CN113253208A - Step frequency radar based on Fourier mode-locked photoelectric oscillator - Google Patents
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- G—PHYSICS
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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Abstract
The invention relates to a step frequency radar based on a Fourier mode locking photoelectric oscillator, which comprises a step frequency signal generating module, a transmitting antenna, a receiving antenna, a step frequency signal demodulating module and a digital signal processing module, wherein the step frequency signal generating module is used for generating a step frequency signal; the output signal generated by the step frequency signal generation module is amplified by a first amplifier and then input into a first power divider, one path of output signal of the first power divider is transmitted by a transmitting antenna, received by a receiving antenna, amplified by a second amplifier and then input into a step frequency signal demodulation module, the other path of output signal of the first power divider is directly input into the step frequency signal demodulation module, and the two paths of input signals are processed by the step frequency signal demodulation module and then output into a digital signal processing module for data processing of radar signals; the step frequency signal generation module forms an annular photoelectric oscillator resonant cavity, and can generate a low-phase noise broadband step frequency output signal when the Fourier domain mode locking condition is met.
Description
Technical Field
The invention relates to a radar device, in particular to a step frequency radar based on a Fourier mode-locked photoelectric oscillator.
Background
Radars are classified into pulse radars and continuous wave radars according to radar signal waveforms. The step frequency continuous wave radar (SFCW) adopts single frequency step continuous wave, has less limitation on the working bandwidth, synthesizes wider bandwidth by using smaller instantaneous bandwidth and can obtain higher distance resolution. The step frequency continuous wave radar is widely applied to the fields of land exploration, micro-deformation monitoring, battlefield monitoring, life detection, vehicle collision prevention and the like.
Thanks to the development of modern frequency synthesis technology, the currently commonly used stepped frequency signal generation technology mainly includes phase-locked loop frequency synthesis technology, direct digital frequency synthesis technology, hybrid frequency synthesis technology, and the like. The indirect frequency synthesis technology formed by the phase-Locked loop (ppl) has the advantages of large output bandwidth, low phase noise and good spurious suppression, and still plays an important role in the field of frequency synthesis until now. However, this frequency synthesis technique has a low frequency resolution and a long frequency switching time, and cannot satisfy the requirements of frequency agility, wavelength parameter agility, and adaptive frequency hopping.
The direct Digital frequency synthesis (DDS) technology appears in the 70 th 20 th century, has the advantages of high frequency resolution, continuous phase change, high frequency conversion speed and the like, and is widely applied to military and civil fields such as radars (frequency agile radars and active phase control radars), communication, medical equipment and the like. However, this frequency synthesis technique also has the disadvantages of low output frequency and poor spurious suppression capability.
Disclosure of Invention
The invention aims to provide a step frequency radar based on a Fourier mode-locked photoelectric oscillator, and aims to solve the problems of low frequency agility speed, high phase noise and poor clutter suppression capability of step frequency in the existing step frequency signal generation technology.
The purpose of the invention is realized as follows: a step frequency radar based on a Fourier mode-locked photoelectric oscillator comprises a step frequency signal generating module, a transmitting antenna, a receiving antenna, a step frequency signal demodulating module and a digital signal processing module;
the output signal generated by the step frequency signal generation module is amplified by a first amplifier and then input into a first power divider, one path of output signal of the first power divider is transmitted by a transmitting antenna, received by a receiving antenna, amplified by a second amplifier and then input into a step frequency signal demodulation module, the other path of output signal of the first power divider is directly input into the step frequency signal demodulation module, and the two paths of input signals are processed by the step frequency signal demodulation module and then output into a digital signal processing module for data processing of radar signals;
the step frequency signal generation module forms an annular photoelectric oscillator resonant cavity, and can generate a low-phase noise broadband step frequency output signal when the Fourier domain mode locking condition is met.
The step frequency signal generating module in the invention comprises:
the laser is connected with the intensity modulator and used for emitting a carrier optical signal;
the intensity modulator is respectively connected with the laser, the photoelectric detector and the broadband filter and is used for modulating the intensity of an optical signal emitted by the laser;
the photoelectric detector is respectively connected with the intensity modulator and the second power divider and is used for performing photoelectric conversion on the carrier optical signal subjected to intensity modulation and demodulating and outputting a radio-frequency signal;
the second power divider is respectively connected with the photoelectric detector, the fast tunable RF filter and the first amplifier and used for outputting radio frequency signals after photoelectric conversion and demodulation;
the fast tunable RF filter is respectively connected with the second power divider and the third amplifier and is used for selecting a radio-frequency signal with tunable frequency from the radio-frequency signals output by the second power divider;
the third amplifier is respectively connected with the fast tunable RF filter and the broadband filter and is used for carrying out gain compensation on the selected radio-frequency signal with tunable frequency so as to compensate attenuation caused by the radio-frequency signal passing through each device and ensure that the gain in the loop is more than 1; and
and the broadband filter is respectively connected with the third amplifier and the intensity modulator, transmits the radio-frequency signal output by the third amplifier to the intensity modulator, and is used for limiting the tuning range of the radio-frequency signal tunable in the resonant cavity of the photoelectric oscillator to be tuned in a certain free spectrum range of the fast tunable RF filter.
The stepping frequency signal generating module forms an annular photoelectric oscillator resonant cavity, and the configuration requirement is that when the Fourier domain mode locking condition is met, a low-phase noise broadband stepping frequency signal can be generated. The optical signal output by the laser is used as an optical carrier, the intensity of the optical signal is modulated by an intensity modulator, the optical signal after intensity modulation is transmitted to the photoelectric detector through a long optical fiber, the signal modulated by the optical signal in the intensity modulator is demodulated after photoelectric conversion, the signal is subjected to low-noise amplification by a third amplifier, the signal is filtered by a broadband filter, and then the signal is input to the intensity modulator, so that a closed positive feedback loop is formed. The optical fiber is used as an energy storage medium due to extremely low transmission loss, so that the photoelectric oscillation loop has a high Q value, and an output radio frequency signal of the photoelectric oscillation loop has good phase noise.
The fast tunable RF filter in the present invention comprises:
the input radio frequency directional coupler is respectively connected with the radio frequency low noise amplifier and the adjustable radio frequency attenuator and is used for receiving the carrier radio frequency signal output by the second power divider and carrying out directional coupling;
the radio frequency low noise amplifier is respectively connected with the input radio frequency directional coupler and the output radio frequency directional coupler and is used for carrying out gain compensation on the carrier radio frequency signals after directional coupling so as to compensate the loss in the ring and enable the gain in the ring to be close to 1 as a whole;
the output radio frequency directional coupler is respectively connected with the radio frequency low noise amplifier and the electric control analog radio frequency phase shifter and is used for carrying out secondary directional coupling and outputting on the carrier radio frequency signal after gain compensation;
the electric control analog radio frequency phase shifter is respectively connected with the output radio frequency directional coupler and the adjustable radio frequency attenuator and is used for enabling the phase of the carrier radio frequency signal of each circulation in the loop to have 2 pi change compared with the phase of the signal of the previous circulation, and after each circulation, the electric control analog radio frequency phase shifter is in coherent superposition with the carrier radio frequency signal output by the output directional coupler, and simultaneously inhibits the carrier radio frequency signals of other phases; and
and the adjustable radio frequency attenuator is respectively connected with the electric control analog radio frequency phase shifter and the input radio frequency directional coupler and is used for controlling gain compensation in the loop so as to enable the gain in the loop to reach an ideal state of infinite approximation to 1.
The signal input into the fast tunable RF filter enters the resonant cavity of the optoelectronic oscillator after being coupled by the input radio frequency directional coupler. A small part of the signal entering the resonant cavity of the optoelectronic oscillator is coupled out through the output radio frequency directional coupler, and the large part of the signal continues to circulate in the resonant cavity of the optoelectronic oscillator. The signal output by n times of cycle coupling and the signal output by n +1 th cycle coupling have 2 pi change on the phase; after each cycle, signals output at the output end of the output radio frequency directional coupler are subjected to coherent superposition, and signals of other phases are suppressed, so that a filtering function is realized.
The step frequency signal demodulation module comprises an I/Q mixer and two signal channels formed by connecting a low-pass filter and an analog-to-digital converter in series; a first input end of the I/Q mixer is connected to one output end of the first power divider, a second input end of the I/Q mixer is connected to an output end of the second amplifier, two outputs of the I/Q mixer are respectively connected to one signal channel, and outputs of the two signal channels are respectively connected to one input end of the digital signal processing module.
The Fourier mode-locked photoelectric oscillator is used as a stepping frequency signal source, the tuning speed of the Fourier mode-locked photoelectric oscillator depends on the tuning speed of the electric control analog phase shifter, and the tuning speed can reach dozens of MHz magnitude or even higher; the frequency tuning width is mainly determined by the tuning range of the fast tunable RF filter, can reach several GHz orders, and the phase noise can be reduced to 10KHz, and the frequency deviation is-140 dBc/Hz.
Drawings
FIG. 1 is a block diagram of the system of the step frequency radar of the present invention.
Fig. 2 is a block diagram of a fast tunable RF filter.
FIG. 3 is a diagram of a fast tunable RF filterS 21And (5) parameter test result graph.
Detailed Description
As shown in fig. 1, the step frequency radar of the present invention includes a step frequency signal generating module, a transmitting antenna, a receiving antenna, a step frequency signal demodulating module, a digital signal processing module, and the like. The output end of the stepping frequency signal generating module is connected with the first power divider through the first amplifier, one output of the first power divider is connected with the transmitting antenna, the other output of the first power divider is connected with the stepping frequency signal demodulating module, the output end of the receiving antenna is connected with the stepping frequency signal demodulating module through the second amplifier, and the two outputs of the stepping frequency signal demodulating module are connected to the digital signal processing module in a shunting mode.
In fig. 1, the stepped frequency signal generating module includes a laser, an intensity modulator, a photodetector, a second power divider, a fast tunable RF filter, a third amplifier, and a wideband filter. The laser, the intensity modulator and the photoelectric detector are connected through optical fiber jumpers; and the photoelectric detector, the second power divider, the fast adjustable RF filter, the third amplifier and the broadband filter are connected through cables.
In fig. 1, a laser is coupled to an intensity modulator for emitting a carrier optical signal. The intensity modulator is respectively connected with the laser, the photoelectric detector and the broadband filter and is used for modulating the intensity of the optical signal emitted by the laser. The photoelectric detector is respectively connected with the intensity modulator and the second power divider and used for performing photoelectric conversion on the carrier optical signal after intensity modulation and demodulating and outputting a radio frequency signal. The second power divider is respectively connected with the photoelectric detector, the fast tunable RF filter and the first amplifier and is used for outputting radio frequency signals after photoelectric conversion and demodulation. The fast tunable RF filter is respectively connected with the second power divider and the third amplifier and is used for performing gain compensation on the selected radio-frequency signal with tunable frequency so as to compensate attenuation caused by the radio-frequency signal passing through each device and ensure that the gain in the loop is more than 1. The third amplifier is respectively connected with the fast tunable RF filter and the broadband filter and is used for carrying out gain compensation on the selected radio frequency signal with tunable frequency so as to compensate attenuation caused by the radio frequency signal passing through each device and ensure that the gain in the loop is more than 1. The broadband filter is respectively connected with the third amplifier and the intensity modulator, and the radio-frequency signals output by the third amplifier are sent to the intensity modulator to limit the tuning range of the radio-frequency signals tunable in the resonant cavity of the photoelectric oscillator to be tuned in a certain free frequency spectrum range of the fast tunable RF filter.
As shown in fig. 1, the stepped frequency signal generating module forms an annular optical-electrical oscillator resonant cavity, and when the fourier domain mode locking condition is satisfied, a low-phase-noise broadband stepped frequency output signal can be generated.
In fig. 1, the carrier optical signal output by the laser is used as an optical carrier, and the intensity of the carrier optical signal is modulated by a signal input through the RF end of the intensity modulator. The carrier optical signal after intensity modulation is transmitted through the long optical fiber, then enters the photoelectric detector for photoelectric conversion, demodulates and outputs a radio frequency signal, selects a radio frequency signal with tunable frequency from the output radio frequency signal through the fast tunable RF filter, performs low noise amplification through the third amplifier, then is filtered through the broadband filter, and loads the carrier radio frequency signal through the RF end of the intensity modulator, thereby forming a closed positive feedback photoelectric oscillation loop. The optoelectronic oscillation loop is a multi-mode oscillator, but in order to realize stable single-mode oscillation and frequency tuning of an output signal of the optoelectronic oscillation loop, a mode selection device of the optoelectronic oscillation loop has the characteristic of adjustable central frequency.
As shown in fig. 2, the fast tunable RF filter includes an input RF directional coupler, an RF low noise amplifier, an output RF directional coupler, an electrically controlled analog RF phase shifter, and an adjustable RF attenuator. The input radio frequency directional coupler, the radio frequency low noise amplifier, the output radio frequency directional coupler, the electric control analog radio frequency phase shifter and the adjustable radio frequency attenuator are sequentially connected to form a closed loop. And the input radio frequency directional coupler is used as the input end of the filter and is used for receiving the carrier radio frequency signal output from the second power divider and carrying out directional coupling. The radio frequency low noise amplifier is used for carrying out gain compensation on the carrier radio frequency signal after directional coupling so as to compensate the loss in the ring and enable the gain in the ring to be close to 1 as a whole. And the output radio frequency directional coupler is used as the output end of the filter and is used for carrying out secondary directional coupling and outputting on the carrier radio frequency signal after gain compensation. The electric control analog radio frequency phase shifter is used for enabling the phase of the carrier radio frequency signal of each circulation in the ring to have 2 pi change compared with the phase of the signal of the previous circulation, and after each circulation, the electric control analog radio frequency phase shifter is in coherent superposition with the carrier radio frequency signal output by the output directional coupler, and meanwhile, the carrier radio frequency signals of other phases are suppressed. The adjustable radio frequency attenuator is used for controlling gain compensation in the loop, so that the gain in the loop reaches an ideal state of infinite approximate 1.
A radio frequency low noise amplifier in the fast tunable RF filter is matched with an adjustable radio frequency attenuator to ensure that the loss of the resonant cavity of the whole photoelectric oscillator is infinitely close to 1 but less than 1, and an electric control analog radio frequency phase shifter controls the center frequency of the fast tunable RF filter through input voltage. An electrically controlled analog radio frequency phase shifter with a large modulation bandwidth (e.g., modulation bandwidth 50M) is preferred.
The period of the control voltage signal driving the fast tunable RF filter in the opto-electronic oscillator cavity must be synchronized with the delay of the opto-electronic oscillator cavity, i.e. the following equation is satisfied:
T 1 = NT 2
in which N is an integer, T1Is the period, T, of the control voltage signal of the fast tunable RF filter2Is the delay of one cycle of transmission of the carrier signal in the optoelectronic oscillator loop. The effect of periodically driving the fast tunable RF filter is that the filter window in the resonator of the optoelectronic oscillator is periodically changed, and only the frequency components that meet the bandpass window of the filter can pass through the filter at any time without loss or with low loss, while other frequency components are filtered out. The passed frequency components come to the fast tunable RF filter again after gain and propagation in the resonant cavity of the optoelectronic oscillator, if the frequency window given by the fast tunable RF filter is the sameThe port just matches it, the signal can pass through again. Therefore, the signal which accords with the window of the periodic filter can be increased in the resonant cavity of the photoelectric oscillator and finally stabilized to realize Fourier mode locking, thereby forming the Fourier mode locking photoelectric oscillator. This fourier-mode-locked opto-electronic oscillator obtains a stable output consistent with the frequency sweep of the fast tunable RF filter, i.e. both have the same frequency sweep range, speed and period.
As shown in fig. 2, the fast tunable RF filter is a resonant filter, has a certain free spectral range, and in order to ensure that the filter outputs only in a certain free spectral range, a wide band filter is provided in the oscillator loop shown in fig. 1, and the band pass range of the wide band filter matches with a certain free spectral range that the fast tunable RF filter needs to ensure (fig. 3).
The output signal of the step frequency signal generation module has the following characteristics:
the full-band frequency agility capability does not need frequency establishment time in the whole Fourier mode locking period, and (very small) stepping frequency continuous wave output can be realized. Signal source output of various systems can be realized through signal control of the analog phase shifter, such as a stepping frequency pulse signal, a stepping frequency continuous wave signal and a frequency modulation continuous wave (including linear frequency modulation, triangular wave frequency modulation and the like) signal;
the full-band carrier phase is continuous: in the whole Fourier mode locking period and when the frequency in the whole frequency band is stepped, the carrier phase can be completely continuous. During coherent demodulation and signal processing, the maximum coherent gain can be brought, so that the detection performance of the system is improved.
In fig. 1, the step-by-step frequency signal demodulation module includes an I/Q mixer and two signal channels formed by a low-pass filter and an analog-to-digital converter connected in series; the input ends of the two signal channels are respectively connected to one output end of the I/Q mixer, and the output ends of the two signal channels are respectively connected to one input end of the digital signal processing module.
The low-phase-noise broadband stepped frequency signal generated by the stepped frequency signal generating module is output to a first amplifier from the output end of a second power divider, is amplified by the first amplifier and then is input to the first power divider, and the first power divider outputs the signal in two paths; one output signal is transmitted to a detection target or a detection area through a transmitting antenna; the receiving antenna receives an echo signal reflected from a detection target or a detection area, and the echo signal is subjected to low-noise amplification by the second amplifier and then input to the stepping frequency signal demodulation module; the other path of output signal of the first power divider is used as an intrinsic signal and is directly input to the stepping frequency signal demodulation module. In a receiving channel, echo signals are received by a receiving antenna, amplified by a low noise amplifier, mixed with intrinsic signals in an I/Q mixer, sampled by an analog-to-digital converter after passing through a low-pass filter (a low-pass filter in quadrature demodulation and a high-pass filter for suppressing direct waves), and then the sampled signals enter a digital signal processing module for digital filtering and software processing, so that the information of a target to be detected is finally obtained.
Claims (4)
1. A step frequency radar based on a Fourier mode-locked optoelectronic oscillator is characterized by comprising a step frequency signal generating module, a transmitting antenna, a receiving antenna, a step frequency signal demodulating module and a digital signal processing module;
the output signal generated by the step frequency signal generation module is amplified by a first amplifier and then input into a first power divider, one path of output signal of the first power divider is transmitted by a transmitting antenna, received by a receiving antenna, amplified by a second amplifier and then input into a step frequency signal demodulation module, the other path of output signal of the first power divider is directly input into the step frequency signal demodulation module, and the two paths of input signals are processed by the step frequency signal demodulation module and then output into a digital signal processing module for data processing of radar signals;
the step frequency signal generation module forms an annular photoelectric oscillator resonant cavity, and can generate a low-phase noise broadband step frequency output signal when the Fourier domain mode locking condition is met.
2. The fourier-modelocked optoelectronic oscillator-based step-frequency radar of claim 1, wherein the step-frequency signal generating module comprises:
the laser is connected with the intensity modulator and used for emitting a carrier optical signal;
the intensity modulator is respectively connected with the laser, the photoelectric detector and the broadband filter and is used for modulating the intensity of an optical signal emitted by the laser;
the photoelectric detector is respectively connected with the intensity modulator and the second power divider and is used for performing photoelectric conversion on the carrier optical signal subjected to intensity modulation and demodulating and outputting a radio-frequency signal;
the second power divider is respectively connected with the photoelectric detector, the fast tunable RF filter and the first amplifier and used for outputting radio frequency signals after photoelectric conversion and demodulation;
the fast tunable RF filter is respectively connected with the second power divider and the third amplifier and is used for selecting a radio-frequency signal with tunable frequency from the radio-frequency signals output by the second power divider;
the third amplifier is respectively connected with the fast tunable RF filter and the broadband filter and is used for carrying out gain amplification on the tunable radio-frequency signal in the photoelectric oscillation cavity; and
and the broadband filter is respectively connected with the third amplifier and the intensity modulator and is used for limiting the tuning range of the tunable radio-frequency signal in the resonant cavity of the photoelectric oscillator to be tuned in a certain free frequency spectrum range of the fast tunable RF filter.
3. The fourier-modelocked optoelectronic oscillator-based step-frequency radar of claim 2, wherein the fast tunable RF filter comprises:
the input radio frequency directional coupler is respectively connected with the radio frequency low noise amplifier and the adjustable radio frequency attenuator and is used for receiving the carrier radio frequency signal output by the second power divider and carrying out directional coupling;
the radio frequency low noise amplifier is respectively connected with the input radio frequency directional coupler and the output radio frequency directional coupler and is used for carrying out gain compensation on the carrier radio frequency signals after directional coupling;
the output radio frequency directional coupler is respectively connected with the radio frequency low noise amplifier and the electric control analog radio frequency phase shifter and is used for carrying out secondary directional coupling and outputting on the carrier radio frequency signal after gain compensation;
the electric control analog radio frequency phase shifter is respectively connected with the output radio frequency directional coupler and the adjustable radio frequency attenuator and is used for enabling the phase of the carrier radio frequency signal of each circulation in the loop to have 2 pi change compared with the phase of the signal of the previous circulation, and after each circulation, the electric control analog radio frequency phase shifter is in coherent superposition with the carrier radio frequency signal output by the output directional coupler, and simultaneously inhibits the carrier radio frequency signals of other phases; and
and the adjustable radio frequency attenuator is respectively connected with the electric control analog radio frequency phase shifter and the input radio frequency directional coupler and is used for gain compensation in the control ring.
4. The fourier-mode-locked optoelectronic oscillator-based step-frequency radar as claimed in claim 1, wherein the step-frequency signal demodulation module comprises an I/Q mixer and two signal channels consisting of a low-pass filter and an analog-to-digital converter connected in series; a first input end of the I/Q mixer is connected to one output end of the first power divider, a second input end of the I/Q mixer is connected to an output end of the second amplifier, two outputs of the I/Q mixer are respectively connected to one signal channel, and outputs of the two signal channels are respectively connected to one input end of the digital signal processing module.
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