High-isolation design method for broadband local oscillator of dechirp imaging system

Abstract

The invention discloses a dechirp imaging system broadband local oscillator high isolation design method. According to the method, broadband local oscillator high isolation of a dechirp system ISAR imaging radar system can be achieved. The dechirp system ISAR imaging radar system transmitting branch comprises a narrowband LFM signal source, an up-conversion and frequency multiplication device and a power amplification device. The receiving branch comprises an amplitude-limiting low-noise amplifier, a down-conversion unit, a filtering unit, an ADC acquisition unit and an imaging processor; wherein broadband pulse emission waveform design and broadband LFM dechirp local oscillator time division generation are generated by sharing the same up-conversion and frequency multiplication module; the frequency difference between the broadband pulse emission waveform and the broadband LFM dechirp local oscillator is kept; and performing digital time-frequency domain leakage cancellation on the signal acquired by the ADC. The dechirping system ISAR imaging radar system broadband local oscillator high-isolation method can simply, conveniently and efficiently meet dechirping system ISAR imaging radar system broadband local oscillator high-isolation implementation, avoids the problem of imaging quality reduction caused by signal leakage, is low in hardware implementation requirement, high in universality and good in expandability, and can meet the long-distance high-resolution imaging requirement.

Description

Technical Field

[0001] The present invention belongs to the field of non-cooperative target ISAR imaging, and relates to a broadband local oscillator high isolation design method for a de-chirp imaging system. Background Art

[0002] The microwave de-chirp ISAR imaging radar system can achieve long-distance high-resolution imaging of non-cooperative targets. The imaging resolution is independent of the distance, and the resolution can reach the centimeter level. The operation is not affected by the illumination conditions.

[0003] The working target of the microwave de-chirp ISAR imaging radar system can be as far as thousands of kilometers away. The system has extremely high imaging sensitivity and can image echoes with an echo power of -168 dBm. Therefore, the isolation requirements for various signals in the system are very high. The isolation degree requirement for the broadband local oscillator signal of its imaging system is as high as 180 dB, and it is very difficult to meet such high isolation requirements only by relying on hardware isolation design.

[0004] After researching the relevant materials on the methods for achieving high isolation in domestic and foreign radar systems that have been publicly disclosed, on the one hand, the publicly available materials found in China are different from the microwave de-chirp ISAR imaging radar system, and the applicability is very limited. Each foreign literature does not give a specific method for achieving high isolation, and fundamentally cannot solve the problem of achieving high isolation in the de-chirp ISAR imaging radar system. Summary of the Invention

[0005] Aiming at the deficiencies existing in the prior art, the purpose of the present invention is to provide a broadband local oscillator high isolation design method for a de-chirp imaging system. Aiming at the problem of high isolation design of the broadband local oscillator in the microwave de-chirp ISAR imaging radar system, a high isolation index is achieved by combining hardware system isolation with software algorithms, which can simply and efficiently meet the high isolation implementation of the broadband local oscillator in the de-chirp ISAR imaging radar system, avoid the problem of image quality degradation caused by leaked signals, and has the advantages of low requirements for hardware implementation, strong versatility, and good scalability. It can meet the requirements of long-distance high-resolution imaging and is of great significance for improving the quality of target images.

[0006] In order to solve the above technical problems, the present invention is implemented by adopting the following technical solutions: A method for designing high isolation of broadband local oscillator of de-slanted imaging system can realize high isolation of broadband local oscillator of de-slanted ISAR imaging radar system; the method comprises: a signal source of the de-slanted ISAR imaging radar system outputs a narrowband LFM signal, modulates the signal to a radio frequency band through an up-conversion and frequency multiplication module, and multiplies the narrowband LFM signal to a broadband LFM signal to generate a broadband pulse transmission waveform, which enters a transceiver switch after power amplification and is output to an antenna to directional illuminate a target, and a target reflection echo is received by an antenna and enters a transceiver switch and is output to a limiting low noise amplifier for amplification; the signal source outputs a narrowband LFM signal again at the same time as the target reflection echo, which is switched to a broadband LFM de-slanted local oscillator signal branch through a switch after up-conversion and frequency multiplication, and is input into a down-conversion module for de-slanted down-conversion, and then into a filtering frequency conversion module for filtering down-conversion, and then into an anti-aliasing filter for filtering, and then into an ADC acquisition module for analog signal acquisition and conversion into a digital signal, and then into an imaging processor for imaging processing to obtain a high-resolution image of the target; wherein: The broadband pulse transmission waveform is time-division-generated with the broadband LFM de-skewed local oscillator, and is generated by sharing the same up-conversion and frequency multiplication module; There is a frequency difference between the broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator, so that the leakage signal and the echo signal before ADC acquisition are both within the passband of the anti-aliasing filter; The signal collected by the ADC is subjected to digital time-frequency domain leakage cancellation.

[0007] The present invention also includes the following technical features: Specifically, if the leakage signal and the echo signal before the ADC acquisition are both within the passband of the anti-aliasing filter, then the following conditions must be met: (a) , , so that the leakage signal is located within the anti-aliasing filter, rather than suppressing the leakage signal out of band; In the above formula, is the anti-aliasing filter front echo center frequency, is the leakage signal frequency before the anti-aliasing filter, is the anti-aliasing filter center frequency, is the -1dB bandwidth of the anti-aliasing filter; (b) = , so that the frequencies of the echo and leakage signals are staggered as far as possible within the passband of the anti-aliasing filter; In the above formula, The center frequency of the broadband LFM de-skewed local oscillator signal is generated by time-sharing the narrowband LFM signal source. Generate the transmit signal center frequency for the narrowband LFM signal source, is the frequency multiplication factor for transmitting up-conversion and frequency multiplication, is the frequency difference between the echo center frequency after de-chirp down-conversion and the leakage signal frequency after de-chirp down-conversion.

[0008] Specifically, the signal after ADC acquisition is subjected to digital time-frequency domain leakage cancellation, including: First, the i-th pulse received signal after acquisition is accumulated in the time domain, where n is the discrete time, n = 1, 2,..., and the number of accumulated pulses is set to The accumulated signal is expressed as:

[0009] Then is Fourier-transformed to the frequency domain for filtering, and the filter response is After filtering, it is inverse Fourier-transformed to the time domain, and the leakage signal is output, and the mathematical expression is:

[0010] Among them, is expressed as:

[0011] Among them, is the leakage signal bandwidth; Subsequently, leakage cancellation is first performed in the time domain, and the signal after cancellation is expressed as: = -

[0012] is the imaging signal after leakage cancellation, and is input to the subsequent imaging processing flow to perform imaging processing.

[0013] Specifically, in the de-chirp ISAR imaging radar system, the frequency synthesizer provides the clock signals required by each frequency conversion module.

[0014] Compared with the prior art, the present invention has the following technical effects: The present invention is applicable to the design of high isolation in the de-chirp ISAR imaging radar system, meets the design requirements of high isolation for broadband signals in the imaging system, and can be extended and applied to the high isolation design of the de-chirp receiving radar system.

[0015] The present invention abandons the traditional direct hardware high isolation design method. By cleverly staggering the frequencies of the leakage signal and the pulse echo signal, while at the same time ensuring that both the leakage signal and the echo signal are collected within the passband of the receiving channel and performing cancellation processing in the digital domain, it reduces the implementation index of the hardware system. At the same time, it improves the implementation index of the system isolation degree, providing a method for high-sensitivity imaging processing of tracking imaging radar. Brief Description of the Drawings

[0016] Figure 1 It is a schematic diagram of the implementation of the broadband local oscillator high isolation design of the dechirping ISAR imaging radar system in the present invention.

[0017] Figure 2 It is a schematic diagram of the specific RF branch design for generating a broadband pulse transmission waveform by sharing the transmission branch in the present invention.

[0018] Figure 3 It is a schematic diagram of the implementation result of the broadband local oscillator high isolation design of the dechirping ISAR imaging radar system in the present invention. Detailed Implementation Manner

[0019] The present invention provides a method for designing a broadband local oscillator with high isolation for a dechirping imaging system. This method can achieve high isolation of the broadband local oscillator in a dechirping ISAR imaging radar system. The imaging resolution of the system is centimeter-level, and the operating range is in the thousands of kilometers. It adopts a broadband LFM dechirping receiving system. The signal source of the dechirping ISAR imaging radar system outputs a narrowband LFM signal, which is modulated to the RF band through an up-conversion and frequency multiplication module, and the narrowband LFM signal is frequency-multiplied to a broadband LFM signal to generate a broadband pulse transmission waveform. After power amplification, it enters the transceiver switch and is output to the antenna for directional illumination of the target. The target reflected echo is received by the antenna, enters the transceiver switch, and is output to the limiting low-noise amplifier for amplification. At the same time as the target reflected echo, the signal source outputs a narrowband LFM signal again. After up-conversion and frequency multiplication, it passes through a switch and is switched to the broadband LFM dechirping local oscillator signal branch. It is input to the down-conversion module together with the echo amplified by the limiting low-noise amplifier for dechirping down-conversion, then input to the filter and frequency conversion module for filtering down-conversion, then input to the anti-aliasing filter for filtering, and then input to the analog-to-digital conversion ADC acquisition module for analog signal acquisition and conversion into digital signals, which are input to the imaging processor for imaging processing to obtain a high-resolution image of the target. The frequency synthesizer provides the clock signals required by each frequency conversion module in the radar system. Among them: (1) The transmission waveform of the dechirping ISAR imaging radar system is a linear frequency modulation (LFM) pulse transmission waveform. The first local oscillator for reception uses a broadband LFM dechirping local oscillator. The broadband pulse transmission waveform and the broadband LFM dechirping local oscillator are generated in a time-division manner. The narrowband LFM signal is generated by the narrowband LFM signal source in the transmission branch and shares the same up-conversion and frequency multiplication module.

[0020] (2) There is a frequency difference between the designed broadband pulse emission waveform and the broadband LFM dechirping local oscillator, so that before the ADC acquisition, both the leakage signal leaked from the emission and the echo signal are within the passband of the anti-aliasing filter, and the anti-aliasing filter does not suppress the leakage signal.

[0021] The narrowband LFM signal source generates an emission signal with a center frequency of , a bandwidth of , and the frequency multiplication factor of the emission up-conversion and frequency multiplication is , then the bandwidth of the emission signal after up-conversion and frequency multiplication is , and let the center frequency of the emission signal after up-conversion and frequency multiplication be .

[0022] The broadband LFM dechirping local oscillator generates a common up-conversion and frequency multiplication branch. Let the frequency of the conversion local oscillator be , and the center frequency of the signal when the narrowband LFM signal source generates the broadband LFM dechirping local oscillator time-divisionally is , then the center frequency of the broadband LFM dechirping local oscillator is .

[0023] The frequency difference between the time-divisionally generated LFM broadband pulse emission waveform and the broadband LFM dechirping local oscillator or the center frequency of the echo after dechirping down-conversion is:

[0024] The center frequency of the leakage signal when generating the broadband LFM dechirping local oscillator is , then the frequency of the leakage signal after dechirping down-conversion is .

[0025] The center frequency of the echo after dechirping down-conversion and the frequency of the leakage signal after dechirping down-conversion , the difference between the two frequencies is . Generally, the echo and the leakage signal after dechirping down-conversion need to be down-converted and filtered again. Denote the center frequency of the echo before the second down-conversion and the anti-aliasing filter as and the frequency of the leakage signal as , and the output echo and signal are sent to the ADC for acquisition after passing through the anti-aliasing filter.

[0026] Let the center frequency of the anti-aliasing filter be , and the -1dB bandwidth of the anti-aliasing filter be , then the design needs to meet: (a) , , characterized in that the leakage signal is located within the anti-aliasing filter, rather than suppressing the leakage signal outside the band; (b) = , characterized in that the design is such that the target echo and the leakage signal are offset by as far a frequency as possible within the passband of the anti-aliasing filter.

[0027] (3)Design digital time-frequency domain leakage cancellation after ADC acquisition.

[0028] Perform cancellation on the signal after ADC acquisition in the digital time-frequency domain. First, accumulate the i-th pulse received signal after acquisition in the time domain, where n is the discrete time, n = 1, 2,..., and the number of accumulated pulses is set to , and the accumulated signal is expressed as:

[0029] Then is Fourier-transformed (FFT) to the frequency domain for filtering, and the filter response is , and after filtering, it is inverse Fourier-transformed (IFFT) to the time domain to output the leakage signal , and the mathematical expression is:

[0030] Among them, is expressed as:

[0031] Among them, is the leakage signal bandwidth, which is obtained by testing with a spectrum analyzer during the system test.

[0032] Subsequently, perform leakage cancellation first in the time domain, and the signal after cancellation is expressed as: = -

[0033] is the imaging signal after leakage cancellation, and is input into the subsequent imaging processing flow to implement imaging processing.

[0034] The present invention is applicable to the design for achieving high isolation in a dechirping ISAR imaging radar system, meets the design requirements for high isolation of broadband signals in the imaging system, and can be extended and applied to the high isolation design of radar systems with a dechirping receiving system.

[0035] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent transformations based on the technical solutions of this application fall within the protection scope of the present invention.

[0036] Embodiment: This embodiment provides a broadband local oscillator high isolation design method for the dechirping imaging system, which can achieve high isolation of the broadband local oscillator in the dechirping ISAR imaging radar system. For a microwave tracking ISAR imaging system with a typical resolution of 5 cm, the bandwidth of the transmitted signal needs to be greater than 4 GHz. The sampling rate of the echo analog-to-digital conversion requires at least 4.8 GHz for quadrature IQ sampling and at least 9.6 GHz for single-channel intermediate frequency acquisition. This poses a huge challenge to the design of the acquisition circuit and is extremely difficult to implement. The tracking ISAR imaging radar utilizes the characteristics of short target echo duration and long duration of each transmitted waveform. By transmitting a broadband linear frequency modulation signal (LFM), the local oscillator uses a broadband linear frequency modulation local oscillator. In the receiver, the received signal is dechirped and compressed, and the input signal entering the acquisition circuit can complete the digital acquisition of the 4 GHz echo dechirping output with a sampling rate of dozens of MHz. The acquired signal is then input into the imaging processor to complete the high-resolution imaging process.

[0037] Specifically, for the microwave dechirping ISAR imaging radar system, as Figure 1 shown, the signal source of the microwave tracking ISAR imaging system outputs a narrowband LFM signal, which is modulated to the radio frequency band through the up-conversion and frequency multiplication links, and the narrowband LFM signal is frequency multiplied to a broadband LFM signal. After power amplification, it enters the transceiver switch and is output to the antenna for directional illumination of the target. The target reflected echo is received by the antenna, enters the transceiver switch, and is output to the limiting low-noise amplifier for amplification. At the same time as the target signal echo, the signal source outputs a narrowband LFM signal again. After up-conversion and frequency multiplication, it passes through the switch and is switched to the broadband LFM dechirping local oscillator signal branch, and is dechirped and mixed with the amplified echo from the limiting low-noise amplifier to output a narrowband signal. After filtering and amplification, it is input into the ADC for acquisition. The digital echo after ADC acquisition is subjected to imaging processing in the imaging processor to obtain a high-resolution image of the target; the frequency synthesizer provides the coherent local oscillator clock required by the system.

[0038] As Figure 2 shown, the three main design implementation methods of this embodiment are as follows: (1) Design of sharing the same up-conversion and frequency multiplication branch: For a microwave tracking ISAR imaging system with a typical resolution of 5 cm, the bandwidth of the transmitted signal needs to be greater than 4 GHz, and it operates at a center frequency of 36 GHz. The bandwidth of the narrowband LFM signal output by the signal source is = 600 MHz, and the up-conversion and frequency multiplication links adopt an 8-fold frequency multiplication design. When generating the broadband LFM dechirping local oscillator, it shares the transmission branch with the broadband pulse transmission waveform. The specific radio frequency branch design is asFigure 3 As shown, the narrowband signal source, up-conversion, Filter 1, 8-fold frequency multiplication, Filter 2 and switching circuit therein are shared.

[0039] When the shared transmit branch generates a broadband pulse transmit waveform, the narrowband LFM signal source outputs a narrowband LFM signal to generate a transmit signal with a center frequency of 1.5 GHz and a bandwidth of 500 MHz; after up-conversion, the center frequency is 4.5 GHz and the bandwidth is 500 MHz; the frequency multiplication factor for transmit up-conversion and frequency multiplication is = 8. After 8-fold frequency multiplication, the output signal has a center frequency of 36 GHz and a broadband pulse transmit waveform with a bandwidth of 4 GHz.

[0040] (2) In the design, a frequency difference is maintained between the LFM pulse transmit waveform and the LFM de-chirp local oscillator. The frequency difference is designed such that before ADC acquisition, both the leakage signal and the echo signal are within the passband of the anti-aliasing filter: The broadband LFM de-chirp local oscillator generates the shared up-conversion and frequency multiplication branches, and the frequency of the local oscillator for frequency conversion is 6 GHz. When the narrowband LFM signal source generates the de-chirp local oscillator signal at different times, the center frequency is = 1.49 GHz. Then the center frequency of the broadband LFM de-chirp local oscillator is = 30.08 GHz. The frequency difference between the LFM pulse transmit waveform and the LFM de-chirp local oscillator generated at different times, or the center frequency of the echo after de-chirp down-conversion is 5.92 GHz. Then the frequency of the leakage signal after de-chirp down-conversion is 6 GHz. The center frequency of the echo after de-chirp down-conversion and the frequency of the leakage signal after de-chirp down-conversion differ by 80 MHz. Generally, the echo and the leakage signal after de-chirp down-conversion still need to be down-converted again and output by the filter. Denote the center frequency of the echo before the second down-conversion and the anti-aliasing filter as = 110 MHz and the frequency of the leakage signal as 190 MHz. The output echo and signal are sent to the ADC for acquisition through the anti-aliasing filter. Assume the center frequency of the anti-aliasing filter is = 150 MHz, and the -1dB bandwidth of the anti-aliasing filter is = 80 MHz. The design needs to meet: (a) , characterized in that the leakage signal is within the anti-aliasing filter, rather than suppressing the leakage signal outside the band; (b) , characterized in that the design makes the target echo and the leakage signal as far apart as possible in the passband of the anti-aliasing filter.

[0041] The specific design results are as Figure 3 shown.

[0042] (3)Digital time-frequency domain leakage implementation for cancellation: The number of accumulated pulses is set to = 1024. During the system test, the leakage signal bandwidth obtained by the spectrum analyzer is 1 MHz, which is expressed as:

[0043] Subsequently, leakage cancellation is first performed in the time domain. The signal after cancellation is expressed as:

[0044] is the imaging signal after leakage cancellation and is input into the subsequent imaging processing flow for imaging processing.

[0045] The present invention can be extended and applied to the high isolation design of pulse dechirping receiving system radars; the content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

[0046] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0047] In addition, it should be noted that, in the above specific embodiments, the various specific technical features described can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0048] Furthermore, any combination can be made between the various different embodiments of the present invention as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. A method for designing broadband local oscillator with high isolation for de-slanted imaging system, characterized in that: The method can realize high isolation of broadband local oscillator of de-slanted ISAR imaging radar system; the method comprises: a signal source of the de-slanted ISAR imaging radar system outputs a narrowband LFM signal, modulates the signal to a radio frequency band through an up-conversion and frequency multiplication module, and multiplies the narrowband LFM signal to a broadband LFM signal to generate a broadband pulse transmission waveform, enters a transceiver switch after power amplification, and outputs the waveform to an antenna for directional illumination of a target, and a target reflection echo is received by an antenna and enters a transceiver switch and outputs the waveform to a limited low noise amplifier for amplification; the signal source outputs a narrowband LFM signal again at the same time as the target reflection echo, and switches the signal to a broadband LFM de-slanted local oscillator signal branch through a switch after up-conversion and frequency multiplication, and inputs the echo amplified by the limited low noise amplifier into a down-conversion module for de-slanted down-conversion, and then inputs the filter frequency conversion module for filtering down-conversion, and then inputs the anti-aliasing filter for filtering, and then inputs the ADC acquisition module for analog signal acquisition and conversion into a digital signal, and then inputs the imaging processor for imaging processing to obtain a high-resolution image of the target; wherein: The broadband pulse transmission waveform is time-division-generated with the broadband LFM de-skewed local oscillator, and is generated by sharing the same up-conversion and frequency multiplication module; There is a frequency difference between the broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator, so that the leakage signal and the echo signal before ADC acquisition are both within the passband of the anti-aliasing filter; The signal collected by the ADC is subjected to digital time-frequency domain leakage cancellation.

2. The method for designing a broadband local oscillator with high isolation for a de-slanted imaging system according to claim 1, characterized in that: Before the ADC is collected, the leakage signal and the echo signal are both within the passband of the anti-aliasing filter, which must satisfy the following conditions: (a) , , so that the leakage signal is located within the anti-aliasing filter, rather than suppressing the leakage signal out of band; In the above formula, is the anti-aliasing filter front echo center frequency, is the leakage signal frequency before the anti-aliasing filter, is the anti-aliasing filter center frequency, is the -1dB bandwidth of the anti-aliasing filter; (b) = , so that the frequencies of the echo and leakage signals are staggered as far as possible within the passband of the anti-aliasing filter; In the above formula, The center frequency of the broadband LFM de-skewed local oscillator signal is generated by time-sharing the narrowband LFM signal source. Generate the transmit signal center frequency for the narrowband LFM signal source, is the number of times of transmitting up-conversion and frequency multiplication, It is the frequency difference between the echo center frequency after de-slanting down-conversion and the leakage signal frequency after de-slanting down-conversion.

3. The method for designing a broadband local oscillator with high isolation for a de-slanted imaging system according to claim 1, characterized in that: The signal collected by the ADC is subjected to digital time-frequency domain leakage cancellation, including: firstly, the i-th pulse receiving signal after the collection is Accumulation is performed in the time domain, where n is the discrete time, n=1,2,..., and the number of accumulated pulses is set to , the accumulated signal It is expressed as: Then Perform Fourier transform to frequency domain for filtering, the filter response is , after filtering, perform inverse Fourier transform to the time domain and output the leakage signal , mathematically expressed as: in, It is expressed as: in, is the leakage signal bandwidth; Then, leakage cancellation is first performed in the time domain, and the signal after cancellation It is expressed as: = - The imaging signal after leakage cancellation is input into the subsequent imaging processing flow to implement imaging processing.

4. The method for designing a broadband local oscillator with high isolation for a de-slanted imaging system according to claim 1, characterized in that: In the de-skewing ISAR imaging radar system, the frequency synthesizer provides the clock signal required by each frequency conversion module.

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