A design method for broadband local oscillator with high isolation for de-slanted imaging system

Abstract

The present invention discloses a method for designing high-isolation broadband local oscillators for a de-skewed imaging system. The method can achieve high-isolation broadband local oscillators for de-skewed ISAR imaging radar systems. The de-skewed ISAR imaging radar system's transmitting branch includes a narrowband LFM signal source, up-conversion and frequency multiplication, and power amplification. The receiving branch includes a limiting low-noise amplifier, down-conversion, filtering, ADC acquisition, and an imaging processor. The broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator are designed for time-division generation, generated by sharing the same up-conversion and frequency multiplication module. A frequency difference is maintained between the broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator. Digital time-frequency domain leakage cancellation is performed on the signal acquired by the ADC. The present invention can simply and efficiently meet the requirements for high-isolation broadband local oscillators in de-skewed ISAR imaging radar systems, avoid the problem of imaging quality degradation caused by leakage signals, has low hardware implementation requirements, strong versatility, and good scalability, and can meet the requirements of long-distance high-resolution imaging.

Description

Technical Field

[0001] The invention belongs to the field of ISAR imaging of non-cooperative targets and relates to a method for designing a broadband local oscillator with high isolation for a de-slanted imaging system. Background Art

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

[0003] The microwave de-skewing ISAR imaging radar system can operate at targets up to a thousand kilometers away. The system has extremely high imaging sensitivity and can image echoes with an echo power of -168dBm. Therefore, the isolation requirements for various signals within the system are very high. The imaging system's broadband local oscillator signal isolation is required to be as high as 180dB, which is difficult to achieve with hardware isolation design alone.

[0004] A review of publicly available domestic and international literature on methods for achieving high isolation in radar systems revealed that the available domestic literature differs from microwave de-slant ISAR imaging radar systems and therefore has limited applicability. Foreign literature also fails to provide specific methods for achieving high isolation, fundamentally failing to address the issue of achieving high isolation in de-slant ISAR imaging radar systems. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method for designing high isolation of broadband local oscillators in a de-slanted imaging system. To address the design difficulties of high isolation of broadband local oscillators in microwave de-slanted ISAR imaging radar systems, the method adopts hardware system isolation combined with software algorithms to achieve high isolation indicators. The method can simply and efficiently meet the implementation of high isolation of broadband local oscillators in de-slanted ISAR imaging radar systems, avoid the problem of imaging quality degradation caused by leakage signals, and has the advantages of low hardware implementation requirements, strong versatility, and good scalability. It can meet the requirements of long-distance high-resolution imaging and is of great significance to improving the quality of target images.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A method for designing high-isolation broadband local oscillators for de-slanted imaging systems is disclosed. The method can achieve high-isolation broadband local oscillators for de-slanted ISAR imaging radar systems. 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 frequency multiplies the narrowband LFM signal to a broadband LFM signal to generate a broadband pulse transmission waveform, which is then amplified and then enters a transceiver switch to output to an antenna for directionally irradiating a target. The target reflected echo is received by the antenna, enters the transceiver switch, and is output to a limiting low-noise amplifier for amplification. The signal source again outputs a narrowband LFM signal at the same time as the target reflected echo. The signal is up-converted and frequency multiplied, and then switched to a broadband LFM de-slanted local oscillator signal branch through a switch. The echo amplified by the limiting low-noise amplifier is input to a down-conversion module for de-slanting down-conversion, then input to a filtering and frequency conversion module for filtering and down-conversion, then input to an anti-aliasing filter for filtering, then input to an ADC acquisition module for analog signal acquisition and conversion into a digital signal, which is then input to an imaging processor for imaging processing to obtain a high-resolution image of the target.

[0008] The broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator are generated in time division and are generated by sharing the same up-conversion and frequency multiplication module;

[0009] 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;

[0010] The signal collected by the ADC is subjected to digital time-frequency domain leakage cancellation.

[0011] The present invention also includes the following technical features:

[0012] Specifically, if the leakage signal and the echo signal before the ADC acquisition are both within the passband of the anti-aliasing filter, the following conditions must be met:

[0013] (a) , , so that the leakage signal is located within the anti-aliasing filter, rather than suppressing the leakage signal out of band;

[0014] 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;

[0015] (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;

[0016] In the above formula, The center frequency of the broadband LFM de-skewed local oscillator signal is generated by the narrowband LFM signal source in a time-sharing manner. Generate the transmit signal center frequency for the narrowband LFM signal source, is the frequency multiplication number of the transmission up-conversion and frequency multiplication, It is the frequency difference between the echo center frequency after de-slant down-conversion and the leakage signal frequency after de-slant down-conversion.

[0017] Specifically, the signal collected by the ADC is subjected to digital time-frequency domain leakage cancellation, including: first, the i-th pulse receiving signal after collection is received 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 Expressed as:

[0018]

[0019] 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:

[0020]

[0021] in, Expressed as:

[0022]

[0023] in, is the leakage signal bandwidth;

[0024] Then the leakage cancellation is first performed in the time domain, and the signal after cancellation Expressed as:

[0025] = -

[0026] The imaging signal after leakage cancellation is input into the subsequent imaging processing flow to perform imaging processing.

[0027] Specifically, in the de-skewing ISAR imaging radar system, a frequency synthesizer provides the clock signal required by each frequency conversion module.

[0028] Compared with the prior art, the present invention has the following technical effects:

[0029] The present invention is applicable to the high isolation design of a de-slant ISAR imaging radar system, meets the high isolation design requirements of the imaging system broadband signal, and can be widely applied to the high isolation design of a de-slant receiving radar system.

[0030] 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 keeping the leakage signal and the echo signal collected simultaneously within the passband of the receiving channel, and performing cancellation processing in the digital domain, the hardware system implementation indicators are reduced, and at the same time, the system isolation implementation indicators are improved, providing a method for high-sensitivity imaging processing of tracking imaging radar. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the implementation of the broadband local oscillator high isolation design of the de-skewed ISAR imaging radar system in the present invention.

[0032] Figure 2 This is a schematic diagram of the specific RF branch design for generating a broadband pulse transmission waveform using a common transmission branch in the present invention.

[0033] Figure 3 This is a schematic diagram of the implementation results of the broadband local oscillator high isolation design of the de-skewed ISAR imaging radar system in the present invention. DETAILED DESCRIPTION

[0034] The present invention provides a method for designing a broadband local oscillator with high isolation for a de-slanted imaging system. The method can realize broadband local oscillator high isolation for a de-slanted ISAR imaging radar system. The system has an imaging resolution of the centimeter level and an operating range of the thousand-kilometer level, and adopts a broadband LFM de-slanted receiving system. The signal source of the de-skewing ISAR imaging radar system outputs a narrowband LFM signal, which is modulated to the RF band through the 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 to irradiate the target in a direction. The target reflection echo is received by the antenna and enters the transceiver switch and is output to the limiting low noise amplifier for amplification. At the same time as the target reflection echo, the signal source outputs a narrowband LFM signal again. After up-conversion and frequency multiplication, it is switched to the broadband LFM de-skewing local oscillator signal branch through the switch. The echo amplified by the limiting low noise amplifier is input to the down-conversion module for de-skewing down-conversion, and then input to the filter frequency conversion module for filtering down-conversion, and then input to the anti-aliasing filter for filtering. After that, it is input to the analog-to-digital conversion ADC acquisition module for analog signal acquisition and conversion into digital signals, and then input to the imaging processor for imaging processing to obtain a high-resolution image of the target. The frequency synthesizer provides the clock signal required by each frequency conversion module in the radar system. Among them:

[0035] (1) The transmit waveform of the de-skewed ISAR imaging radar system is a linear frequency modulation (LFM) pulse transmit waveform. The first local oscillator of the receiver adopts a broadband LFM de-skewed local oscillator. The broadband pulse transmit waveform and the broadband LFM de-skewed local oscillator are designed to be time-division generated. The narrowband LFM signal source in the transmit branch is used to generate a narrowband LFM signal and the same up-conversion and frequency multiplication module is used for generation.

[0036] (2) There is a frequency difference between the broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator, so that before the ADC is collected, the leakage signal and the echo signal from the transmission are both within the passband of the anti-aliasing filter, and the anti-aliasing filter does not suppress the leakage signal.

[0037] The narrowband LFM signal source generates a transmission signal with a center frequency of , the bandwidth is , the frequency multiplication times of transmitting up-conversion and frequency multiplication are , then the bandwidth of the transmitted signal after up-conversion and frequency multiplication is , and assume that the center frequency of the transmitted signal after up-conversion and frequency multiplication is .

[0038] The broadband LFM de-skewed local oscillator generates a common up-conversion and frequency multiplication branch. Assume that the frequency of the frequency conversion local oscillator is When the narrowband LFM signal source generates the broadband LFM de-skewed local oscillator in time-sharing mode, the center frequency of the signal is , then the broadband LFM de-skewed local oscillator center frequency is .

[0039] The frequency difference between the time-division generated LFM wideband pulse transmission waveform and the wideband LFM de-skewed local oscillator or the echo center frequency after de-skewed down-conversion for:

[0040]

[0041] The center frequency of the leakage signal when generating broadband LFM de-skewed local oscillator is , then the leakage signal frequency after de-slanting down-conversion is for .

[0042] Echo center frequency after de-slanting and down-conversion and the leakage signal frequency after de-sloping down-conversion , the frequency difference between the two The echo after de-slanting down-conversion and the leakage signal after de-slanting down-conversion generally need to be down-converted and filtered again. The center frequency of the echo before down-conversion and anti-aliasing filter is and the leakage signal frequency is , the output echo and signal are sent to the ADC for acquisition through the anti-aliasing filter.

[0043] Assume the center frequency of the anti-aliasing filter is , the -1dB bandwidth of the anti-aliasing filter is , the design needs to meet:

[0044] (a) , , the characteristic is that the leakage signal is located in the anti-aliasing filter, rather than suppressing the leakage signal out of band;

[0045] (b) = , the characteristic is that it is designed so that the target echo and the leakage signal are staggered as far as possible in the passband of the anti-aliasing filter.

[0046] (3) Design ADC to perform digital time-frequency domain leakage cancellation after acquisition.

[0047] The signal collected by ADC is cancelled in the digital time-frequency domain. First, the i-th pulse receiving signal after 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 Expressed as:

[0048]

[0049] Then Perform Fourier transform (FFT) to the frequency domain for filtering, and the filter response is , after filtering, perform inverse Fourier transform (IFFT) to the time domain and output the leakage signal , mathematically expressed as:

[0050]

[0051] in, Expressed as:

[0052]

[0053] in, This is the leakage signal bandwidth, obtained by spectrum analyzer testing during system testing.

[0054] Then the leakage cancellation is first performed in the time domain, and the signal after cancellation Expressed as:

[0055] = -

[0056] The imaging signal after leakage cancellation is input into the subsequent imaging processing flow to perform imaging processing.

[0057] The present invention is applicable to the high isolation design of a de-slant ISAR imaging radar system, meets the high isolation design requirements of the imaging system broadband signal, and can be widely applied to the high isolation design of a de-slant receiving radar system.

[0058] 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 modifications made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0059] Example:

[0060] This embodiment provides a design method for high-isolation broadband local oscillators (LOs) in a de-skewing imaging system. This method achieves high isolation of broadband LOs in de-skewing ISAR imaging radar systems. Microwave tracking ISAR imaging systems typically have a resolution of 5 cm, requiring a transmit signal bandwidth greater than 4 GHz. The sampling rate for the echo analog-to-digital conversion (ADC) must be at least 4.8 GHz for quadrature IQ sampling, and at least 9.6 GHz for single-channel intermediate frequency acquisition. This presents a significant challenge for acquisition circuit design and is extremely difficult to implement. Tracking ISAR imaging radars exploit the short duration of target echoes and the long duration of each transmitted waveform. By transmitting a broadband linear frequency modulated (LFM) signal, the LO uses a broadband LFM LO. The receiver de-skewing and compresses the input signal entering the acquisition circuit. This allows digital acquisition of the de-skewing 4 GHz echo output with a sampling rate of tens of MHz. The acquired signal is then fed into the imaging processor for high-resolution imaging processing.

[0061] Specifically for microwave de-slant ISAR imaging radar system, such as Figure 1 As shown in the figure, the signal source of the microwave tracking ISAR imaging system outputs a narrowband LFM signal, which is modulated to the RF band through up-conversion and frequency multiplication links, and the narrowband LFM signal is frequency multiplied to a broadband LFM signal after power amplification, 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, which is up-converted and multiplied, and then switched to the broadband LFM de-skewed local oscillator signal branch through a switch. The de-skewed mixed output narrowband signal is filtered and amplified and input to the ADC for acquisition. The digital echo after ADC acquisition is imaged and processed 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.

[0062] like Figure 2 As shown, the three main design implementations of this embodiment are as follows:

[0063] (1) Sharing the same up-conversion and frequency multiplication branch design:

[0064] The typical resolution of the microwave tracking ISAR imaging system is 5cm. The transmission signal bandwidth needs to be greater than 4GHz. It operates at a center frequency of 36GHz. The signal source outputs a narrowband LFM signal with a bandwidth of =600MHz, the up-conversion and frequency multiplication chain adopts 8-fold frequency design. When generating broadband LFM de-skewed local oscillator, it shares the transmission branch with the broadband pulse transmission waveform. The specific RF branch design is as follows Figure 3 As shown, the narrowband signal source, up-conversion, filter 1, 8-times frequency multiplication, filter 2 and switch circuit are shared.

[0065] When the common transmitting branch generates a broadband pulse transmitting waveform, the narrowband LFM signal source outputs a narrowband LFM signal to generate a transmitting signal with a center frequency of is 1.5GHz, and the bandwidth is 500MHz; after up-conversion, the center frequency is 4.5GHz, and the bandwidth is 500MHz; the number of times of transmitting up-conversion and frequency multiplication is =8, output signal center frequency after 8-fold frequency multiplication The waveform is a broadband pulse transmission with a frequency of 36 GHz and a bandwidth of 4 GHz.

[0066] (2) The design maintains a frequency difference between the LFM pulse transmission waveform and the LFM de-skewed local oscillator. The frequency difference is designed so that both the leakage signal and the echo signal are within the passband of the anti-aliasing filter before the ADC is collected:

[0067] The broadband LFM de-skewed LO generates a common up-conversion and frequency multiplication branch, and the frequency conversion LO frequency is The narrowband LFM signal source generates the de-skewed local oscillator in time-sharing mode at a center frequency of 6 GHz. =1.49GHz, then the broadband LFM de-skewed local oscillator center frequency is =30.08GHz. The frequency difference between the time-division generated LFM pulse transmission waveform and the LFM de-skewed local oscillator or the echo center frequency after de-skewed down-conversion is 5.92GHz, then the leakage signal frequency after de-sloping down-conversion is 6GHz. The echo center frequency after de-slanting and down-conversion and the leakage signal frequency after de-sloping down-conversion The frequency difference between the two is 80MHz. The echo after de-slant down-conversion and the leakage signal after de-slant down-conversion generally need to be down-converted and filtered again. The center frequency of the echo before down-conversion and anti-aliasing filter is =110MHz and leakage signal frequency The output echo and signal are sent to the ADC for acquisition through the anti-aliasing filter. Assume that the center frequency of the anti-aliasing filter is =150MHz, the -1dB bandwidth of the anti-aliasing filter is =80MHz, the design needs to meet:

[0068] (a) , the characteristic is that the leakage signal is located in the anti-aliasing filter, rather than suppressing the leakage signal out of band;

[0069] (b) , the characteristic is that it is designed so that the target echo and the leakage signal are staggered as far as possible in the passband of the anti-aliasing filter.

[0070] Specific design results such as Figure 3 shown.

[0071] (3) Digital time-frequency domain leakage cancellation:

[0072] The number of accumulated pulses is set to =1024, the spectrum analyzer is used to obtain the leakage signal bandwidth during system testing. 1MHz, Expressed as:

[0073]

[0074] Then the leakage cancellation is first performed in the time domain, and the signal after cancellation Expressed as:

[0075]

[0076] The imaging signal after leakage cancellation is input into the subsequent imaging processing flow to perform imaging processing.

[0077] The present invention can be extended to apply to the high isolation design of a pulse de-skewed receiving radar system; matters not described in detail in the specification of the present invention belong to the common knowledge of professionals in this field.

[0078] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0079] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0080] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for designing a broadband local oscillator with high isolation for a de-skewed imaging system, characterized in that: The method can achieve high isolation of broadband local oscillators of a 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 frequency multiplies the narrowband LFM signal to a broadband LFM signal to generate a broadband pulse transmission waveform, which is then amplified and then enters a transceiver switch to output to an antenna for directionally irradiating a target, a target reflection echo is received by the antenna, enters the 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 up-converted and frequency multiplied and then switched to a broadband LFM de-slanted local oscillator signal branch through a switch, and the echo amplified by the limiting low-noise amplifier is input to a down-conversion module for de-slanting down-conversion, then input to a filtering and frequency conversion module for filtering and down-conversion, then input to an anti-aliasing filter for filtering, and then input to an ADC acquisition module for analog signal acquisition and conversion into a digital signal, which is then input to an imaging processor for imaging processing to obtain a high-resolution image of the target; wherein: The broadband pulse transmission waveform and the broadband LFM de-skewed local oscillator are generated in time division and are 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; Performing digital time-frequency domain leakage cancellation on the signal collected by the ADC; Before the ADC is collected, the leakage signal and the echo signal are both within the passband of the anti-aliasing filter. This must satisfy the following requirements: (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 the narrowband LFM signal source in a time-sharing manner. Generate the transmit signal center frequency for the narrowband LFM signal source, is the frequency multiplication number of the transmission up-conversion and frequency multiplication, It is the frequency difference between the echo center frequency after de-slant down-conversion and the leakage signal frequency after de-slant down-conversion.

2. The method for designing a broadband local oscillator with high isolation for a de-slanted imaging system according to claim 1, wherein: The signal collected by the ADC is subjected to digital time-frequency domain leakage cancellation, including: first, the i-th pulse receiving signal after the collection is received 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 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, Expressed as: in, is the leakage signal bandwidth; Then the leakage cancellation is first performed in the time domain, and the signal after cancellation Expressed as: = - The imaging signal after leakage cancellation is input into the subsequent imaging processing flow to perform imaging processing.

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

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