Key Technology Of VLF Communication Reconnaissance System

The Very Low Frequency(VLF)is a very characteristic frequency band in the radio frequency spectrum.This kind of signal can penetrate the sea water and go deep into the rock stratum with small attenuation,strong anti-interference ability and long propagation distance.Therefore,it is increasingly widely used in submarine communication,ocean communication and geological prospecting.At the same time,with the escalation of the arms race,the VLFS plays an increasingly critical role in various special communications.In the detection and reception system,the receiving front end,as the decisive module of receiver performance,determines the receiver performance to a large extent and is the core part of receiver design.In this paper,the VLF high resolution detection and reception system is studied and designed as an anti-noise,high dynamic range VLF signal detection and reception front end.The main contents of the paper are as follows:Firstly,according to the characteristics of the input signal,the technical specifications of the receiving front-end circuit are proposed.The circuit structure of Automatic Gain Control(AGC)is analyzed and discussed according to the requirements of the indicators.By comparing the performance of the commonly used single-feedback AGC circuit,the single-operational amplifier and double-feedback AGC solution is finally adopted.The scheme can select corresponding control modes according to different signal states,expand the dynamic range of the receiver,and well integrate the advantages of the front and back feedback AGC structure.In the aspect of detection algorithm,by consulting the relevant data,it is finally determined that the detection mode is implemented by the Cordic+FFT operation mode.Compared with other conventional detection algorithms,the algorithm has a greater advantage in reconnaissance of narrowband signals in broadband,and can display signals in the entire frequency band,and control a certain signal.Then the following digital signal processing unit is studied,which mainly includes channelization,signal reconstruction and carrier bandwidth identification.After analyzing the theory of each part,the specific implementation scheme is given.In order to overcome the drawbacks of traditional channelized structure design,which is cumbersome and resource-intensive,this paper adopts a multi-phase structure-based channelization scheme,which transforms complex filter bank design into a single prototype filter design,effectively reduces the design complexity and computational pressure,and introduces an approximate complete reconstruction unit to realize dynamic channelization.By comparing various filter design methods,a prototype filter is designed by single parameter optimization method,which makes the stopband attenuation of the filter reach-110 dB,and effectively reduces the aliasing distortion introduced in the process of signal reconstruction.In order to improve the system function,the carrier frequency and bandwidth estimation unit is added in this paper.The carrier frequency and bandwidth estimation of the input modulation signal is realized by the square frequency doubling method and the energy concentration method,and its effectiveness is confirmed by the simulation of MATLAB.Finally,according to the above schemes and engineering indicators,PCB and FPGA at board level are developed.Through the actual development and verification,the test results are analyzed,which proves the correctness of hardware circuit design and meets the expected requirements.