基于优势效应理论的双绞线简化建模

在对双绞线电磁干扰进行研究中,一些学者建立了精确的干扰模型,但并不适合工程应用.本文根据电磁兼容优势效应理论,推导出双绞线传导干扰主要由差模电流引起,辐射干扰主要由共模电流引起.分别利用广义二端口网络模型和电流元分析方法建立双绞线串扰和辐射的简化模型,从而有效估算双绞线串扰电压和对外辐射场的大小,为工程人员采取有效措施抑制互连设备电磁干扰提供了理论帮助.     

新能源汽车电力线缆串扰仿真分析与抑制方法

新能源汽车主要依靠电力驱动,其电力线缆传输高电压、大功率的瞬变信号,加之线缆本身的天线效应,使其成为电动汽车中最主要的电磁干扰发射源。因此电力线缆极易对周围线缆产生串扰。通过分析电力线缆辐射电磁干扰的来源,利用FEKO仿真工具建立了电力线缆和相邻线缆的简化模型。通过改变仿真模型中线缆屏蔽层及金属线槽的设置,对比分析了这些因素对抑制串扰和干扰发射的影响。根据仿真结果,得出了降低线缆屏蔽层阻抗和增加金属线槽的使用,有助于抑制线缆串扰的结论。     

机载线缆间的串扰耦合分析

串扰是机载设备间互联线缆干扰耦合的重要因素。以混合模S参数为基础,建立单线-双绞线模型以模拟机载设备之间的动力线缆对信号线产生的串扰耦合。在此模型基础上提出串扰耦合测试方案,搭建串扰耦合测试系统,并根据测试获得的耦合系数评估单线-双绞线线间串扰耦合强弱。通过测试比较,对影响因素进行分析,结果表明,距地高度对线间串扰影响不大,线间距对线间串扰耦合影响显著,因此在条件允许下尽可能增加设备间互联线缆的间距可有效抑制串扰耦合。     

刍议双绞线的抗干扰及抗串扰原理

双绞线是一种广为人知,由两互相绝缘的铜导线按一定密度的螺旋结构绞合成的绞线对,双绞线传输形式在模拟和数字等许多领域得到了广泛的应用.双绞线的使用,不仅有效降低了外界电磁场对双绞线的干扰,以及双绞线线对之间的串音干扰,而且有效降低了非平衡型传输线之间的互电容,使传输线的特性阻抗能够保持恒定.本文对双绞线的抗干扰和抗串扰原理进行了初步分析和探讨.     

电缆的干扰问题与对策

电缆是电磁兼容设计中的关键环节,大部分电磁干扰敏感问题、电磁干扰发射问题、信号串扰问题是电缆产生的。电缆之所以容易产生各种电磁干扰问题,主要有以下几个原因。 （1）电缆本身是一根高效的接收天线,能够接收到空间的电磁干扰,将干扰能量传进设备电路,造成干扰; （2）电缆是一根高效的辐射天线,能够将电路中的干扰辐射到空间,造成辐射发射超标; （3）电缆中的导线相互平行,并且靠得很近,导线之间具有很大的寄生电容和互感,这些电容和互感是导致串扰的根本原因; （4）电缆连接的设备接地电位不同,电缆屏蔽层引进地线干扰。 前三个情况是很直观的,第四个情况如图1所示。     

同轴电缆串扰的仿真计算与测试

鉴于互连系统中电缆的电磁干扰问题,本文首先论述了由传输线理论对同轴电缆串扰的计算仿真,进而根据此模型自制了同轴电缆电磁耦合测试装置,测试若干结构参数对同轴电缆串扰电压的影响,得出了同轴电缆间串扰电压大小随电缆长度、干扰源电压和频率的增大而增大,最后比较仿真结果和测试结果,基本上达到了很好的一致性,此测试方法和结论对电气工程师测试和解决电缆串扰问题具有一定的实用价值.     

同轴电缆串扰的仿真计算与测试

鉴于互连系统中电缆的电磁干扰问题,首先论述了由传输线理论对同轴电缆串扰的计算仿真,进而根据此模型自制了同轴电缆电磁耦合测试装置,测试若干结构参数对同轴电缆串扰电压的影响,得出了同轴电缆的串扰电压大小随电缆长度、干扰源电压和频率的增大而增大,最后比较仿真结果和测试结果,基本上达到了很好的一致性,此测试方法和结论对电气工程师测试和解决电缆串扰问题具有一定的实用价值。     

电动车辆电磁兼容性整车测量与分析

基于电磁兼容理论,指出了电动车辆系统中的电磁干扰源与干扰途径,详细说明了电动车辆驱动系统中高功率变换器件和高频率陡沿等干扰源;同时对电动车辆中电磁干扰中的耦合与串扰、共地干扰等进行建模与分析。最后通过整车干扰的测试,总结了电动车辆电磁骚扰在电磁干扰方面的特性。     

PCB板级双绞互连结构减小感性和容性串扰噪声

非屏蔽双绞线已经被广泛的运用于网络的互联中,双绞的结构能够提高对串扰和辐射发射的抵抗能力。在高速数字电路的PCB板上差分信号变得越来越多。随着上升时间的加快,差分信号的信号完整性问题变得越来越重要。最近,一种新的双绞差分传输线被引入到布线中。文章通过理论和仿真分析了这种双绞差分线结构如何减小串扰和辐射发射。     

基于电路参数模型的辐射电磁干扰噪声预估方法

针对Hubing电流驱动模型中认为电流在辐射线缆中是均一分布的, 幅值和相位保持不变即未考虑辐射线缆共模电流频率效应给辐射电磁干扰噪声预估带来的误差的问题.文中利用电流传输波动特性建立了辐射线缆长度与共模电流波长为同一数量级时的辐射线缆共模电流分布模型, 并设计电路模型进行测试预估.实验结果表明:采用文中方法预估辐射电磁干扰噪声与Hubing电流驱动模型预估方法相比能提高20.12 dBμV/m, 更加接近标准暗室测试结果, 从而为辐射电磁干扰(Electro Magnetic Interference, EMI)测试与分析提供理论依据.     

开关电源散热器的辐射发射

开关电源电路中,接地散热器提供了一条共模骚扰高频谐波电流的通路,使共模传导电磁干扰增加;非接地散热器切断了共模传导电磁干扰的通路,但增强了散热器辐射发射,射频辐射的增强能够引起设备误操作或者违反目前的电磁兼容性标准.文中分析功率开关管散热器产生辐射发射和共模传导电磁干扰的机理.采用有限元方法,通过电磁场数值计算,探讨非接地散热器的几何形状、尺寸和安装方式对其辐射发射的影响.数值计算结果表明:散热器面积越大、高次谐波频率越高,辐射电场幅值就越大,但是辐射电场幅值与散热器面积、高次谐波频率无线性关系;相同面积的散热器中,圆形散热器的散热效果最佳、辐射电场最小.因此,开关电源设计中,应根据散热效果、辐射EMI和共模传导EMI,综合考虑散热器面积、形状的选择,以及散热器是否接地.     

Electromagnetic Design Techniques for Liquid Crystal Display Driver ICs

A simple method of verifying electromagnetic interference (EMI) reduction effects for liquid crystal display (LCD) driver integrated circuits (ICs) is proposed. In this paper, we discuss correlations between radiated emissions and high-frequency currents of power system at three different levels: print circuit board (PCB) level, chip level, and functional circuit level. The EMI design points for LCD driver ICs are presented too. Simulated and measured results prove that our EMI design effectively reduces LCD EMI noise.     

Radiated emissions and immunity of microstrip transmission lines:theory and reverberation chamber measurements

The increasing complexity of electronic systems has introduced an increased potential for electromagnetic interference (EMI) between electronic systems. We analyze the radiation from a microstrip transmission line and calculate the total radiated power by numerical integration. Reverberation chamber methods for measuring radiated emissions and immunity are reviewed and applied to three microstrip configurations. Measurements from 200 to 2000 MHz are compared with theory, and excellent agreement is obtained for two configurations that minimize feed cable and finite ground plane effects. Emissions measurements are found to be more accurate than immunity measurements because the impedance mismatch of the receiving antenna cancels when the ratio of the microstrip and reference radiated power measurements is taken. The use of two different receiving antenna locations for emissions measurements illustrates good field uniformity within the chamber     

Line code selection for 155.52 Mb/s data transmission on category 5cable plant

EMI/EMC is a dominant problem encountered in high bit-rate (>100 Mb/s) transmission over unshielded twisted-pair cable (UTP) which leads to a novel set of line-code dependent tradeoffs affecting transceiver and cable plant complexity. To understand the tradeoffs, the authors examined the factors affecting RF emissions and susceptibility in both trellis-configured cable plant models and installed cable plant. An analytical model is presented that describes mode conversion by discontinuities in multi-pair UTP cable plant. Termination of the three dominant propagating modes produces consistently lower radiated emissions than other standard cable termination procedures. Cable plant parameters which predict good transmission do not assure satisfactory EMI performance. Radiated emissions from the trellis model occur in broad bands at 43, 53, 60, 70, and 80 MHz, and show a positive correlation with the transmit signal spectrum. For a given transmit level at 155 Mb/s, the emissions with MLT3 and BPR1 were 8-13 dB (10 dB typ) and 4-20 dB (13 dB typ) below NRZ levels, respectively. The authors also compared the performance of NRZ, MLT3, BPR1, and BPR4 line codes in an experimental 155 Mb/s link with 100 m of UTP5 by measuring the BER sensitivity to both injected noise and pseudo-random data sequence length. When the measured receiver penalty associated with three levels is included, MLT3, BPR1, and BPR4 offer 5-15 dB better performance over NRZ at 155 Mb/s, although implementation complexity is greater     

A comparison of the contributions of common-mode anddifferential-mode currents in radiated emissions

It is shown that radiated emissions due to common-mode currents on printed circuit board lands can greatly exceed those due to differential-mode currents. It is concluded that predictions of radiated emissions based solely on differential-mode (transmission line) currents can bear little, if any, resemblance to actual measured emissions. Therefore, radiated emission prediction models that use only differential-mode (transmission-line) currents and ignore common-mode (antenna) currents are not adequate for the prediction of radiated emissions from printed circuit boards (as well as cables)     

Multilayer power printed structures suitable for controlling EMI noises generated in power converters

Electromagnetic interference (EMI) noises generated in power converters are diffused on the surface of conductors. This means influences occur from radiated EMI noises emitted from power transmission lines as well as conducted EMI noises transmitted from them. EMI noises diffusing on the surface of conductors are generally difficult to control using conventional concentrated constant theory. Thus, a new approach based on distributed constant circuit theory is needed in order to control EMI noises. A power converter structure to control EMI noises using multilayer power printed circuit technology is studied in this paper. A structure which can control EMI noises should simultaneously satisfy two conditions, i.e., one to shut down and one to attenuate EMI noises. The structure satisfying these conditions is studied through simulations using the Transmission-Line Modeling Method. The simulations show that the diffusion of EMI noises is controlled by dividing the flow of currents produced by EMI noises into the horizontal and perpendicular directions. That is, the horizontal current flow is controlled inside using the differences in the resistance produced from differences between inner and outer diameter of power transmission lines and the perpendicular current flow can be controlled by properly designing the thickness of the dielectric layer sandwiched between P-and N-power transmission lines with the symmetrical structure. Moreover, it is confirmed by simulations and experiments that the attenuation of EMI noises is affected by the width of the power transmission lines. It is expected that the results obtained in this paper can provide important rules when designing power converters with EMI noise control functions which use the multilayer power printed circuit technology.     

A generalized expansion for radiated and scattered fields

At a given frequency every perfectly conducting obstacle has associated with it a particular set of surface currents and corresponding radiated fields which are characteristic of the obstacle shape and independent of any specific excitation. These characteristic modes form a useful basis set in which to expand fields radiated or scattered at a great distance from the obstacle. Once these modes are known for a given obstacle, the scattering of plane waves incident from arbitrary source directions into arbitrary receiver directions may be evaluated concisely. To support the theory, a method is described for determining characteristic mode currents on thin wires of general shape and is applied to several shapes to generate certain backscattering and input admittance data. Wherever possible comparison is made with existing data.     

EMI mitigation with multilayer power-bus stacks and via stitchingof reference planes

General methods for reducing printed circuit board (PCB) emissions over a broad band of high frequencies are necessary to meet EMI requirements, as processors become faster and more powerful. One mechanism by which EMI can be coupled off a PCB or multichip module (MCM) structure is from high-frequency fringing electric fields on the DC power and reference planes at the substrate periphery. An approach for EMI mitigation by stitching multiple ground planes together along the periphery of multilayer PCB power-bus stacks with closely spaced vias is reported and quantified in this paper. Power-bus noise induced EMI and coupling from the board edges is the major concern herein. The EMI at 3 m for different via stitch spacing and layer thickness is modeled with the finite-difference time domain (FDTD) method. Design curves and an empirical equation are extracted from a parametric study to summarize the variation of the radiated EMI as a function of layer thickness and stitch spacing     

Characterizing the Electric Field Coupling from IC Heatsink Structures to External Cables Using TEM Cell Measurements

One method for evaluating the unintentional radiated emissions from integrated circuits (ICs) involves mounting the IC on a printed circuit board (PCB) embedded in the wall of a transverse electromagnetic (TEM) cell. The signal voltages on the IC and its package produce electric fields that can couple to cables and other structures attached to the PCB, inducing common-mode currents that can be a primary source of unintentional radiated emissions. The signal currents in the IC and its package produce magnetic fields that can also result in common-mode currents on larger radiating structures. This paper describes a TEM cell measurement method employing a hybrid to separate the electric field coupling and the magnetic field coupling. The results of this measurement can be used to determine the product of the IC's self-capacitance and the effective voltage that drives this capacitance. This voltage-capacitance product characterizes the IC's ability to drive common-mode currents onto cables or enclosures due to electric field coupling. This information can then be used to estimate the resulting radiated emissions.     

Some electromagnetic aspects of coreless PCB transformers

In this paper, some EMI aspects of using coreless PCB transformers are addressed. Based on the antennas theory, the radiated power of a coreless PCB transformer is estimated and found to be negligible. The electromagnetic field plot of a power electronic circuit using a gate drive circuit isolated by a coreless PCB transformer has been recorded. The major radiated EMI source in the frequency range of 30 MHz to 300 MHz is found to be the copper tracks of the power circuit, where switching transients occur, rather than the coreless PCB transformer. Coreless PCB transformers essentially operate at relatively low frequency (8 MHz in this case) by near-field magnetic coupling. Experimental results have confirmed that the application of coreless PCB transformer in gate drive circuit will not impose any serious EMI problem on the power electronic circuit