Study On Realization Methods Of Electromagnetic Time Reversal Based On Analog Signal Processing

Owing to the synchronously temporal and spatial focusing characteristics,time reversal technology,which has been introduced in electromagnetic regime,has been found many applications in recent years,such as high resolution imaging,target detection,localization,high-speed wireless communication,and wireless power transfer.As the key part of an electromagnetic time reversal system,how to realize time reversal of electromagnetic signals efficiently,real-time and accurately remains challenges.In this dissertation,methods to realize the time reversal of a high frequency signal based on analog signal processing have been studied.The main contents are as follows:In Chapter One,the research background and significance of this study are briefly introduced.The literature reviews are performed that are focused on the analog signal processing based time reversal at home and abroad.The contents and contributions of this work are presented.Chapter Two discusses the methods to implement the time reversal that is based on time-varying guided wave system.A time-varying guided wave system,based on time-varying transmission line loaded high-speed microwave switches,is described and discussed.Based on theoretical analysis of the equivalent model,it is found the time reversal of the input signal can be achieved.The time-varying guided wave system is evaluated numerically.The microwave switches controlled by a sinusoidal pulse signal are utilized to implement time-varying of the system.Experiments on the prototype show that an amplitude-modulated saw signal can be time-reversed effectively,where the signal has a pulse width of 7 ns and the carrier is 3.5 GHz.Experiments match well with numerical simulations,meanwhile,high transformation efficiency is realized.Further,some physical parameters related to the system are analyzed and discussed,such as carrier frequency,pulse width,switch numbers,dielectric constant of the substrate,amplitude and waveform of the control signal,and so on.Conclusions that effect on the system performance are drawn and some improvements are suggested.Based on these,some other schemes are presented.For the substrate integrated waveguide,the scattering parameters can be time-varied by many groups of trapezoid open-circuit branches connected with microwave switches controlled by sinusoidal signals.The corresponding topology is described and analyzed numerically under different input signals.Results show that this system can realize time reversal of electromagnetic signals.The important controlling component,multi-way differential controlling network,is detailed.Two different prototypes are provided.Compared with other similar contributions,the proposed network exhibits simple and compact architecture,and good performance.In Chapter Three,the spoof surface plasmon polaritons and its application to time reversal of electromagnetic signals are discussed.A type of planar surface plasmon polariton structure based on microstrip is presented.The characteristic parameters of this structure are analyzed and further,the corresponding scheme is proposed.Starting with Maxwell equation,the propagation of electromagnetic wave in a time-varying media is analyzed theoretically.The reflection wave generated in the time-varying medium is corresponding to the time reversal of an incident wave.The characteristic impedance of the surface plasmon polariton system can be time-varied by microwave switches periodically loaded,thus realizing the time reversed waveform of the input signal.Numerical analyses for different signals are carried out,and experiments show that a saw signal with a pulse width of 10 ns and the amplitude-modulated carrier of 3.3 GHz can be time reversed effectively.In Chapter Four,the time reversal system based on phase conjugating is studied.The operation principles of a chirp signal and a sinusoidal signal are analyzed based on the Fourier transformation.Results show the two kinds of signals can be time reversed by using the phase-conjugating methods.A prototype network,composed of ring hybrid and branch line balun,is developed.By loading short-circuited stub or using different impedance of the quarter or half wavelength branch-lines,the amplitude and phase bandwidth of the passive component can be enhanced effectively.To verify the analyses,the phase-conjugating network is examined experimentally.Measurements on a chirp signal with a bandwidth of 40 MHz are performed and good performance is observed.Further,a method called synchronous phase conjugation mixing is proposed.Radio frequency signals sweeping from 5.4 to 6.25 GHz with dynamically synchronized local oscillation frequencies from 10.8 to 12.5 GHz are investigated experimentally,and results validate this study.In addition,realizing phase-conjugating of a single-frequency signal in a cavity is studied.The incident wave can be absorbed by a metamaterial structure placed in the metal waveguide cavity,and then phase-conjugated by the mixing component.The model of the system is given,and numerical analyses of a 2.4GHz signal are carried out.Two different systems are proposed;the scattering parameters are analyzed preliminarily.This method also provides another way to achieve the time reversal of single-frequency signal.In Chapter Five,time reversal based on temporal-spatial conversion is discussed.After the analysis of a signal in time and frequency domain,time reversal can be achieved by using twice Fourier transformations.A network based on this approach is presented and analyzed.This method provides another solution for the time reversal of high frequency signal using analog signal processing technologies.Chapter Six summarizes this work,and further presents the later studies that are based on the analog signal processing to realize the time reversal of electromagnetic signals.