In the high speed target environment,there exists serious Doppler effect in the low pulse repetition frequency（LPRF） modulated frequency stepped frequency（MFSF） radar signal.The velocity range of the target is lar...In the high speed target environment,there exists serious Doppler effect in the low pulse repetition frequency（LPRF） modulated frequency stepped frequency（MFSF） radar signal.The velocity range of the target is large and the velocity is high ambiguous,so the single method is difficult to satisfy the velocity measurement requirement.For this problem,a novel method is presented,it is a combination of cross-correlation inner frame velocity measurement and range-Doppler coupling velocity measurement.The cross-correlation inner frame method,overcoming the low Doppler tolerance of the cross-correlation between frames,can obtain the coarse velocity of the high speed target,and then the precision velocity can be obtained with the range-Doppler coupling method.The simulation results confirm the method is effective,and also it is well real-time and easy to the project application.展开更多
With appropriate geometry configuration, helicopter- borne rotating synthetic aperture radar （ROSAR） can break through the limitations of monostatic synthetic aperture radar （SAR） on forward-looking imaging. With ...With appropriate geometry configuration, helicopter- borne rotating synthetic aperture radar （ROSAR） can break through the limitations of monostatic synthetic aperture radar （SAR） on forward-looking imaging. With this capability, ROSAR has extensive potential applications, such as self-navigation and self-landing. Moreover, it has many advantages if combined with the frequency modulated continuous wave （FMCW） technology. A novel geometric configuration and an imaging algorithm for helicopter-borne FMCW-ROSAR are proposed. Firstly, by per- forming the equivalent phase center principle, the separated trans- mitting and receiving antenna system is equalized to the case of system configuration with antenna for both transmitting and receiving signals. Based on this, the accurate two-dimensional spectrum is obtained and the Doppler frequency shift effect in- duced by the continuous motion of the platform during the long pulse duration is compensated. Next, the impacts of the velocity approximation error on the imaging algorithm are analyzed in de- tail, and the system parameters selection and resolution analysis are presented. The well-focused SAR image is then obtained by using the improved Omega-K algorithm incorporating the accurate compensation method for the velocity approximation error. FJnally, correctness of the analysis and effectiveness of the proposed al- gorithm are demonstrated through simulation results.展开更多
By adding frequency modulated signals to the intensity equation of gain noise model of the single-mode laser driven by two coloured noises which are correlated, this paper uses the linear approximation method to calcu...By adding frequency modulated signals to the intensity equation of gain noise model of the single-mode laser driven by two coloured noises which are correlated, this paper uses the linear approximation method to calculate the power spectrum and signal-to-noise ratio （SNR） of the laser intensity. The results show that the SNR appears typical stochastic resonance with the variation of intensity of the pump noise and quantum noise. As the amplitude of a modulated signal has effects on the SNR, it shows suppression, monotone increasing, stochastic resonance, and multiple stochastic resonance with the variation of the frequency of a carrier signal and modulated signal.展开更多
A simple model is developed to study the mechanism of stimulated Brillouin scattering（SBS） suppression with frequency-modulated laser in optical fiber. By taking into account the laser frequency distribution along t...A simple model is developed to study the mechanism of stimulated Brillouin scattering（SBS） suppression with frequency-modulated laser in optical fiber. By taking into account the laser frequency distribution along the fiber induced by frequency modulation, the average effective Brillouin gain is calculated to determine the SBS threshold. Experimental results show agreement with the numerical analysis. The application for SBS suppression in interferometric fiber sensing system is also discussed in this paper. The results show that the maximum input power can be increased effectively by frequency modulation method.展开更多
A method to evaluate the influence of the laser linewidth on the linearly frequency-modulated(LFM)signals generated by heterodyning two free-running laser diodes(LDs)is proposed.The Pearson correlation coefficient bet...A method to evaluate the influence of the laser linewidth on the linearly frequency-modulated(LFM)signals generated by heterodyning two free-running laser diodes(LDs)is proposed.The Pearson correlation coefficient between the instantaneous frequency of the generated LFM signal and that of an ideal LFM signal is introduced to quantify the quality of the generated LFM signal.The closed-form solution of the correlation coefficient is given,which shows that the correlation coefficient is determined by the ratio of the LFM signal bandwidth to the square root of the total linewidth of the two LDs when the observation interval is fixed.Simulation results are also given,which proves the correctness of the theoretical results.展开更多
We propose an arrayed waveguide grating(AWG)-based 10 Gbit/s per channel full duplex wavelength division multiplexing passive optical network(WDM-PON).A chirp managed directly modulated laser with return-to-zero(RZ) d...We propose an arrayed waveguide grating(AWG)-based 10 Gbit/s per channel full duplex wavelength division multiplexing passive optical network(WDM-PON).A chirp managed directly modulated laser with return-to-zero(RZ) differential phase shift keying(DPSK) modulation technique is utilized for downlink(DL) direction,and then the downlink signal is re-modulated for the uplink(UL) direction using intensity modulation technique with the data rate of 10 Gbit/s per channel.A successful WDM-PON transmission operation with the data rate of 10 Gbit/s per channel over a distance of 25 km without any optical amplification or dispersion compensation is demonstrated with low power penalty.展开更多
In response to the issue of determining the appropriate capacity when hybrid energy storage systems(HESS)collaborate with thermal power units(TPU)in the system’s secondary frequency regulation,a configuration method ...In response to the issue of determining the appropriate capacity when hybrid energy storage systems(HESS)collaborate with thermal power units(TPU)in the system’s secondary frequency regulation,a configuration method for HESS based on the analysis of frequency regulation demand analysis is proposed.And a corresponding simulation platform is developed.Firstly,a frequency modulation demand method for reducing the frequency modulation losses of TPU is proposed.Secondly,taking into comprehensive consideration that flywheel energy storage features rapid power response and battery energy storage has the characteristic of high energy density,a coordinated control strategy for HESS considering the self-recovery of state of charge(SOC)is put forward.Then,to measure the economic and technical performance of HESS in assisting the secondary frequency modulation of TPU,an optimized configurationmodel considering the full-life-cycle economy and frequency modulation performance of TPU and HESS system is constructed.Finally,a visual simulation platform for the combined frequency modulation of TPU and HESS is developed based on Matlab Appdesigner.Theresults of calculation examples indicate that the proposed configuration method can improve the overall economic efficiency and frequency modulation performance of TPU and HESS;The control strategy can not only prolong the service life of battery energy storage but also enhance the continuous response ability of HESS;The visual simulation platform is easy to use,and the simulation results are accurate and reliable.展开更多
A pulse frequency modulation（PFM） circuit for retinal prosthesis,which generates electrical pulses with frequency proportional to the intensity of incident light, is presented. The fundamental characteristic of the ...A pulse frequency modulation（PFM） circuit for retinal prosthesis,which generates electrical pulses with frequency proportional to the intensity of incident light, is presented. The fundamental characteristic of the circuit is described and analyzed. The circuit is realized in 0.6μm CMOS process,and the simulation results testify to the possibility of sub-retinal implantation.展开更多
Although envelope spectrum does not involve complicated sideband,thus has a much simpler structure than the common Fourier spectrum,it is still subject to the efect of planets passing or time variant vibration transfe...Although envelope spectrum does not involve complicated sideband,thus has a much simpler structure than the common Fourier spectrum,it is still subject to the efect of planets passing or time variant vibration transfer pams.The presence of planets passing frequency,sun gear rotating frequency,or planet carrier rotating frequency in the envelope spectrum may confuse the analysis in fault diagnosis.Therefore,it is important to look for an approach to remove the interferences caused by the efect of planets passing or time variant vibration transfer paths.展开更多
We introduce a new method of simultaneously implementing frequency stabilization and frequency shift for semiconductor lasers. We name this method the frequency tunable modulation transfer spectroscopy （FTMTS）. To r...We introduce a new method of simultaneously implementing frequency stabilization and frequency shift for semiconductor lasers. We name this method the frequency tunable modulation transfer spectroscopy （FTMTS）. To realize a stable output of 780 nm semiconductor laser, an FTMTS optical heterodyne frequency stabilization system is constructed. Before entering into the frequency stabilization system, the probe laser passes through an acousto-optical modulator （AOM） twice in advance to achieve tunable frequency while keeping the light path stable. According to the experimental results, the frequency changes from 120 MHz to 190 MHz after the double-pass AOM, and the intensity of laser entering into the system is greatly changed, but there is almost no change in the error signal of the FTMTS spectrum. Using this signal to lock the laser frequency, we can ensure that the frequency of the laser changes with the amount of AOM shift. Therefore, the magneto-optical trap （MOT）-molasses process can be implemented smoothly.展开更多
Recent advances in electronics have increased the complexity of radar signal modulation.The quasi-linear frequency modulation(quasi-LFM)radar waveforms(LFM,Frank code,P1−P4 code)have similar time-frequency distributio...Recent advances in electronics have increased the complexity of radar signal modulation.The quasi-linear frequency modulation(quasi-LFM)radar waveforms(LFM,Frank code,P1−P4 code)have similar time-frequency distributions,and it is difficult to identify such signals using traditional time-frequency analysis methods.To solve this problem,this paper proposes an algorithm for automatic recognition of quasi-LFM radar waveforms based on fractional Fourier transform and time-frequency analysis.First of all,fractional Fourier transform and the Wigner-Ville distribution(WVD)are used to determine the number of main ridgelines and the tilt angle of the target component in WVD.Next,the standard deviation of the target component's width in the signal's WVD is calculated.Finally,an assembled classifier using neural network is built to recognize different waveforms by automatically combining the three features.Simulation results show that the overall recognition rate of the proposed algorithm reaches 94.17%under 0 dB.When the training data set and the test data set are mixed with noise,the recognition rate reaches 89.93%.The best recognition accuracy is achieved when the size of the training set is taken as 400.The algorithm complexity can meet the requirements of real-time recognition.展开更多
A new method of estimating the frequency-known signals from the strong background noise was presented first. Then the new method was used in the demodulation of the digital frequency modulation (FSK) signals. The new ...A new method of estimating the frequency-known signals from the strong background noise was presented first. Then the new method was used in the demodulation of the digital frequency modulation (FSK) signals. The new demodulation method can complete the demodulation of the FSK signals only with the carrier frequency and without any carrier phase information. The simulation results show that the performance of anti-noise of the new method is better than that of the incoherent demodulation method and the fluctuation of the carrier phase has little effect on the new method. So the new demodulation method has a fine prospect in the practical applications.展开更多
A new type of window called combined window is designed to get higher Ratio of Mainlobe to Sidelobes (RMS) and lower Mainlobe Widening Factor (MWF). Simulation results prove that the new window can solve the contradic...A new type of window called combined window is designed to get higher Ratio of Mainlobe to Sidelobes (RMS) and lower Mainlobe Widening Factor (MWF). Simulation results prove that the new window can solve the contradiction between RMS and MWF better than classic windows.展开更多
Based on the theory of the passive hydrogen maser, along with the technology of frequency modulation and modulation transfer spectroscopy, the theoretical expression of the single frequency modulation for the passive ...Based on the theory of the passive hydrogen maser, along with the technology of frequency modulation and modulation transfer spectroscopy, the theoretical expression of the single frequency modulation for the passive hydrogen maser and the function of the cavity and H line error signals separation are derived, which are basically coincident with the experiment. The absorption and dispersion spectrum curves with different resonance widths show that the cavity and hydrogen transition serve as discriminators, and the two error signals can be separated. Through the calculations of the two error signals in the passive hydrogen maser, it analyzes the traditional method of the two error signals separation, and then describes a new improved method for the passive hydrogen servo loops consisting in the use of a single modulation frequency and frequency discrimination. A null interaction of the two error signals for the new selection of the phase setting is deduced theoretically and validated by the simulation. The preliminary experimental result confirms the feasibility of this new approach, which can reduce the influence from the cavity frequency variety on the crystal oscillator and contribute significantly to the long term performance of the passive hydrogen maser.展开更多
In view of the low resolution and accuracy of traditional magnetometer,a method of microwave frequency modulation technology based on nitrogen-vacancy(NV)center in diamond for magnetic detection was proposed.The magne...In view of the low resolution and accuracy of traditional magnetometer,a method of microwave frequency modulation technology based on nitrogen-vacancy(NV)center in diamond for magnetic detection was proposed.The magnetometer studied can reduce the frequency noise of system and improve the magnetic sensitivity by microwave frequency modulation.Firstly,ESR spectra by sweeping the microwave frequency was obtained.Further,the microwave frequency modulated was gained through the mixed high-frequency sinusoidal modulation signal generated by signal generator.In addition,the frequency through the lock-in amplifier was locked,and the signal which was proportional to the first derivative of the spectrum was obtained.The experimental results show that the sensitivity of magnetic field detection can reach 17.628 nT/Hz based on microwave frequency modulation technology.The method realizes high resolution and sensitivity for magnetic field detection.展开更多
Ultrahigh resolution synthetic aperture radar(SAR)imaging for ship targets is significant in SAR imaging,but it suffers from high frequency vibration of the platform,which will induce defocus into SAR imaging results....Ultrahigh resolution synthetic aperture radar(SAR)imaging for ship targets is significant in SAR imaging,but it suffers from high frequency vibration of the platform,which will induce defocus into SAR imaging results.In this paper,a novel compensation method based on the sinusoidal frequency modulation Fourier-Bessel transform(SFMFBT)is proposed,it can estimate the vibration errors,and the phase shift ambiguity can be avoided via extracting the time frequency ridge consequently.By constructing the corresponding compensation function and combined with the inverse SAR(ISAR)technique,well-focused imaging results can be obtained.The simulation imaging results of ship targets demonstrate the validity of the proposed approach.展开更多
A wideband dipole signal is required for dipole dispersion correction and nearborehole imaging. To obtain the broadband flexural wave dispersion, we use a nonlinear frequency modulation （NLFM） signal and propose a s...A wideband dipole signal is required for dipole dispersion correction and nearborehole imaging. To obtain the broadband flexural wave dispersion, we use a nonlinear frequency modulation （NLFM） signal and propose a segment linear frequency modulation （SLFM） signal as the dipole excitation signal to compensate for the excitation intensity. The signal-to-noise ratio （SNR） of the signal over the entire frequency band is increased. The finite-difference method is used to simulate the responses from a Ricker wavelet, a linear frequency modulation （LFM） signal, an NLFM signal, and an SLFM signal in two borehole models of a homogeneously hard formation and a radially stratified formation. The dispersion and radial tomography results at low SNR of the sound source signals are compared. Numerical modeling suggests that the energy of the flexural waves excited by the Ricker wavelet source is concentrated near the Airy phase. In this case, the dispersion is incomplete and information regarding the formation near or far from the borehole cannot be obtained. The LFM signal yields dispersion information near the Airy phase and the high-frequency range but not in the low-frequency range. Moreover, the information regarding the formation far from the borehole is not accurate. The NLFM signal extends the frequency range of the flexural waves by compensating for the excitation intensity and yields information regarding the formation information, but it is not easy to obtain. The SLFM signal yields the same results as the NLFM signal and is easier to implement. Consequently, the dipole detection range expands and the S-wave velocity calculation accuracy improves.展开更多
Numerical experiments on forcing dissipation and heating response of dipole (unipole) are carried out using global spectral models with quasi-geostrophic barotropic vorticity equations. For each experiment model integ...Numerical experiments on forcing dissipation and heating response of dipole (unipole) are carried out using global spectral models with quasi-geostrophic barotropic vorticity equations. For each experiment model integration is run for 90 days on the condition of three-wave quasi-resonance. The results are given as follows: Under the effects of dipole (unipole) forcing source and basic flow intensity, there exist strong interactions among the three planetary waves and quasi-biweekly and intraseasonal oscillation of the three planetary waves. In the meantime, the changes in the intensity of dipole or unipole forcing source and basic flow have different frequency modulation effects on LFO in the middle and higher latitudes. The results of the stream function field of three quasi-resonant waves evolving with time confirm that the low-frequency oscillation exists in extratropical latitude.展开更多
Conventional parameter estimation methods for pseudo-random binary code-linear frequency modulation(PRBC-LFM)signals require prior knowledge,are computationally complex,and exhibit poor performance at low signal-to-no...Conventional parameter estimation methods for pseudo-random binary code-linear frequency modulation(PRBC-LFM)signals require prior knowledge,are computationally complex,and exhibit poor performance at low signal-to-noise ratios(SNRs).To overcome these problems,a blind parameter estimation method based on a Duffing oscillator array is proposed.A new relationship formula among the state of the Duffing oscillator,the pseudo-random sequence of the PRBC-LFM signal,and the frequency difference between the PRBC-LFM signal and the periodic driving force signal of the Duffing oscillator is derived,providing the theoretical basis for blind parameter estimation.Methods based on amplitude method,short-time Fourier transform method,and power spectrum entropy method are used to binarize the output of the Duffing oscillator array,and their performance is compared.The pseudo-random sequence is estimated using Duffing oscillator array synchronization,and the carrier frequency parameters are obtained by the relational expressions and characteristics of the difference frequency.Simulation results show that this blind estimation method overcomes limitations in prior knowledge and maintains good parameter estimation performance up to an SNR of-35 dB.展开更多
Modified implementation architecture for sinusoidal frequency modulation is introduced to extract the range information from the received radar echo. Range ambiguity problem arises because the range is calculated from...Modified implementation architecture for sinusoidal frequency modulation is introduced to extract the range information from the received radar echo. Range ambiguity problem arises because the range is calculated from the estimated phase of the received signal which is wrapped into （0, 2π]. By integrating Doppler frequency shifts, the variation of range can be estimated and used as an auxiliary information to help eliminating the corresponding range ambiguity. The performance of the new technique is evaluated by simulations. The results show that this technique is robust to sever phase noise and can be used effectively for ambiguity elimination of the modified sinusoidal frequency modulated continuous wave radar.展开更多
文摘In the high speed target environment,there exists serious Doppler effect in the low pulse repetition frequency（LPRF） modulated frequency stepped frequency（MFSF） radar signal.The velocity range of the target is large and the velocity is high ambiguous,so the single method is difficult to satisfy the velocity measurement requirement.For this problem,a novel method is presented,it is a combination of cross-correlation inner frame velocity measurement and range-Doppler coupling velocity measurement.The cross-correlation inner frame method,overcoming the low Doppler tolerance of the cross-correlation between frames,can obtain the coarse velocity of the high speed target,and then the precision velocity can be obtained with the range-Doppler coupling method.The simulation results confirm the method is effective,and also it is well real-time and easy to the project application.
基金supported by the National Basic Research Program of China(2011CB707001)the Fundamental Research Funds for the Central Universities(106112015CDJXY500001CDJZR165505)
文摘With appropriate geometry configuration, helicopter- borne rotating synthetic aperture radar （ROSAR） can break through the limitations of monostatic synthetic aperture radar （SAR） on forward-looking imaging. With this capability, ROSAR has extensive potential applications, such as self-navigation and self-landing. Moreover, it has many advantages if combined with the frequency modulated continuous wave （FMCW） technology. A novel geometric configuration and an imaging algorithm for helicopter-borne FMCW-ROSAR are proposed. Firstly, by per- forming the equivalent phase center principle, the separated trans- mitting and receiving antenna system is equalized to the case of system configuration with antenna for both transmitting and receiving signals. Based on this, the accurate two-dimensional spectrum is obtained and the Doppler frequency shift effect in- duced by the continuous motion of the platform during the long pulse duration is compensated. Next, the impacts of the velocity approximation error on the imaging algorithm are analyzed in de- tail, and the system parameters selection and resolution analysis are presented. The well-focused SAR image is then obtained by using the improved Omega-K algorithm incorporating the accurate compensation method for the velocity approximation error. FJnally, correctness of the analysis and effectiveness of the proposed al- gorithm are demonstrated through simulation results.
基金supported by the Key Project Scientific Research Foundation from the Education Department of Hubei Province of China(Grant No D200725001)
文摘By adding frequency modulated signals to the intensity equation of gain noise model of the single-mode laser driven by two coloured noises which are correlated, this paper uses the linear approximation method to calculate the power spectrum and signal-to-noise ratio （SNR） of the laser intensity. The results show that the SNR appears typical stochastic resonance with the variation of intensity of the pump noise and quantum noise. As the amplitude of a modulated signal has effects on the SNR, it shows suppression, monotone increasing, stochastic resonance, and multiple stochastic resonance with the variation of the frequency of a carrier signal and modulated signal.
基金Project supported by the National Natural Science Foudation of China(Grant No.6177073)
文摘A simple model is developed to study the mechanism of stimulated Brillouin scattering（SBS） suppression with frequency-modulated laser in optical fiber. By taking into account the laser frequency distribution along the fiber induced by frequency modulation, the average effective Brillouin gain is calculated to determine the SBS threshold. Experimental results show agreement with the numerical analysis. The application for SBS suppression in interferometric fiber sensing system is also discussed in this paper. The results show that the maximum input power can be increased effectively by frequency modulation method.
基金supported by the National Key R&D Program of China(No.2017YFE0121500)the National Natural Science Foundation of China(Nos.61971193 and 61601297)+1 种基金the Open Fund of State Key Laboratory of Advanced Optical Communication Systems and Networks,Peking University,China(No.2020GZKF005)the Fundamental Research Funds for the Central Universities。
文摘A method to evaluate the influence of the laser linewidth on the linearly frequency-modulated(LFM)signals generated by heterodyning two free-running laser diodes(LDs)is proposed.The Pearson correlation coefficient between the instantaneous frequency of the generated LFM signal and that of an ideal LFM signal is introduced to quantify the quality of the generated LFM signal.The closed-form solution of the correlation coefficient is given,which shows that the correlation coefficient is determined by the ratio of the LFM signal bandwidth to the square root of the total linewidth of the two LDs when the observation interval is fixed.Simulation results are also given,which proves the correctness of the theoretical results.
基金supported by the National Basic Research Program of China (No.2010CB328300)the Fundamental Research Funds for the Central Universities (No.2009RC0314)+3 种基金the National Natural Science Foundation of China (Nos.61077050,61077014 and 60932004)the BUPT Young Foundation (No.2009CZ07)the Fundamental Research Funds for the Central Universitiesthe Open Foundation of State Key Laboratory of Optical Communication Technologies and Networks (WRI)(No.2010OCTN-02)
文摘We propose an arrayed waveguide grating(AWG)-based 10 Gbit/s per channel full duplex wavelength division multiplexing passive optical network(WDM-PON).A chirp managed directly modulated laser with return-to-zero(RZ) differential phase shift keying(DPSK) modulation technique is utilized for downlink(DL) direction,and then the downlink signal is re-modulated for the uplink(UL) direction using intensity modulation technique with the data rate of 10 Gbit/s per channel.A successful WDM-PON transmission operation with the data rate of 10 Gbit/s per channel over a distance of 25 km without any optical amplification or dispersion compensation is demonstrated with low power penalty.
基金supported by a Key Project of the National Natural Science Foundation of China under Grant 52337004.
文摘In response to the issue of determining the appropriate capacity when hybrid energy storage systems(HESS)collaborate with thermal power units(TPU)in the system’s secondary frequency regulation,a configuration method for HESS based on the analysis of frequency regulation demand analysis is proposed.And a corresponding simulation platform is developed.Firstly,a frequency modulation demand method for reducing the frequency modulation losses of TPU is proposed.Secondly,taking into comprehensive consideration that flywheel energy storage features rapid power response and battery energy storage has the characteristic of high energy density,a coordinated control strategy for HESS considering the self-recovery of state of charge(SOC)is put forward.Then,to measure the economic and technical performance of HESS in assisting the secondary frequency modulation of TPU,an optimized configurationmodel considering the full-life-cycle economy and frequency modulation performance of TPU and HESS system is constructed.Finally,a visual simulation platform for the combined frequency modulation of TPU and HESS is developed based on Matlab Appdesigner.Theresults of calculation examples indicate that the proposed configuration method can improve the overall economic efficiency and frequency modulation performance of TPU and HESS;The control strategy can not only prolong the service life of battery energy storage but also enhance the continuous response ability of HESS;The visual simulation platform is easy to use,and the simulation results are accurate and reliable.
文摘A pulse frequency modulation（PFM） circuit for retinal prosthesis,which generates electrical pulses with frequency proportional to the intensity of incident light, is presented. The fundamental characteristic of the circuit is described and analyzed. The circuit is realized in 0.6μm CMOS process,and the simulation results testify to the possibility of sub-retinal implantation.
文摘Although envelope spectrum does not involve complicated sideband,thus has a much simpler structure than the common Fourier spectrum,it is still subject to the efect of planets passing or time variant vibration transfer pams.The presence of planets passing frequency,sun gear rotating frequency,or planet carrier rotating frequency in the envelope spectrum may confuse the analysis in fault diagnosis.Therefore,it is important to look for an approach to remove the interferences caused by the efect of planets passing or time variant vibration transfer paths.
基金Project supported by the National Key Scientific Instrument and Equipment Development Project,China(Grant No.2014YQ35046103)
文摘We introduce a new method of simultaneously implementing frequency stabilization and frequency shift for semiconductor lasers. We name this method the frequency tunable modulation transfer spectroscopy （FTMTS）. To realize a stable output of 780 nm semiconductor laser, an FTMTS optical heterodyne frequency stabilization system is constructed. Before entering into the frequency stabilization system, the probe laser passes through an acousto-optical modulator （AOM） twice in advance to achieve tunable frequency while keeping the light path stable. According to the experimental results, the frequency changes from 120 MHz to 190 MHz after the double-pass AOM, and the intensity of laser entering into the system is greatly changed, but there is almost no change in the error signal of the FTMTS spectrum. Using this signal to lock the laser frequency, we can ensure that the frequency of the laser changes with the amount of AOM shift. Therefore, the magneto-optical trap （MOT）-molasses process can be implemented smoothly.
基金This work was supported by the National Natural Science Foundation of China(91538201)the Taishan Scholar Project of Shandong Province(ts201511020)the project supported by Chinese National Key Laboratory of Science and Technology on Information System Security(6142111190404).
文摘Recent advances in electronics have increased the complexity of radar signal modulation.The quasi-linear frequency modulation(quasi-LFM)radar waveforms(LFM,Frank code,P1−P4 code)have similar time-frequency distributions,and it is difficult to identify such signals using traditional time-frequency analysis methods.To solve this problem,this paper proposes an algorithm for automatic recognition of quasi-LFM radar waveforms based on fractional Fourier transform and time-frequency analysis.First of all,fractional Fourier transform and the Wigner-Ville distribution(WVD)are used to determine the number of main ridgelines and the tilt angle of the target component in WVD.Next,the standard deviation of the target component's width in the signal's WVD is calculated.Finally,an assembled classifier using neural network is built to recognize different waveforms by automatically combining the three features.Simulation results show that the overall recognition rate of the proposed algorithm reaches 94.17%under 0 dB.When the training data set and the test data set are mixed with noise,the recognition rate reaches 89.93%.The best recognition accuracy is achieved when the size of the training set is taken as 400.The algorithm complexity can meet the requirements of real-time recognition.
基金the National Natural Science Foundation of China (60272077) the Science Foundation of Aeronautics (02F53030).
文摘A new method of estimating the frequency-known signals from the strong background noise was presented first. Then the new method was used in the demodulation of the digital frequency modulation (FSK) signals. The new demodulation method can complete the demodulation of the FSK signals only with the carrier frequency and without any carrier phase information. The simulation results show that the performance of anti-noise of the new method is better than that of the incoherent demodulation method and the fluctuation of the carrier phase has little effect on the new method. So the new demodulation method has a fine prospect in the practical applications.
文摘A new type of window called combined window is designed to get higher Ratio of Mainlobe to Sidelobes (RMS) and lower Mainlobe Widening Factor (MWF). Simulation results prove that the new window can solve the contradiction between RMS and MWF better than classic windows.
基金supported by the Next Generation of Beidou Navigation Satellite(GFZX0301020104)
文摘Based on the theory of the passive hydrogen maser, along with the technology of frequency modulation and modulation transfer spectroscopy, the theoretical expression of the single frequency modulation for the passive hydrogen maser and the function of the cavity and H line error signals separation are derived, which are basically coincident with the experiment. The absorption and dispersion spectrum curves with different resonance widths show that the cavity and hydrogen transition serve as discriminators, and the two error signals can be separated. Through the calculations of the two error signals in the passive hydrogen maser, it analyzes the traditional method of the two error signals separation, and then describes a new improved method for the passive hydrogen servo loops consisting in the use of a single modulation frequency and frequency discrimination. A null interaction of the two error signals for the new selection of the phase setting is deduced theoretically and validated by the simulation. The preliminary experimental result confirms the feasibility of this new approach, which can reduce the influence from the cavity frequency variety on the crystal oscillator and contribute significantly to the long term performance of the passive hydrogen maser.
基金National Natural Science Foundation of China(Nos.51635011,61503346,51727808)National Science Foundation of Shanxi Province(No.201701D121080)
文摘In view of the low resolution and accuracy of traditional magnetometer,a method of microwave frequency modulation technology based on nitrogen-vacancy(NV)center in diamond for magnetic detection was proposed.The magnetometer studied can reduce the frequency noise of system and improve the magnetic sensitivity by microwave frequency modulation.Firstly,ESR spectra by sweeping the microwave frequency was obtained.Further,the microwave frequency modulated was gained through the mixed high-frequency sinusoidal modulation signal generated by signal generator.In addition,the frequency through the lock-in amplifier was locked,and the signal which was proportional to the first derivative of the spectrum was obtained.The experimental results show that the sensitivity of magnetic field detection can reach 17.628 nT/Hz based on microwave frequency modulation technology.The method realizes high resolution and sensitivity for magnetic field detection.
基金supported by the National Natural Science Foundation of China(61871146)the Fundamental Research Funds for the Central Universities(FRFCU5710093720)。
文摘Ultrahigh resolution synthetic aperture radar(SAR)imaging for ship targets is significant in SAR imaging,but it suffers from high frequency vibration of the platform,which will induce defocus into SAR imaging results.In this paper,a novel compensation method based on the sinusoidal frequency modulation Fourier-Bessel transform(SFMFBT)is proposed,it can estimate the vibration errors,and the phase shift ambiguity can be avoided via extracting the time frequency ridge consequently.By constructing the corresponding compensation function and combined with the inverse SAR(ISAR)technique,well-focused imaging results can be obtained.The simulation imaging results of ship targets demonstrate the validity of the proposed approach.
基金This work was supported by the National Natural Science Foundation of China （Nos. 11574347, 11734017, 91630308, and 11374322）, the Youth Talent Project of the Institute of Acoustics of Chinese Academy of Sciences （No. QNYC201619）, and the PetroChina Innovation Foundation （No. 2016D-5007-0304）.
文摘A wideband dipole signal is required for dipole dispersion correction and nearborehole imaging. To obtain the broadband flexural wave dispersion, we use a nonlinear frequency modulation （NLFM） signal and propose a segment linear frequency modulation （SLFM） signal as the dipole excitation signal to compensate for the excitation intensity. The signal-to-noise ratio （SNR） of the signal over the entire frequency band is increased. The finite-difference method is used to simulate the responses from a Ricker wavelet, a linear frequency modulation （LFM） signal, an NLFM signal, and an SLFM signal in two borehole models of a homogeneously hard formation and a radially stratified formation. The dispersion and radial tomography results at low SNR of the sound source signals are compared. Numerical modeling suggests that the energy of the flexural waves excited by the Ricker wavelet source is concentrated near the Airy phase. In this case, the dispersion is incomplete and information regarding the formation near or far from the borehole cannot be obtained. The LFM signal yields dispersion information near the Airy phase and the high-frequency range but not in the low-frequency range. Moreover, the information regarding the formation far from the borehole is not accurate. The NLFM signal extends the frequency range of the flexural waves by compensating for the excitation intensity and yields information regarding the formation information, but it is not easy to obtain. The SLFM signal yields the same results as the NLFM signal and is easier to implement. Consequently, the dipole detection range expands and the S-wave velocity calculation accuracy improves.
基金Natural Science Foundation of China (40275016)Operation "Six Kinds of Talents Summit"Foundation for Jiangsu Province
文摘Numerical experiments on forcing dissipation and heating response of dipole (unipole) are carried out using global spectral models with quasi-geostrophic barotropic vorticity equations. For each experiment model integration is run for 90 days on the condition of three-wave quasi-resonance. The results are given as follows: Under the effects of dipole (unipole) forcing source and basic flow intensity, there exist strong interactions among the three planetary waves and quasi-biweekly and intraseasonal oscillation of the three planetary waves. In the meantime, the changes in the intensity of dipole or unipole forcing source and basic flow have different frequency modulation effects on LFO in the middle and higher latitudes. The results of the stream function field of three quasi-resonant waves evolving with time confirm that the low-frequency oscillation exists in extratropical latitude.
基金the National Natural Science Foundation of China(Grant Nos.61973037 and 61673066).
文摘Conventional parameter estimation methods for pseudo-random binary code-linear frequency modulation(PRBC-LFM)signals require prior knowledge,are computationally complex,and exhibit poor performance at low signal-to-noise ratios(SNRs).To overcome these problems,a blind parameter estimation method based on a Duffing oscillator array is proposed.A new relationship formula among the state of the Duffing oscillator,the pseudo-random sequence of the PRBC-LFM signal,and the frequency difference between the PRBC-LFM signal and the periodic driving force signal of the Duffing oscillator is derived,providing the theoretical basis for blind parameter estimation.Methods based on amplitude method,short-time Fourier transform method,and power spectrum entropy method are used to binarize the output of the Duffing oscillator array,and their performance is compared.The pseudo-random sequence is estimated using Duffing oscillator array synchronization,and the carrier frequency parameters are obtained by the relational expressions and characteristics of the difference frequency.Simulation results show that this blind estimation method overcomes limitations in prior knowledge and maintains good parameter estimation performance up to an SNR of-35 dB.
基金Sponsored by the Ministerial Level Advanced Research Foundation (323010101-50)
文摘Modified implementation architecture for sinusoidal frequency modulation is introduced to extract the range information from the received radar echo. Range ambiguity problem arises because the range is calculated from the estimated phase of the received signal which is wrapped into （0, 2π]. By integrating Doppler frequency shifts, the variation of range can be estimated and used as an auxiliary information to help eliminating the corresponding range ambiguity. The performance of the new technique is evaluated by simulations. The results show that this technique is robust to sever phase noise and can be used effectively for ambiguity elimination of the modified sinusoidal frequency modulated continuous wave radar.
