Research On The Key Technology Of Down-conversion

At present,fast broadband radio frequency spectrum detection has become an important technology in modern communication and information processing,especially in civil,military and scientific research.It has an important strategic position and role.With the development of radar,electronic warfare,radio astronomy,wireless communications and satellite communications,the requirements of microwave operating frequency is becoming higher and higher.However,the frequency interception of fast high-frequency signals is very hard,and it is impossible for conventional electronic technology to achieve the detection and analysis of ultra-wideband fast radio frequency signal frequencies.Therefore,the fast broadband radio frequency spectrum measurement technology based on microwave photonics has been a hot and difficult topic in recent years.High speed,high sensitivity,low loss,large bandwidth,strong anti-interference ability and other advantages will become an important development trends of information technology research in the future.Down-conversion is a technology of analysing and processing,which can convert high-frequency signals into mid-low frequency signals.What's more,its aim is to reduce the harsh conditions on electronic technology and systems at high frequencies.This paper conducts a study to realize the key technology of down conversion which is real-time detection of radio frequency signals based on fiber loops.In the detection of broadband radio frequency signals,the optical frequency comb and the optical carrier signal loaded with radio frequency information can be used to beat the frequency,and the entire frequency spectrum is divided into multiple small frequency bands,and then the coherent detection of the broadband radio frequency signal is performed by down-conversion technology.In the method of dividing the frequency spectrum by generating an optical frequency comb and passing through a tunable filter,whose accuracy of the filter is relatively high.Therefore,the method of directly generating a frequency scanning laser is proposed for dividing the frequency spectrum in the experiment and a coherent receiver based on an optical mixer to achieve the down-conversion technology.The main contents of this article are as follows:1.Through theoretical analysis,a scheme of generating a frequency scanning laser directly based on acousto-optic modulation is proposed.The laser uses a ring cavity structure,which has some features such as fast frequency sweep,high frequency sweep accuracy,good signal-to-noise ratio and so on.The principle is elaborated and theoretical derivation,simulation analysis and experimental verification are carried out in this paper.2.Optimizing the existing frequency scanning laser which includes the parameters of cavity length,EDFA,acousto-optic modulation frequency of the frequency scanning laser on the flatness and signal-to-noise ratio of the generated frequency scanning laser.By analyzing the influence of parameters such as the signal-to-noise ratio of the laser and the length of the erbium-doped fiber in the erbium-doped fiber amplifier on the performance of the frequency-swept laser,simulations and experiments are performed to achieve optimization.During the experiment,when the external modulation signal injected by the FPGA is a pulse signal with a pulse width of 200 ns and a period of 8 ?s,a frequency scan in the negative 0.3 nm range happens.3.The coherent detection of the down-conversion system built by a 90° mixer,considering the influence of signal-to-noise ratio and gain,an ultra-wideband scanning radio frequency receiver based on microwave photonics is implemented,and the simulation results are used to analyze and verification.The frequency-sweeping laser system and the coherent detection system built in this paper have lower cost and simpler structure,therefore,they have achieved preliminary results and provided new ideas for frequency scanning and RF detection schemes.Researches on microwave spectrum measurement based on microwave photonics is at a stage of rapid development,and relevant researches and innovations are of great practical significance and value.