Application Of Time-streched Structure In Instantaneous Frequency Measurement Receiver

Determining the instantaneous frequency of a high-frequency,ultra-wideband signal accurately is the primary task required for many fields ranging from radio detection to astronomy.Especially in the electronic warfare,the carrier frequency is a very important one among the virous parameters of radar signals because the frequency information can be used for signal sorting,direction finding,threat identification and other further work,which can obtain the enemy target information quickly and accurately and implement effectively disrupt and accurately strike.However,the characteristics of large signal density,serious overlap in time domain and wide frequency coverage in modern electronic warfare environment requires a digital receiver which has the processing capabilities of ultra-wideband and ultra-high-speed signal.As a bridge to convert analog signals into digital signals,the analog-digital converter plays a crucial role in the electronic system.However,subject to the electronic bottleneck,the limitations of analog-to-digital converters in the processing of high-frequency broadband signals have also become the bottleneck for implementing high-speed digital signal processing,which makes researchers have to try to find ways to solve the problem in other fields.In recent research,photoelectric analog-digital conversion has shown tremendous advantages with high-speed and large-bandwidth signals,which can break through the predicament in processing certain signals,so as to realize high-speed and high-precision analog-digital conversion.However,most researches at home and abroad have studied the structure of photoelectric analog-to-digital conversion independently,and the comprehensive research on specific application fields still relatively few.So the purpose of this paper is to study the application of Photonic Time-Stretched(PTS)model in the instantaneous frequency measurement receiver.The model is only a preprocessing method and the final frequency measurement is still carried out in the electric field.The main principle of the photonic time-stretched model is to modulate the detected high-speed electrical signals to the super continuum through the photoelectric modulator and process them in the optical domain.When the optical signal is dispersing in the optical fiber,the time required for signals of different wavelengths to pass through the same length of optical fiber varies.Therefore,the optical modulated signal carrying the electric signal has a certain degree stretched in time domain at the output.As the signal is expanded in the time domain,the rate of the signal decreases correspondingly,which eventually reaches the purpose of treating high frequency signal with low speed ADC.The mathematic model of the time-stretched structure is demonstrated in detail,as so to analyze the influence of the dispersion of the optical fiber in detail when the optical pulse is transmitting in it.Through the establishment of the mathematical model,the effectiveness of the structure is demonstrated in detail.The application scope,stretched factor and influences on the SNR of the time-stretched structure are also analyzed.In order to define the performance indicators of the photonics-assisted instantaneous frequency measurement receiver,numerical simulation will be carried out to analyze the influence of structure parameters of the optical fiber and the photoelectric modulation mode on the inputtable bandwidth of the time-stretched structure.Through the derivation of the mathematical model,the effects of high-order dispersion and limited optical bandwidth on the system output and frequency measurement results are analyzed in detail.In order to improve the computing speed and precision of traditional frequency measuring algorithms FFT,this paper adopted the instantaneous frequency measurement based on the autocorrelation method.Through the mathematical model,the IFM algorithm based on the autocorrelation function is deduced in detail,and the validity of the algorithm is verified with the Cramer-Rao lower bound.Because of the single-bit receiver's advantages in devices and processing speed,this paper will further analyze the frequency measurement performance of the algorithm under the condition of single-bit quantization and analyze the sources of error.Finally,an optical simulation link is set up on optical simulation software.By analyzing the principle of Mach-Zehnder,the influence of fiber dispersion on different modulation modes is analyzed amply.And the time-stretched structure based on DSB and SSB are set up by the various Mach-Zehnders to pre-process high-speed signals and the frequency of the output signal is measured with Matlab.The condition under which SNR can reach effective measurement is tested,and the performance of the system is also tested.