Time-domain study of a shielding cable system with a nonlinear load

An impulse current of several kiloamperes was injected to the shield of a shielded cable, which was terminated by a varistor. The induced voltage on the inner conductor caused by this impulse current reaches an amplitude in excess of the varistor's threshold level. The clamped voltage across the varistor and the injected current have been studied for different termination conditions of the shielding cable. Furthermore, this paper also presents the use of a lumped circuit to simulate the transfer impedance of an “electrically short” shielded cable in the time domain. In combination with the varistor circuit model, the shielded cable with the nonlinear load, a varistor, was also simulated straightforwardly in the time domain. Good agreement was found between the measured voltage and current oscillograms and the calculated waveforms. It is thereby demonstrated the validity of the developed lumped circuit model for the transfer impedance of a shielded cable     

Overvoltage suppression filter design methods based on voltage reflection theory

To reduce voltage overshoot at the motor terminal, RLC filters are used at the inverter side with an objective of increasing the rise time, while RC filters are used at the motor side as a means of reducing the load impedance at high frequency. However, no clear optimal method for determining the filter parameters has appeared. In this work, we propose filter design methods that fully utilize given conditions such as cable length, cable inductance, cable capacitance, and the reflection coefficient at the inverter side. For determining the parameters of the RLC filter, the filter transfer function is utilized to make the rise time long enough to achieve desirable overshoot level at motor terminals. In choosing the parameters of the RC filter, the reflection coefficient is regarded as a transfer function between the incident and reflected voltages, and the capacitance is chosen so that cancellation occurs between the reflected voltage and its resulting incident voltage. The validity of the proposed design method is supported by simulation results, which are also compared with the experimental results.     

Current flow in coaxial braided cable shields

In the last few years, much effort has been made to describe the behavior of shielded cables. Many researchers have attempted to understand how an electromagnetic field couples into a braided coaxial cable. There are some important publications on this topic. Nevertheless, up to now, it has not been possible to predict analytically the coupling through a braid shield. An electromagnetic field outside a cable induces a disturbance current in the cable shield. The coupling from the current in the shield into the cable can be described by the transfer impedance. How the current flows in the cable shield is an important quantity in this coupling process. Therefore, to understand the coupling mechanism into a cable, it is necessary to understand the behavior of the current flow in such a braided shield. The paper discusses the current flow in a braided cable shield. The assumption often made in the literature, that a braided shield behaves like a homogeneous tube with apertures, is shown to be inaccurate. It is also shown that the standard braid of the shield used had the same properties as a braid made with insulated wires.     

New method for measuring transfer impedance and transfer admittanceof shields using a triaxial cell

Two performance parameters of a cable or connector shield are its surface transfer impedance Z_T and its surface transfer admittance Y_T. A new method for measuring these properties is presented. The use of two different terminations for the cable or conductor under test (CUT) allows one to determine both Z_T and Y_T. Through characterization of the inner and outer transmission lines of the triaxial cell, using time domain reflectrometry, Z_T and Y_T can be determined in amplitude as well as in phase. The phase is obtained by de-embedding the measured S-parameters up to the CUT. The de-embedding of the measurements also allows one to extend the frequency range up to 3 GHz. To illustrate this method a solid shield with a circular aperture and a coaxial cable with a braided shield have been measured and compared, respectively, with theoretical predictions and published results     

The transfer impedance of cables with a nearby return conductor anda noncentral inner conductor

The transfer impedance Z_t of a cable is often assumed to be a characteristic of the shield only. We investigate the limits of this assumption by calculations and measurements. The first test cable was formed by a solid copper tube (“shield”) and a wire inside; several positions, also non-central, were chosen for the wire. The common mode current (between 10 Hz and 100 kHz) through the tube had as return path a single wire, which was placed at several positions, near and at some distance from the shield. A second cable with a braided shield was tested as well. The results show that both the differential mode and the common mode circuit have to be carefully defined for a particular Z_t. Varying either circuit may alter Z_t drastically. Consequently, the Z_t of a particular cable measured in a triaxial setup, is a characteristic of that setup and cannot always be used in another setup as in for instance cable bundles     

Electromagnetic coupling to and from shielded cables-optimizingfactors

Judging the shielding effectiveness of shielded cables often means in practice that only the transfer impedance is considered. The transfer impedance essentially characterizes the coupling via the magnetic field; the coupling via the electric field, the transfer admittance, is mostly neglected. This may be correct for shields with high optical coverage but for optimized single braided shields (coverage ≈0.8 . . . 0.9), the transfer admittance has to be taken into account. In practice, the cable shields are mostly grounded or open-ended at the line ends. With regard to the shield connections, the electromagnetic coupling to a cable by a plane wave and coupling from a cable are investigated. From the results, optimizing factors for the coupling parameters of shielded cables are deduced. By means of these optimizing factors the coupling to and from a cable can be minimized in certain applications     

屏蔽腔内任意高度线缆端接TVS管电路的电磁耦合时域分析

该文基于时域有限差分(FDTD)方法和传输线方程,结合插值技术和牛顿迭代法,提出一种新的时域混合算法,能够快速模拟屏蔽腔内任意高度线缆端接瞬态电压抑制(TVS)管电路的电磁耦合问题,并实现空间电磁场与线缆和电路瞬态响应的同步计算。该算法首先利用FDTD方法结合STL网格剖分技术实现屏蔽腔结构的快速建模以及腔体内空间电磁场分布的准确模拟。然后利用传输线方程结合插值技术建立腔体内线缆的场线耦合模型,结合FDTD方法,迭代求解出线缆上的电压和电流响应。对于线缆端接的TVS管电路,列写电压电流方程,采用牛顿迭代法计算得到电路端口的电压响应。通过与电磁仿真软件的计算结果进行对比,验证了所提时域混合算法的正确性。研究表明,该算法能够很好地应用于屏蔽腔内线缆端接负载的TVS管限幅防护设计。     

Cable Trays in EMC: Measurement and Modeling to 30 MHz

Common mode (CM) currents are a major source of interference in electrical and electronic systems. Cable trays are often used to shield cables from unwanted CM electromagnetic interference, and their shielding characteristics are defined in terms of transfer impedance. We present the measurement and modeling of nonmagnetic U-shaped cable trays from 300 kHz to 30 MHz. A calibrated vector network analyzer in a screened environment is required for the high dynamic range measurements. We use method of moments simulations to determine the transfer impedance and mutual inductance within the interior region of a cable tray. We refined the modeling after detailed attention to the code. The computational and measured data are in good agreement. We propose the simulation as a means to predict the magnetic fields, mutual inductance, and transfer impedance associated with victim cable loops in the cross section of nonmagnetic cable trays to frequencies well beyond our studied range of 30 MHz.     

A rapid method for measuring the transfer impedance of connectors

An inexpensive, simple, and sensitive workbench setup has been developed to determine the transfer impedance Z_t of shielded connectors. The injection current is measured by an inductive sensor integrated in the setup. The induced voltage is determined by a spectrum/network analyzer. The overall sensitivity is 3 μΩ or better; the frequency range is up to 1 GHz. No special calibration samples are needed. As a demonstration the Z_t is measured for both a coaxial tube with a small hole in the shield which provides a mutual inductance of 0.1 pH. Improvements in the design of the shield of a telecommunication connector are presented. Results for some common connectors are given; the transfer impedances for these connectors ranges from 1 mΩ to more than 100 mΩ at 1 GHz     

Analyzing electromagnetic pulse coupling by combining TLT, MoM, andGTD/UTD

An important problem in electromagnetic compatibility (EMC) analysis is to determine the coupling of an electromagnetic field into a shielded cable. Using the transmission line theory (TLT), the disturbance voltage induced inside the cable is easily calculated from the current distribution on its shield. This current distribution depends on the incident electromagnetic field and is efficiently determined by the method of moments (MoM). Extending the MoM with the geometrical and uniform theory of diffraction (GTD/UTD) makes it possible to solve scattering problems that are too large and too complex for the plain MoM. The combination of the three approaches-TLT, MoM, and GTD/UTD-allows calculation of the disturbance voltage inside a shielded cable, which is part of a complex scattering structure. The fundamentals of each method and the way of putting them together are shown in this paper. The application of the proposed method is demonstrated by an example: the pulse coupling between a monopole antenna and a shielded cable is analyzed, taking into account a large conducting structure in the vicinity     

On the Effective Transfer Impedance of Thin Coaxial Cable Shields

It is shown that the effective transfer impedance per unit length of a thin coaxial cable shield is given by Zs h2/Ys, where ZS and Ys are, respectively, the cable's inductive transfer impedance per unit length and capacitive transfer admittance per unit length, and h is the axial propagation constant. This general result is illustrated by consideration of a specific shield model, the M-filar filamentary helix.     

Modélisation de l’impédance de transfert des câbles coaxiaux à blindages multiples

The shielding effectiveness of multishielded coaxial cables is determinated through the concept of the equivalent transfer impedance. The transfer impedance is computed from the main parameters of the coaxial structure. We describe in this paper the theoretical formulation to evaluate the amplitude of the disturbing voltage at the end of the cable flowed by the disturbing current. This result is used for the computation of the equivalent transfer impedance when the cable is made of various shields for exemple: homogeneous screens or braids. A comparison with the experimental results is also described.     

Techniques de calibration et de traitement des signaux pour la mesure de I’efficacite de blindage des cables au moyen de methodes temporelles

The purpose of this paper is to show the influence of the various time domain parameters on the precision of the result in the frequency domain for the determination of the transfer impedance of the coaxial cable. In the first part of the paper the conversion is made through the discrete Fourier transform. The ratio of the spectrum of the cross talk voltage and the spectrum of the disturbing current is computed to obtain the transfer impedance of the cable. In the second part we purpose some techniques for increasing the dynamic range of the characteristic of the transfer impedance. The average and the determination of the Dirac response of the cable are commented on few examples which summarize the main important point of this method.     

Computations of the passive electrical parameters of neurons usinga system model

Time-domain analysis of voltage responses to current pulse stimulation has been used to estimate the electrotonic parameters of neurons using the signal model. Errors are likely to accumulate from various steps of the analysis due to noise and electrode artifacts. A system model, which has inherent noise immunity and filtering properties, is presented here. This model employs frequency-domain analysis of the input impedance of a neuronal model (an RC cable). The resistances and capacitances of the system model are estimated from the cell-input impedance using an optimization strategy. Using the expression for the input impedance, any specified number of equalizing time constants can be computed exactly. The accessibility to these equalizing time constants 1) provides greater insight into the charge equalization along the length and circumference of the cable, and 2) improves the estimation of all other passive parameters including the electrotonic length. Thus, the system model approach allows information to be extracted more directly and accurately than the signal model approach.     

Screening of cables and connectors

Cable and connector screening is given as transfer impedance, which relates a voltage seen inside the component to the current on its outside. One transient disturbance comes from switching on the local mains network, which results in large RF currents flowing in an instrument cable nowhere near the original source. The effect of this current on cables and connectors is assessed using their transfer impedance and it is explained why application of this test in real life depends critically on the electrical length of the cable. The screening performance of different components is compared and an attempt made to explain why some are much better than others. This is extended to outlining the effect of cable construction and detailing the importance of connector design. In conclusion an outline is given of the problems which are found in braid optimisation and how the measurement of cable and connector screening can be used in system design-not just to show one to be better or worse than another     

Resonance Properties of the Shield of a Coaxial Cable over a Ground Plane

In situations where several high-power transmitters and their antennas are to be used near one another, a certain amount of mutual interference can be expected. An instance of particular interest is that of high-intensity radiation inducing standing waves between the shields of nearby coaxial cables and a metal deck of ground plane. Standing waves induced may cause high potentials and possible breakdown at the ends of the cable, damaging connectors and antennas. There may also be some reduction of the shielding effectiveness of the coaxial cable when high-voltage standing waves are present in the shield. It has been common practice to eliminate such standing waves by periodic grounding of the outer conductor of the coaxial cable. This, however, requires penetration of the insulation material on the cable and formation of metal-to-metal joints on the shield. This is not only an inconvenient method of installation, but is also undesirable around salt water. Copper shielding will corrode, and corrosion at the joint of the dissimilar metal can cause nonlinear interference effects. The standing waves induced in the transmission system formed by the cylindrical shield of a coaxial cable and a conducting plane are examined theoretically and experimentally as a function of the shield-to-ground impedance at the end points only (Z1 and Z2 of Fig. 1). Ordinarily, standing waves are eliminated by terminating a guiding system in its characteristic impedance. In this situation, however, the exciting source (i.e., incident radiation) is distributed along the length of the transmission system.     

无源二端口网络的综合

本文论述了双端接阻容负载RC二端口网络转移电压函数的极点与RC阻抗和导纳乘积的零,极点位置分布关系定理.导出了双端接载与等效单端接载网络转移电压函数之间关系的数学模型,利用一次归并和两次分解途径给出了双端接任意阻容负载网络的实现方法.     

A three dimensional analysis of slotted tube resonator for MRI

A three dimensional model of a slotted tube resonator (STR) used as a probe in the magnetic resonance imaging (MRI), which is loaded by a dielectric body and surrounded by a conducting shield, is analyzed by using the variational method and the dyadic Green's function of a circular waveguide having a dielectric core. Three surface current modes are properly assumed to expand the currents on the STR. The characteristics such as the input impedance, the resonance frequency, the Q value, and the magnetic field distribution are obtained to show the effects of the dielectric body and the conducting shield. Some theoretical results are compared with the measured data to confirm the validity of the present analysis.     

Time-Domain Investigation on Cable-Induced Transient Coupling Into Metallic Enclosures

A hybrid time-domain method is proposed for characterizing electromagnetic interference (EMI) signals coupled into some composite structures with metallic enclosures, braided shielded cable, printed circuit boards, and even lumped active devices. In order to rapidly capture the induced interior EMI, the finite-difference time-domain, modified node analysis, and multiconductor transmission lines methods are combined together and implemented successfully. Numerical investigation is carried out to demonstrate the frequency-dependent transfer impedance of the coaxial cable, the induced voltage at the place of active loaded element in the transmission line network, and the enclosure shielding effectiveness of these composite enclosures. The captured transient response information is useful for further designing electromagnetic protection of the inner circuits against the impact of voltage or current surge caused by nonintentional as well as intentional electromagnetic interference.      

Electromagnetic Theory of the Loosely Braided Coaxial Cable: Part II--Numerical Results

The general modal equation obtained in Part I is solved numerically for the propagation constants of both the monofilar and bifilar modes. For the special case of an air-filled cable, only one mode is supported. Numerical results are also presented for the surface transfer impedance of the shield which, in general, depends on the propagation constant. The properties of the counterwound helical shield are found to be qualitatively similar to those of the unidirectional helical shield.