Analysis Of Radiation Characteristics Of Microstrip Line In High Speed Digital Circuit

In high speed digital circuits,the faster and faster switching speed of transistors makes the frequency of high harmonic component of digital signal higher and higher,which causes more serious electromagnetic radiation problem,and severely affects the EMC performance of the system.In order to solve the potential EMC problems in the circuit and interconnection design.In this paper,a mathematical model of the radiation field intensity of the microstrip line under digital excitation source is established based on the analytical approximation method for the structure of the microstrip line in high speed digital circuit.Taking ideal square wave and ideal trapezoidal wave as examples,the radiation field intensity of each harmonic wave and its related factors are analyzed.The main research contents and results of this paper are as follows:Firstly,the radiation field of microstrip line under sinusoidal excitation source is deduced in this paper.the current distribution of the microstrip line is determined according to the power supply and impedance boundary conditions of the lossless transmission line,and then the radiation field of the microstrip line is obtained by superposition of the far field of the current element according to the current distribution.The influence of the microstrip line return plane is treated by mirror principle.The influence of dielectric layer and conductor loss is neglected in the derivation.In this paper,the traveling standing wave field of the microstrip line is derived when the load ends of the microstrip line are matched,then the standing wave field of the microstrip line is derived when the load ends of the microstrip line are mismatched,finally the moving standing wave field of the microstrip line is derived under any termination according to the linear superposition principle.For the digital excitation source,this paper expands it into the superposition of sinusoidal waves of different frequencies through Fourier series,and then superpositions the radiation fields of sinusoidal waves of different frequencies to obtain the radiation fields of microstrip lines under the digital excitation source.Through the analysis of ideal square wave and ideal trapezoidal wave,it is found that there are three kinds of factors affecting the field intensity under the digital excitation source: The first is the excitation source,including amplitude,frequency and rising edge time.The second is the geometric length,including conductor length and medium thickness;The third is the impedance,including the source impedance,characteristic impedance and load impedance.In this paper,the relationship between the radiation field intensity of each harmonic component and the excitation source and geometric length is analyzed under all matching,source matching and load matching.The influence of source impedance and load impedance on the series termination and the parallel termination is also analyzed.For the ideal square wave excitation source,in source matching,the harmonic field intensity first increases with harmonic frequency and then oscillates around a certain value.Under load matching and both matching,the harmonic field intensity increases monotonically with harmonic frequency and then approaches to a certain value.From the point of view of reducing EMI,both matching are optimal.Under three kinds of matching,the harmonic field intensity first increases and then approaches to a certain value with the conductor length.The higher the harmonic frequency is,the shorter the conductor length will be when the harmonic field intensity approaches a fixed value.The harmonic field intensity increases monotonically and linearly with the increase of medium thickness under three matching conditions.Under the premise of impedance control,minimizing the length of microstrip line and medium thickness is conducive to reducing EMI.Under the premise of meeting the signal quality requirements,larger source impedance is beneficial to reduce EMI.For parallel termination the selection of high-precision resistor is beneficial to improve the signal quality and reduce EMIFor the ideal trapezoidal wave excitation source,the radiation is the minimum when both are matching.The harmonic field intensity of the three matching first increases with the harmonic frequency and then decreases after reaching the maximum value corresponding to the turning frequency.The turning frequency is related to the rising edge time,the longer the rising edge time is,the smaller the turning frequency is,and the faster the field intensity decays with frequency after the turning frequency is exceeded.The relationship between harmonic field intensity and conductor length and medium thickness is the same as that of ideal square wave.The effect of impedance variation is also similar to that of an ideal square wave.It is difference that with the increase of the rising edge time,the influence of the change of source impedance and load impedance on the harmonic radiation field decreases.When the rising edge time reaches one tenth of the signal cycle,the effect of the load impedance on the harmonic radiation field is almost negligible.Therefore,while the signal quality is satisfied,the selection of low-rate devices is helpful to reduce EMI.