The Capacitances and Surface Charge Distributions of a Shielded Unbalanced Pair (Short Papers)

The capacitance matrix of an unbalanced shielded pair cable is determined theoretically. The wires of the cable are asymmetrically located about the axis of the shield and have different radii; however, the axes of the wires are restricted to lie on a line passing through the axis of the shield. The elements of the capacitance matrix are determined as particular elements of the inverse of a truncated intinite matrix, which relates the Fourier coefficients of the surface charge densities on the inner conductors and the shield to the applied voltage excitations on the cable conductors. The capacitances and surface charge distributions are evaluated numerically for a shielded pair cable, which, due to inaccuracies in the cable manufacturing processes, has one wire with a smaller or larger radius than the other wire of the pair and/or has one wire closer to or farther from the axis of the shield than the other wire of the pair.     

On Reducing the Lightning Transients in Buried Shielded Cables Using Follow-On Earth Wire

This paper investigates the importance of a follow-on buried bare earth wire for the lightning protection of buried shielded cables. The use of follow-on bare wires for lightning protection of communication towers was suggested as a recommendation in certain standards, without being complemented either by theory or experiments. When lightning transients couple to the cable shields, it induces large currents (depending on the type of coupling) causing transient overvoltages between the inner conductors and the shield. It is shown by simulations based on multiconductor transmission line theory that if the follow-on bare earth conductor is placed in parallel with the shielded cable with the bare earth wire connected to the shield at the current injection end, then the shield current, and thereby, the internal transient voltages of the cable are reduced considerably.     

Double-layer microstrip structures—lumped capacitances and open-circuit end correction

The analysis is reported of the lumped capacitance and open-circuit end effects of finite-length strip conductors in double-layer microstrip structures. Two specific configurations, namely double-layer microstrip and microstrip-with-overlay configurations, are considered. The analytical approach uses the variational technique in the Fourier transform domain in conjunction with the transverse transmission line technique. It is identified that the only parameter needed to analyse the lumped capacitances and open-circuit end effects of these microstrip structures is the admittance at the charge plane. This parameter can easily be determined from the two-wire transmission line equivalent circuit. Extensive numerical data are generated for the lumped capacitances and open-circuit end effects of the finite-length strip conductor in double-layer microstrip and microstrip-with-overlay configurations. The data presented should be useful in designing lumped elements and filters in these configurations.     

Capacitance calculations for cable harnesses using the method ofmoments

A method of moments procedure to obtain the capacitance matrix for a cable harness is presented. The geometry consists of multiple, parallel, insulated wires, possibly above a ground plane. The method uses Fourier harmonic expansion functions for the total charge and Galerkin testing functions. The capacitance matrix is obtained by the relationship of free charge to potential. Computed and experimental results are presented and compared to exact analytical solutions when possible     

Interperiod capacitance calculations for three-dimensionalmulticonductor systems

An approach is proposed to facilitate the capacitance calculations for periodic three-dimensional multiconductor systems. Based on the Fourier transform technique, the approach requires only the conductors inside one period and solves the charge distribution in the spectral domain by the integral equation method. The resultant spectral capacitances are then inverse transformed to give the capacitances between any two conductors, which may even be inside different periods. The approach is applied to the capacitance analysis for connector pins in a packaging board design     

Computation of the Transmission Line Inductance and Capacitance Matrices from the Generalized Capacitance Matrix

In a recent paper [1], a method for computing the per-unitlength generalized capacitance matrix of a system of dielectric-insulated wires was given. In this-paper, a method for computing the per-unitlength inductance and capacitance matrices used in multiconductor transmission-line models in terms of the elements of the generalized capacitance matrix is given. Certain approximate formulas for large wire separations are also given. Rome Air Development Center.     

Partial capacitance calculation for shielded twisted pair cables

This brief presents a method for the calculation of the capacitance of shielded twisted pair cables. The method presented allows the adoption of a realistic geometry of the shield in the analysis and does not limit the analysis to nonpractical geometrical primitives. The analysis is based on the combination of partial capacitances obtained through two-conformal mappings and by making a simple assumption on part of the geometry combined with the use of potentials. Comparisons with measurements, simulations, and with an analytical model deriving the capacitance for the pair in a cylindrical shield demonstrate a high level of agreement.     

Application of phase detection frequency domain reflectometry for locating faults in an F-18 flight control harness

The performance of a phase-detection frequency-domain reflectometer (PD-FDR) for locating open and short circuits (hard faults) in a Navy F-18 flight control harness has been tested, and the analytical expressions for accuracy verified. Nine different types of aircraft wires appear in this harness: twisted pair, shielded wires with 1-4 inner conductors, "filter wire," and bundles of individual wires. PD-FDRs in a variety of frequency ranges (12-25, 100-220, 150-300, and 180-400 MHz) are compared. Signal processing techniques are utilized to remove the reflections where the PD-FDR is connected to the wire harness, which is critical to obtaining accurate measurements, particularly for short lengths of wire. For this specific application, open and short circuits are located to within 2.5 cm (1 in) for PD-FDR200 and 11 cm (5.5 in) for PD-FDR25 for wires ranging from 9 cm to 9.15 m (6-360 in).     

Capacitance matrix calculation of a wire conductor line: a new FEMapproach

An original finite-element approach is presented to calculate the capacitance matrix of a uniform multiconductor wire line. The examined two-dimensional (2-D) domain is discretized by nodal-based triangular elements where the Laplace equation is solved. A new procedure is developed to take into account the presence of the wires, which are assumed to be located in the vertex nodes of the FEM mesh. Through the proposed procedure, the physical dimensions of the wire cross sections are considered modifying the terms of the local stiffness matrix in the finite elements surrounding the wires. A further modification of the local FEM matrices allows one to consider the logarithmic variation of the electrical potential around the wires. The procedure is efficient from a numerical point of view since it avoids the fine discretization of the nonconductive region surrounding the wire while achieving a good numerical accuracy. Numerical examples are given and compared with the analytical solutions for canonical configurations, including wires with a dielectric cover     

The Determination of an Excess Capacitance (Short Papers)

In the evaluation of "fringing capacitances" one is required to map the specified geometry in the z plane onto the upper half t plane and then to determine the limiting value of the capacitance between the two segments of the real axis---corresponding to the two conductors in the z plane---when one or both of the gaps between them approaches zero. The usual procedure of mapping the upper half t plane onto an infinite parallel plate configuration, which is often more involved than the first mapping, can be eliminated if one recognizes that the capacitance obtained by mapping the upper half t plane onto a rectangle by means of a well-known elliptic function exceeds, in the limit, the correct value by (log 2)//spl pi/, for each gap involved.     

Excitation of surface waves on a unidirectionally conducting screen by a phased line source

The excitation of surface waves on a unidirectionally conducting screen produced by a phased line source located above the screen and perpendicular to the wire elements is considered. The screen consists of an infinite number of straight, perfectly conducting, parallel wires and conducts only in the direction of the wire elements. The phased line source consists of a periodic line current with an electric charge distributed along its length. The complete electromagnetic field is determined exactly and simple expressions are given for the scattered far field. It is shown that surface waves exist and simple expressions for the amplitudes are given. Another principal result is the determination of the magnitude of the complex Poynting vector for the radiated power. It is found that the pattern function lies on a cone independent of the presence of the screen and that the cone angle depends only on the phasing of the source. The pattern function at points below the screen is independent of the location of the source above the screen. Furthermore, the pattern function vanishes in the direction of the screen and this seems concomittant to the existence of surface waves. Two pattern functions are drawn for typical cases of interest. The power propagated by the surface waves is also determined. The method employed to solve the problem is based on the deduction that the scattered magnetic field component in the direction of the wire elements is zero. A consequence of this deduction is that the electromagnetic field can be derived from a single scalar wave function that satisfies a partial differential equation in the plane of the screen and a jump condition across the screen. This method is quite general and can be applied to a large class of interesting propagation problems arising from different types of excitation. The scattered far field is obtained using another method that is algebraic in character and does not require a complete solution of the problem.     

VLSI circuit reconstruction from mask topology

This paper discusses whether and how parasitic circuit elements must be included in the circuit simulator source file to obtain reliable simulation results. In particular, attention is paid to fabrication tolerances, wire capacitance (including fringing effects), wire resistance (dispersive line effects), coupling capacitances and capacitances associated with contacts and the aspect ratio of (non-rectangular) transistors.     

Analysis of a four parallel wire antenna system by the spatialFourier transform method

The electromagnetic fields and currents associated with an infinite four parallel wire transmission line are analyzed through the use of the spatial Fourier transform method. The near and far electromagnetic fields and currents that are associated with frill and gap voltage excitations are analyzed by the Fourier transform method. Possible VHF compact range applications of a four parallel wire antenna system are discussed, including the possibility of simulating an off-axis EM plane wave by the appropriate adjustment of the exciting voltage phase on each of the four parallel wires. Comparisons of Fourier transform method solutions with method-of-moments solutions and finite-difference-time-domain solutions are made for an infinite four parallel wire antenna system     

Calculation of the characteristics of two-wire lines in multiconductor cables

A rigorous method is proposed for calculating the parameters of two-wire lines (twisted pairs) surrounded by a single metal shield and the mutual coupling between these lines. It is shown that coupling between the lines in multiconductor cables results in electromagnetic interference (crosstalk) in communications channels and that inphase currents in the lines are caused by the asymmetry of excitation and loads. The stray voltages induced across the impedances placed at the beginning and the end of an adjacent line are determined at a given power in the main line. The effect produced by the loads placed between the wires and the shield is considered. The proposed method allows generalization of the obtained results to the case of lossy multiconductor cables.     

Propagation Along a Coaxial Cable with a Helical Shield

A leaky coaxial cable is modelled by a dielectric coated conductor shielded by a finite number of unidirectional helical wires. A modal equation is derived and soIved numerically for the propagation constants of both the monofilar and bifilar modes. Numerical results are also presented for the effective surface transfer impedance of the shield. This parameter is found to depend, in general, on the propagation constant.     

Simulation théorique et expérimentale du fonctionnement d’un banc de mesure à structure triaxiale appliquée à la mesure de l’impédance de transfert des câbles multifilaires blindés

We are interested in the present paper in the measure of the transfer impedance and the near-end crosstalk voltages of multi-wire shielded cable by applying the well-known transmission-line theory. Using the impedance and admittance transfer measurement set up, we show that conductors inside the shield play an important role in the evaluation of the internal voltages. We equally show that when the conduction coupling appear, we can not neglect the reaction of the interior wires with regard to the exterior.     

Effect of Pigtails on Crosstalk to Braided-Shield Cables

An investigation of the effect of pigtail sections (terminal segments of braided-shield cables in which the shield braid has been stripped back exposing the interior wire) on the electromagnetic coupling to braided-shield cables from adjacent wires is presented. Experimental and computed data indicate that, even though the lengths of these pigtail sections may be only a very small portion of the total cable length, they may, over certain frequency ranges, constitute the dominant coupling mechanism for the braided-shield cable. For situations in which pigtail coupling is dominant, the shield simply serves to reduce the exposed section of the interior wire from what it would be if no shield were present. Thus the shield provides some reduction in coupling, but the effectiveness of the shield in reducing crosstalk is shown to be as much as 30 dB (over certain frequency ranges) less than it would be if the pigtail sections were eliminated. A low-frequency model which explains this phenomenon is given. The effect of various shield grounding configurations is also investigated.     

High frequency cable connector for twisted pair cables

A new cable connector for twisted pair cable usable for high frequency applications is presented in this paper. An elastic conductive matrix as an interface between cable and printed wiring board (PWB) is pressed against the ends of the copper wires of the cable, and a land grid array on the PWB, thereby making the connections. The shielding braid of the cable is lengthened by a tube structure up to the same plane as the end of the copper wires, where the shielding is connected to an earth plane on the PWB. This not only gives a sound basis for good electromagnetic interference (EMI) behavior, but can also serve as an adequate structure for a dc barrier of common-mode currents in the shielding of twisted pair cables. A washer-formed capacitor between the earth of the PWB and the shielding tube structure would probably be the only addition needed. Measurements, performed on two connectors and the connected twisted pair cable, confirmed the hypothesis of how the performance of the new cable would be improved in the high frequency range compared to the SOFIX cable connector.     

TEM modes in parallel-line transmission structures

A transmission structure of many parallel conductors is capable of propagating many distinct quasi-TEM modes. A field theory of such modes is developed for a theoretically infinite number of very thin conductors. These possess infinitely many distinct propagation modes, which are the eigenfunctions of a pair of integral operators with Green's function kernels. The modes depend strongly on the structure cross-sectional shape and size, only slightly on the number and placement of individual conductors. The theory is illustrated by a structure comprising many thin parallel wires on a dielectric substrate backed by a ground plane, whose modal functions and velocity distributions are used to solve a simple crosstalk problem     

Maximizing throughput over parallel wire structures in the deep submicrometer regime

In a parallel multiwire structure, the exact spacing and size of the wires determine both the resistance and the distribution of the capacitance between the ground plane and the adjacent signal carrying conductors, and have a direct effect on the delay. Using closed-form equations that map the geometry to the wire parasitics and empirical switch factor based delay models that show how repeaters can be optimized to compensate for dynamic effects, we devise a method of analysis for optimizing throughput over a given metal area. This analysis is used to show that there is a clear optimum configuration for the wires which maximizes the total bandwidth. Additionally, closed form equations are derived, the roots of which give close to optimal solutions. It is shown that for wide buses, the optimal wire width and spacing are independent of the total width of the bus, allowing easy optimization of on-chip buses. Our analysis and results are valid for lossy interconnects as are typical of wires in submicron technologies.