal strips to realize the regulation of all polarized incident electromagnetic waves and have polarization-insensitive electromagnetic switchable characteristics.

[0018] The active tunable metasurface is composed of a plurality of active tunable metasurface units arranged in a two-dimensional periodic array. The active tunable metasurface units are electrically connected to each other and adopt a series-parallel combination mode for PIN diode feeding. The active tunable metasurface unit structure itself is used as a bias feed line for the PIN diode, which reduces the additional feeding network and avoids the negative impact of the additional feeding network on the overall performance of the metasurface.

[0019] The time modulation corner reflector can realize two opposite scattering state switches. By using the FPGA time modulation control system output to control the bias voltage time sequence required by the time modulation corner reflector, the time modulation corner reflector can quickly and dynamically switch between the low scattering state and the high scattering state. The time modulation corner reflector has a double-bit encoding function, in which the binary encoding state: "0" represents the off state of the diode without bias, and "1" represents the on state of the diode with forward bias.

[0020] The application further provides a method for realizing continuous harmonic spectrum transformation of incident electromagnetic waves in a wide angular domain. Specifically, based on the design of the time modulation corner reflector, the time modulation corner reflector is controlled by using a non-periodic time modulation sequence, and the time modulation corner reflector which is quickly and dynamically tuned with time can manipulate the echo signal in a wide angular domain, thereby realizing the effect of wide angular domain reflection type continuous harmonic spectrum transformation.

[0021] To realize the above-mentioned continuous harmonic spectrum transformation method in a wide angular domain, the application uses the FPGA time modulation control system to generate a non-periodic time modulation waveform to control the time modulation corner reflector to modulate signals incident from different directions, so as to realize wide angular domain harmonic control. And through random coding time modulation, the echo signal produces continuous spectrum distribution.

[0022] Specifically, a continuous harmonic spectrum transformation method based on a non-periodic time modulation corner reflector includes the following steps:

[0023] Step 1: Wide angular domain time modulation corner reflector design

[0024] The wide angular domain time modulation corner reflector device is mainly composed of an active tunable metasurface, a three-sided corner reflector structure, and a FPGA time modulation control system matched therewith. By replacing the traditional corner reflector with one or more metal plate structures with an active tunable metasurface, and then being driven by the FPGA time modulation control system in real time, the incident electromagnetic waves can be quickly coded and controlled. It can not only obtain the ability to control electromagnetic waves in real time, but also has the advantage of being able to control the target single station RCS in a wide angular domain.

[0025] The FPGA time modulation control system is mainly composed of a host computer and a FPGA controller. In order to generate a time modulation coding sequence for controlling the corner reflector, first, the host computer generates a binary coding instruction containing "0" and "1" sequence, and then transmits it to the FPGA controller through Ethernet.

[0026] Secondly, after receiving the encoding instruction, the FPGA controller generates the time-varying DC bias voltage required to control the change of the reflection coefficient of the corner reflector to drive the PIN diode. In the FPGA controller, with respect to the binary encoding sequence, the encoding "1" corresponds to the forward bias voltage of the PIN diode in the on state, and the encoding "0" corresponds to the zero bias voltage of the PIN diode in the off state. Therefore, the time-varying bias voltage can make the PIN diode quickly switch between the on and off states (the switching rate can reach the level of hundreds of nanoseconds). The FPGA time modulation control system includes an expansion interface circuit for connecting to control the active tunable metasurface of the corner reflector.

[0027] Finally, the time-varying bias voltage generated by the FPGA time modulation control system is loaded to the positive and negative poles of the new type of corner reflector to drive the new type of corner reflector to quickly switch between the low scattering and high scattering states. Among them, the on state of the PIN diode corresponds to the low scattering state of the corner reflector, and the off state corresponds to the high scattering state of the corner reflector.

[0028] Step two: establishment of aperiodic time modulation model based on wide-angle domain time modulation corner reflector

[0029] The new type of corner reflector loaded with an active tunable metasurface has a wide-band electromagnetic modulation characteristic. By changing the bias voltage loaded at both ends, the amplitude of the reflected echo can be modulated to work in two opposite scattering states (strong scattering state and weak scattering state). Therefore, according to the amplitude modulation characteristics of the proposed new type of corner reflector and the time modulation theory, an equivalent time modulation model for amplitude modulation is established.

[0030] At the same time, the frequency range in which the amplitude modulation difference of the reflection coefficient of the new type of corner reflector under different bias states is greater than 10 dB is taken as the measurement standard of the working bandwidth, and the corresponding "1-a" amplitude modulation model is established in combination with time modulation. Wherein "1" represents the normalized amplitude coefficient of the new type of corner reflector in the strong scattering state, and "a" represents the amplitude coefficient of the new type of corner reflector in the weak scattering state.

[0031] The relationship between the amplitude coefficient "a" and the reflection coefficient amplitude change difference ΔR is:

[0032] a=10 ΔR / 20

[0033] Wherein, the reflection coefficient amplitude change difference ΔR=R1-R2, wherein R1 represents the reflection coefficient amplitude of the new type of corner reflector in the weak scattering state, and R2 represents the reflection coefficient amplitude of the new type of corner reflector in the strong scattering state.

[0034] The time-modulated corner reflector controls the amplitude variation of the reflection coefficient of the corner reflector by using a time function, so that the corner reflector exhibits electromagnetic switching characteristics varying with time, and then the corner reflector is used on the incident signal to form a target echo signal varying with time.

[0035] In the time-modulated metasurface, the amplitude-frequency characteristics of the echo signal based on periodic modulation have been studied a lot. The present application draws on the idea of non-periodic modulation in signal processing, adjusts the reflection characteristics of the time-modulated corner reflector by generating a pseudo-random code sequence, non-periodically modulates the incident electromagnetic wave, and analyzes the effect of non-periodic modulation on the incident electromagnetic wave.

[0036] Step three: continuous harmonic spectrum transformation method based on wide-angle domain time-modulated corner reflector

[0037] For continuous harmonic spectrum transformation, a non-periodic coded modulation waveform is applied to the wide-angle domain time-modulated corner reflector, and the spectrum of the modulation signal contains a series of continuous harmonic frequency components, and the amplitude envelope obeys the sinc function distribution. When the incident signal is irradiated on the time-modulated corner reflector, the incident signal is modulated by the time-modulated corner reflector. Assuming that the non-periodic coded modulation is controlled by a random code sequence A k ∈{1,a}, the random coded rectangular pulse modulation sequence is applied to the time-modulated corner reflector. The spectrum of the random coded modulation is continuous, and the spectrum of the echo signal after modulation is no longer a single pulse located at the center frequency, but a harmonic spectrum envelope containing a series of continuous frequency components, and the spectral amplitude distribution still obeys the sinc function distribution.

[0038] Random coded modulation can realize continuous Doppler modulation, and in the frequency domain, the spectral energy is no longer concentrated at the original center frequency, but is continuously distributed on both sides of the original center frequency, and the spectral energy distribution obeys the sinc function distribution. Random modulation can be regarded as continuous spectrum shift of the incident signal, and compared with the periodic modulation method, the modulation effect on the incident signal is no longer to generate discrete false harmonic peaks, but to generate a continuous strip.

[0039] The present application has the following advantages:

[0040] (1) A non-periodic time-modulated corner reflector is first proposed, which realizes fast and real-time regulation of the incident electromagnetic wave, solves the problem of insufficient flexibility and diversity of traditional corner reflectors in electromagnetic regulation, and effectively improves the wide-angle stability of the time-modulated metasurface by combining the time-modulated metasurface with the corner reflector, so that the harmonic spectrum distribution of the back-reflected electromagnetic wave can be controlled in a wide angle range.

[0041] (2) The continuous harmonic spectrum transformation is realized by using the non-periodic time modulation corner reflector, and it is verified that the non-periodic time modulation can generate continuous false harmonic peaks in fast time. Compared with the mechanical motion modulation of the target, the switching rate of the time modulation corner reflector is faster, generally reaching hundreds of nanoseconds, and the intrapulse regulation of the radar echo can be realized, and different target feature transformation effects are formed in the distance direction.

[0042] (3) Compared with the existing polarization-insensitive electromagnetic regulation materials, most of the units realize the polarization-insensitive electromagnetic characteristics by loading multiple diodes, and the active adjustable metasurface unit designed by the application only contains two PIN diodes, greatly simplifying the structure design, reducing the number of lumped elements, reducing the processing difficulty, and saving the cost. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 It is a three-dimensional topological structure diagram of the active adjustable metasurface and the metasurface unit with angle and polarization insensitivity proposed by the application.

[0044] Figure 2a It is the reflection coefficient of the active adjustable metasurface under the on / off state under the TE polarization for different incident angles.

[0045] Figure 2b It is the reflection coefficient of the active adjustable metasurface under the on / off state under the TM polarization for different incident angles.

[0046] Figure 3a It is the reflection coefficient of the active adjustable metasurface under the on / off state under the TE polarization for different polarization angles.

[0047] Figure 3b It is the reflection coefficient of the active adjustable metasurface under the on / off state under the TM polarization for different polarization angles.

[0048] Figure 4 It is a schematic diagram of the time modulation corner reflector, which has the ability to regulate the incident signal in a wide angle domain.

[0049] Figure 5a It is the reflection coefficient measurement result of the new type of corner reflector prototype loaded with the active adjustable metasurface proposed by the application under different azimuth angles.

[0050] Figure 5b It is the reflection coefficient measurement result of the new type of corner reflector prototype loaded with the active adjustable metasurface proposed by the application under different elevation angles.

[0051] Figure 6It is a schematic diagram of the non-periodic time modulation corner reflector with wide-angle regulation proposed in the application, which can realize continuous harmonic spectrum conversion effect.

[0052] Figure 7 It is the amplitude modulation model of the non-periodic coding waveform of the time modulation corner reflector established by the application.

[0053] Figure 8 It is the single-frequency incident signal and echo signal spectrum, and the single-frequency signal after non-periodic modulation shows continuous Doppler spectrum.

CONCRETE IMPLEMENTATION

[0054] In order to better understand the method of the application, the technical solutions of the application are further described below in combination with the drawings and specific embodiments.

[0055] The application designs and manufactures a novel corner reflector with wide-angle amplitude regulation capability. Based on the amplitude modulation model, the application first needs to design an active tunable metasurface, and then combine it with the corner reflector structure to realize the wide-angle amplitude regulation function.

[0056] The corner reflector can generate a very strong echo signal, and its working principle is mainly that the electromagnetic wave incident to the corner reflector in any direction in space can be reflected back along the original incident direction after two or three reflections. For the electromagnetic control metasurface loaded on the corner reflector, it must have good angle stability and polarization insensitivity, so as to play a good electromagnetic control effect in the corner reflector. Therefore, an electromagnetic control metasurface with good angle stability and polarization insensitivity needs to be designed first.

[0057] Figure 1 The three-dimensional topological structure of the proposed active tunable metasurface and its unit is shown, which is mainly composed of a metal bending line 1, a metallized via 2, a dielectric substrate 3 and a PIN diode 4. The metal bending line is printed on the F4BM dielectric substrate with a relative dielectric constant ε r = 2.2 and a thickness h = 0.25 mm. The curved metal line structure can increase the equivalent resonance length of the metal strip, making the unit structure more compact, and the size of the unit is only 4.1 mm (0.12 λ0), realizing the miniaturization of the unit, so as to improve the angle stability of the metasurface. Four metal bending lines are printed on both sides of the dielectric substrate and are connected through four metallized vias with a diameter of 0.2 mm. Four metal bending lines can be rotated by 90°, 180° and 270° respectively through one of the bending lines. Therefore, the designed unit structure has good rotational symmetry, which can make the metasurface structure show excellent polarization insensitivity.

[0058] Two PIN diodes are integrated on the upper and lower sides of the dielectric substrate, respectively, for connecting the horizontal and vertical metal strips, realizing the control of two orthogonal polarizations. Since all polarized incident electromagnetic waves can be decomposed into horizontal and vertical polarized waves, the super surface unit structure designed in this paper can regulate all polarized incident electromagnetic waves, has polarization-insensitive electromagnetic switchable characteristics, and can realize the switching of two opposite scattering states. Among them, the binary coding state: "0" represents the OFF state of the diode without bias, and "1" represents the ON state of the diode with forward bias. In addition, compared with the existing polarization-insensitive electromagnetic regulation materials, most of the units achieve polarization-insensitive electromagnetic characteristics by loading multiple diodes, while the unit structure designed in this paper only contains two PIN diodes, greatly simplifying the structure design, reducing the number of lumped elements, reducing the processing difficulty, and saving the cost.

[0059] The CST microwave studio software is used to design the geometry of the proposed active tunable metasurface. The geometric parameters of the proposed unit are: a = 4.1 mm, b = 4.1 mm, c = 2.98 mm, d = 2.98 mm, e = 1.6 mm, f = 0.34 mm, g = 0.3 mm, w = 0.22 mm, f = 2.98 mm, k = 0.34 mm, h = 0.25 mm.

[0060] As shown in Figure 1 The total size of the active tunable metasurface prepared by the application is 263 mm x 263 mm, which is composed of 60 x 60 units. Surface mount technology is used to solder the PIN diode. The PIN diode adopts BAR64-02 model with the smallest package TSLP-2-19.

[0061] The active tunable metasurface is composed of a plurality of active tunable metasurface units arranged in a two-dimensional periodic array, and the active tunable metasurface units are electrically connected to each other and adopt a combination of series and parallel to feed the PIN diode. The super surface unit structure itself is used as the bias feed line of the PIN diode, which reduces the additional feed network and avoids the negative impact of the additional feed network on the overall performance of the super surface.

[0062] To demonstrate the electromagnetic modulation characteristics of the proposed active tunable metasurface more intuitively, Fig. 2 shows the reflection coefficient modulation performance of the metasurface for TE and TM polarized waves at different incident angles (the incident wave electric field vector E is perpendicular to the incident plane, and the incident wave magnetic field vector H is parallel to the incident plane, which is a TE polarized wave, also known as a horizontally polarized wave. The incident wave magnetic field vector H is perpendicular to the incident plane, and the incident wave electric field vector E is parallel to the incident plane, which is a TM polarized wave, also known as a vertically polarized wave). As can be seen from the figure, for TE and TM polarized incident waves, the proposed active metasurface can switch between low scattering and high scattering states, with excellent broadband electromagnetic switchable performance. As shown in Fig. 2(a), when the top PIN diode is in the ON state, for TE polarized electromagnetic waves at normal incidence, it shows a low scattering state, and the -10 dB reflection coefficient bandwidth is greater than 3.9 GHz (8 GHz-11.9 GHz). On the contrary, when the PIN diode is in the OFF state, the proposed active tunable metasurface shows the opposite broadband high reflection state. Moreover, the switchable performance of TM polarization is consistent with that of TE polarization, as shown in Fig. 2(b).

[0063] For radar target feature modulation applications, the key factor determining the performance of time-modulated reflectors is the reflection coefficient modulation depth. Therefore, the frequency bandwidth where the difference between the reflection coefficients of the PIN diode in the ON and OFF states is greater than 10 dB is used as a measure of the operating bandwidth. As can be seen from Fig. 2, when the PIN diode is switched between the ON and OFF states, the proposed active metasurface has a working frequency range of 8 GHz-11 GHz with a reflection coefficient difference greater than 10 dB. At the same time, a maximum reflection coefficient difference greater than 40 dB is obtained at 9.79 GHz. Fig. 2 also analyzes the reflection coefficient variation characteristics of the proposed active metasurface at different oblique incident angles. When the incident angle reaches 45°, the frequency response of its reflection coefficient can still maintain stable broadband electromagnetic switchable performance. Therefore, the proposed active metasurface has excellent angle stability.

[0064] To further analyze the applicability of the active tunable metasurface proposed in this paper to different polarized incident electromagnetic waves, the reflection coefficients of the metasurface under different polarization angles are simulated, as shown in FIG. 3. It can be seen from the figure that the proposed structure has stable frequency response to any polarized incident electromagnetic wave. When the PIN diode is switched between the ON and OFF states, the working frequency range with a reflection coefficient difference greater than 10 dB is still 8 GHz-11 GHz. Therefore, the structure can obtain excellent polarization insensitivity characteristics. In summary, by regulating the switching state of the PIN diode embedded in the metasurface structure, broadband electromagnetic switchable characteristics can be achieved, and due to the symmetric miniaturized unit structure form of the proposed active tunable metasurface, excellent angle stability and polarization insensitivity can be obtained. Therefore, the stable electromagnetic regulation characteristics of the active tunable metasurface lay a foundation for the design of the time-modulated angular reflector.

[0065] The planar active metasurface can only realize regulation of the backscattering characteristics of the target in the normal incidence direction due to the mirror principle. To expand the angular response of the planar active tunable metasurface, realize single-station RCS regulation in a wide angular domain, and solve the problem of insufficient flexibility of the electromagnetic regulation effect of the existing angular reflector, the present application further designs a new type of trihedral corner reflector loaded with an active tunable metasurface, and measures the backscattering characteristics thereof.

[0066] Figure 4 The structure of the designed new type of trihedral corner reflector loaded with an active tunable metasurface is shown, mainly including an active tunable metasurface 5 and a trihedral corner reflector structure 6. By measuring the reflection coefficients of the backscattering characteristics of the new type of angular reflector under different bias voltages, the amplitude regulation depth is obtained, the electromagnetic regulation performance is evaluated, and the frequency range with a reflection coefficient difference greater than 10 dB is taken as the working bandwidth. At the same time, the reflection coefficients of the new type of angular reflector under different incident angles are measured, and then the applicability of the new type of angular reflector under wide-angle electromagnetic wave incidence is evaluated.

[0067] The time-modulated angular reflector can realize switching between two opposite scattering states. By using the FPGA time-modulation control system to output the bias voltage time sequence required to control the time-modulated angular reflector, the time-modulated angular reflector can be quickly and dynamically switched between the low scattering state and the high scattering state. The time-modulated angular reflector has a double-bit coding function, in which the binary coding states are: “0” represents the off state of the diode without bias, and “1” represents the on state of the diode with forward bias.

[0068] In order to observe the electromagnetic regulation characteristics of the new corner reflector at different angles, the electromagnetic switching performance of the new corner reflector at different incident angles in the azimuth direction and the elevation direction is measured, as shown in Figure 5. Figure 5 (a) shows the measurement results of the reflection coefficient of the designed new corner reflector in the range of 0-75° in the azimuth angle under different bias states of the PIN diode. As can be seen from the figure, the new corner reflector loaded with an active adjustable super surface has a reflection coefficient amplitude modulation difference of more than 10 dB in the range of 0-75° in the azimuth angle, and can realize good switchable reflection characteristics and good angle stability. In the main lobe direction of the backscattering of the corner reflector (when the azimuth angle is 45°), the frequency range of the reflection coefficient amplitude regulation difference of more than 10 dB is 9-10.5 GHz, and the bandwidth is 1.45 GHz.

[0069] Figure 5 (b) shows the measurement results of the reflection coefficient of the designed new corner reflector in the range of 0-45° in the elevation angle under different bias states of the PIN diode when the azimuth angle is 45°. As can be seen from the figure, the new corner reflector has a reflection coefficient amplitude modulation difference of more than 10 dB in the range of 0-45° in the elevation angle, and can realize good electromagnetic switching characteristics. In the main lobe direction of the corner reflector (when the azimuth angle is 45° and the elevation angle is 45°), the frequency range of the reflection coefficient modulation amplitude difference of more than 10 dB is 9.4-10.7 GHz, and the bandwidth is 1.3 GHz. The above experimental test results show that the new three-face corner reflector loaded with an active adjustable super surface still has excellent electromagnetic regulation function, and can realize good wide-angle domain electromagnetic regulation effect on the monostatic RCS of the target, verifying the effectiveness of the design.

[0070] In order to realize the continuous harmonic spectrum transformation method in a wide angle domain, the time-modulated corner reflector is used to realize the continuous spectrum transformation process. The application first proposes a time-modulated corner reflector with wide-angle domain regulation. A non-periodic time modulation sequence is generated by using an FPGA time modulation control system to control the time-modulated corner reflector to modulate signals incident from different directions, and the effect of wide-angle domain reflection type continuous harmonic spectrum transformation is realized.

[0071] The application relates to three specific steps of wide-angle domain time-modulated corner reflector design, non-periodic time modulation model establishment based on the wide-angle domain time-modulated corner reflector, and continuous harmonic spectrum transformation method based on the wide-angle domain time-modulated corner reflector, and the specific implementation is as follows:

[0072] Step one: wide-angle domain time-modulated corner reflector design

[0073] Figure 6The structure diagram of the designed wide-angle non-periodic time modulation corner reflector is shown. The wide-angle time modulation corner reflector device is mainly composed of an active adjustable metasurface, a three-face corner reflector structure and a matching FPGA time modulation control system.

[0074] The time modulation control system is mainly composed of a host computer and an FPGA controller.

[0075] Secondly, after receiving the coding instruction, the FPGA controller generates the time-varying direct current bias voltage required to control the change of the reflection coefficient of the corner reflector to drive the PIN diode.

[0076] Finally, the time-varying bias voltage generated by the FPGA controller is loaded to the positive and negative electrodes of the new corner reflector to drive the new corner reflector to quickly switch between the low scattering and high scattering states.

[0077] Step two: non-periodic time modulation model based on the wide-angle time modulation corner reflector

[0078] The new corner reflector loaded with the active adjustable metasurface has a wideband electromagnetic modulation characteristic, and by changing the bias voltage loaded at both ends, the amplitude of the reflected echo can be modulated to work in two opposite scattering states (strong scattering state and weak scattering state). Figure 7The new corner reflector is in a high scattering state in the OFF state and in a low scattering state in the ON state. By using the time modulation control system to output time-varying control voltages, the new corner reflector can be switched between the OFF state and the ON state, and thus can be controlled to be quickly and dynamically switched between the low scattering state and the high scattering state.

[0079] Meanwhile, the frequency range in which the difference between the reflection coefficient modulation amplitudes of the new corner reflector in different bias states is greater than 10 dB is taken as a measurement standard of the working bandwidth, and a corresponding "1-a" amplitude modulation model is established in combination with time modulation. "1" represents the normalized amplitude coefficient of the new corner reflector in the strong scattering state, and "a" represents the amplitude coefficient of the new corner reflector in the weak scattering state.

[0080] The relationship between the amplitude coefficient "a" and the reflection coefficient amplitude difference ΔR is as follows:

[0081] a = 10 ΔR / 20

[0082] wherein the reflection coefficient amplitude difference ΔR = R1-R2, wherein R1 represents the reflection coefficient amplitude of the new corner reflector in the weak scattering state, and R2 represents the reflection coefficient amplitude of the new corner reflector in the strong scattering state.

[0083] The time-modulated corner reflector controls the reflection coefficient amplitude of the corner reflector by using a time function, so that the corner reflector exhibits time-varying electromagnetic switching characteristics, and then the corner reflector is applied to an incident signal to form a time-varying target echo signal.

[0084] In the time-modulated metasurface, the amplitude-frequency characteristics of the echo signal based on periodic modulation have been studied a lot. In the present application, the idea of non-periodic modulation in signal processing is used to adjust the reflection characteristics of the time-modulated corner reflector by generating a pseudo-random code sequence, to non-periodically modulate the incident electromagnetic wave, and to analyze the effect of non-periodic modulation on the incident electromagnetic wave.

[0085] Step three: continuous harmonic spectrum transformation method based on wide-angle domain time-modulated corner reflector

[0086] Suppose that the incident electromagnetic wave is a single-frequency signal with a frequency of f c , and the time-domain expression is

[0087]

[0088] wherein A is the amplitude coefficient of the incident electromagnetic wave, t is the working time, e is the exponential function, and j represents the imaginary part.

[0089] When the incident signal s(t) illuminates on the time-modulated corner reflector, the incident signal is modulated by the time-modulated corner reflector. Assuming that the non-periodic coded modulation is controlled by a random coding sequence A k ∈{1,a}, the random coded rectangular pulse modulation sequence is applied to the time-modulated corner reflector, and the modulation waveform is as shown in Figure 7 The reflection coefficient time-domain response Γ p (t) of the non-periodic time-modulated corner reflector can be represented as

[0090]

[0091] where τ is the symbol width in the random coding sequence strong scattering state, K is the coding number for generating the pseudo-random sequence, denotes the convolution operation, a represents the amplitude coefficient of the new corner reflector in the weak scattering state, n is a positive integer, and δ is an impulse function.

[0092] The Fourier transform of the above equation can obtain the reflection coefficient spectrum Γ p (f) expression is

[0093]

[0094] where f s =1 / τ represents the modulation frequency, c is the speed of light, n is a positive integer, and δ is an impulse function. For random coded modulation, because the random coding sequence A k exists, an analytical expression in the frequency domain cannot be obtained.

[0095] The modulated reflection signal behaves as a time-varying echo signal, and the echo signal r(t) can be represented as:

[0096]

[0097] According to the Fourier transform relationship, the echo signal r(t) spectrum can be represented as Γ p (f) is brought into it, and the spectrum response of the echo signal can be obtained as:

[0098]

[0099] where S(f) represents the spectrum distribution of the incident electromagnetic wave, and δ is an impulse function.

[0100] Through analysis of the above equation, it can be concluded that the spectrum of the random coded modulation is continuous, and the spectrum of the modulated echo signal is no longer a single pulse located at the center frequency, but a harmonic spectrum envelope containing a series of continuous frequency components, and the spectrum amplitude distribution still obeys the sinc function distribution.

[0101] In order to analyze the modulating effect of the non-periodic time modulation on the incident signal more intuitively, a single frequency signal with a center frequency of 10 GHz is used as the incident signal for simulation. Assuming that the time modulation corner reflector is randomly coded modulated with a symbol width τ = 500 ns, the frequency spectrum of the reflected echo signal after being modulated by the non-periodic time modulation corner reflector is shown in FIG. 3. Figure 8 As can be seen from the figure, in addition to the original echo frequency, a series of continuous harmonic frequency envelopes are generated on both sides of the original echo signal, wherein the main lobe width is 4 MHz, and the overall amplitude envelope is subject to a sinc function distribution.

[0102] Therefore, the random coded modulation can realize continuous Doppler modulation, and in the frequency domain, the spectral energy is no longer concentrated at the original center frequency, but is continuously distributed on both sides of the original center frequency, and the spectral energy distribution is subject to a sinc function distribution. Random modulation can be regarded as continuous spectrum shift of the incident signal, and compared with the periodic modulation method, the modulating effect on the incident signal is no longer to generate discrete false harmonic peaks, but to generate a continuous strip.

Claims

1. A method of harmonic spectral transformation of a non-periodic time-modulated corner reflector with wide angular domain control, characterized by, The specific steps are as follows: Step one: Constructing wide-angle domain regulated non-periodic time modulation corner reflector The wide-angle domain regulated non-periodic time modulation corner reflector is composed of an active adjustable metasurface, a three-sided corner reflector structure, and a FPGA time modulation control system; The overall structure of the active adjustable metasurface includes an upper surface metal layer, a dielectric substrate, a lower surface metal layer, and a metallized via connecting the upper and lower surface metal layers and two PIN diodes welded on the upper and lower surfaces; the upper surface metal layer and the lower surface metal layer are both composed of a plurality of metasurface metal units arranged in a two-dimensional periodic array and printed on the upper and lower surfaces of the dielectric substrate; The three-sided corner reflector structure is composed of two or three mutually perpendicular metal plates or metal frames, and by mounting the active adjustable metasurface on the metal plate or metal frame, the radar incident signal can be realized in a large angle range. The back reflection regulation function; for the electromagnetic wave incident in any direction in space to the corner reflector, after two or three reflections, it can be reflected back along the original incident direction; The FPGA time modulation control system is composed of an upper computer and a FPGA controller; in order to generate the time modulation code sequence for controlling the corner reflector, first, the upper computer generates a binary code instruction containing "0" and "1" sequence, and then transmits it to the FPGA controller through Ethernet; Secondly, after receiving the code instruction, the FPGA controller generates the time-varying DC bias voltage required to control the change of the reflection coefficient of the corner reflector to drive the PIN diode; in the FPGA controller, for the binary code sequence, the code "1" corresponds to the forward bias voltage of the PIN diode in the on state, and the code "0" corresponds to the zero bias voltage of the PIN diode in the off state; therefore, the time-varying bias voltage makes the PIN diode quickly switch between the on and off states; the FPGA time modulation control system includes an expansion interface circuit for connecting the corner reflector loaded with the active adjustable metasurface; Finally, the time-varying bias voltage generated by the FPGA time modulation control system is loaded to the positive and negative electrodes of the new corner reflector to drive the new corner reflector to quickly switch between the low scattering and high scattering states; the on state of the PIN diode corresponds to the low scattering state of the corner reflector, and the off state corresponds to the high scattering state of the corner reflector; Step two: Establishing a non-periodic time modulation model based on the wide-angle domain time modulation corner reflector The frequency range of the difference in reflection coefficient modulation amplitude change of the reflector in different bias states greater than 10 dB is used as the measurement standard of the working bandwidth, and a corresponding "1-a" amplitude modulation model is established combined with time modulation; wherein "1" represents the normalized amplitude coefficient of the new corner reflector in the strong scattering state, and "a" represents the amplitude coefficient of the new corner reflector in the weak scattering state; The relationship between the amplitude coefficient "a" and the reflection coefficient amplitude change difference ΔR is: a＝10 ΔR / 20 Wherein, the reflection coefficient amplitude change difference ΔR = R1-R2, wherein R1 represents the new type of corner reflector in weak scattering state of the reflection coefficient amplitude, R2 represents the new type of corner reflector in strong scattering state of the reflection coefficient amplitude; The time modulation corner reflector is used to control the reflection coefficient amplitude change of the corner reflector by using time function, so that it shows the electromagnetic switching characteristics changing with time, and then it is applied to the incident signal to form the target echo signal changing with time; Step three: continuous harmonic spectrum transform method based on wide angle domain time modulation corner reflector For continuous harmonic spectrum transform, aperiodic coded modulation waveform is applied to wide-angle time-modulated corner reflector, whose modulation signal spectrum contains a series of continuous harmonic frequency components, and the amplitude envelope obeys the sinc function distribution; when the incident signal illuminates on the time-modulated corner reflector, the incident signal is modulated by the time-modulated corner reflector; assuming that the aperiodic coded modulation is controlled by a random coding sequence A k ∈{1,a} control, the random coding rectangular pulse modulation sequence is applied to the time-modulated corner reflector; the spectrum of the random coding modulation is continuous, and the spectrum of the modulated echo signal is no longer a single pulse at the center frequency, but a harmonic spectrum envelope containing a series of continuous frequency components, and the spectral amplitude distribution still obeys the sinc function distribution.

2. A method of harmonic spectral transformation of aperiodic time-modulated angular reflector with wide-angle control according to claim 1, characterized by that: The random coding modulation realizes continuous Doppler modulation, and in the frequency domain, the spectrum energy is no longer concentrated at the original center frequency, but is continuously distributed on both sides of the original center frequency, and the spectrum energy distribution obeys the sinc function distribution; Random modulation is regarded as continuous spectrum shift of the incident signal, compared with the periodic modulation method, the modulation effect of the incident signal is no longer to generate discrete false harmonic peaks, but to generate a continuous strip.

3. A method of harmonic spectral transformation of aperiodic time-modulated corner reflector with wide-angle control according to claim 1, characterized in that: Let the incident electromagnetic wave be a single-frequency signal with frequency f c whose time-domain expression is Wherein, A is the amplitude coefficient of the incident electromagnetic wave, t is the working time, e is the exponential function, and j represents the imaginary part; When the incident signal s(t) is irradiated on the time-modulated corner reflector, the incident signal is modulated by the time-modulated corner reflector; assuming that the non-periodic coded modulation is subjected to a random coding sequence A k ∈{1,a} control, the random coding rectangular pulse modulation sequence is applied to the time-modulated corner reflector; the reflection coefficient time domain response Γ of the non-periodic time-modulated corner reflector p (t) is expressed as where τ is the symbol width at the strong scattering state of the random code sequence, K is the code number for generating the pseudo-random sequence, denotes the convolution operation, a denotes the amplitude coefficient of the new corner reflector at the weak scattering state, n is a positive integer, and δ is an impulse function; Taking the Fourier transform of the above equation, the reflection coefficient spectrum Γ is obtained p (f) the expression is where f s = 1 / τ represents the modulation frequency, c is the speed of light, n is a positive integer, and δ is the impulse function; for random coding modulation, because the random coding sequence A k exists, and the frequency domain analytical expression cannot be obtained; The modulated reflection signal shows the time-varying echo signal, and the echo signal r(t) is represented as: According to the Fourier transform relation, the echo signal r(t) is represented in the frequency domain as Substituting Γ p (f) into which gives the spectral response of the echo signal as Wherein, S(f) represents the spectrum distribution of the incident electromagnetic wave, and δ is the impulse function.

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