Showing papers in "IEEE Transactions on Electromagnetic Compatibility in 1968"

New Theoretical Expressions for Predicting Shielding Effectiveness for the Plane Shield Case

Abstract: The problem of the electromagnetic shielding effectiveness of a thin, plane metal sheet of infinite extent on the electromagnetic field generated by a circular loop field source with uniform current I is solved by application of the quasi-near and near field approximations to the exact integral expressions. The results are shown to be in close agreement with experimental data as well as numerical integration results. Also, they are similar to the results obtained from the plane wave shielding theory of Schelkunoff.

Predicting the Magnetic Fields from a Twisted-Pair Cable

Abstract: A theory that predicts the magnitude of low-frequency magnetic fields near a current-carrying twisted-pair cable is developed. By asymptotically expressing the theoretical results, it is shown that the magnetic fields from a twisted-pair cable of pitch distance p decrease exponentially with the radial distance from the center of the cable. The asymptotically expressed result is verified experimentally for a radial distance as large as (3/2)p. At such a distance, the maximum fields from the cable are shown to be 50 dB below that from a two-wire line (two parallel wires), even though both the cable and the wire line are carrying the same amount of current.

Frequency-Selective Limiters and Their Application

Abstract: The frequency-selective limiter is presented as an adaptive filter system element. The operating mechanisms of various types of frequency-selective limiters are described and classified. The transient response, small-signal suppression, and intermodulation characteristics are presented for comparison. The application of frequency-selective limiters in particular EMI situations is discussed, and required device parameters are shown to depend on the application.

Magnetic Shielded Enclosure Design in the DC and VLF Region

Abstract: A review of magnetic, shielding concepts and equations as applied to ideal shield configurations has provided a basis for the analysis of the shielding effectiveness of practical shielded enclosures to dc and VLF magnetic fields. The permeability of the shield material is considered as a function of the induction, and a significantly improved method of estimating the induction and permeability of the shield is presented. The effects of multiple shell geometry are given by the equations of this analysis, which are indeterminable with a transmission line analysis. The degrading effects of other departures from ideal shield materials and configurations are analyzed qualitatively; equations for estimating the magnitude of those effects are developed where possible.

ELF and VLF Shielding Effectiveness of High-Permeability Materials

Abstract: Research was conducted to determine the low-frequency shielding effectiveness of magnetic materials. Both analytical and experimental approaches were used. This work is unique in that it provides a technique for experimental separation of the various terms in the shielding expression. Expressions of shielding effectiveness of flat sheets for very low frequencies are derived, and the results of experiments with the following are given: 1) sheet materials, including AMPB-65,1 HyMu 80, conetic AA, mumetal, copper-plated AMPB-65, and galvannealed steel; 2) AMPB-65 sheets perforated with various size holes and various numbers of 0.125-inch diameter holes; 3) an overlap junction of two AMPB-65 sheets with a) various numbers of fastening screws and b) various depths of overlap; 4) overlap junctions of copper-plated AMPB-65 sheets; 5) AMPB-65 sheets clamped in Lindsay structure; and 6) honeycomb-core stainless steel sandwiches.

Review of Circuit Approach to Calculate Shielding Effectiveness

Abstract: The shielding effectiveness of an enclosure at low frequencies can be readily computed using a circuit approach. Not only does this technique include the effects of the properties of the shield material, but it also includes the details of the geometry of the enclosure. Furthermore, this approach allows a nonempirical consideration of mesh enclosures and the effects of resistive seams in enclosure walls. By working with the circuit analogue, penetration by transient fields can also be computed. Essentially the enclosure is viewed as an antenna. In the case of magnetic shielding effectiveness, the enclosure is viewed as a short circuited loop antenna. In the case of electric field penetration, the enclosure is viewed as a fat electric dipole. Using this characterization and exact solutions where available, the current distribution on the outside of the enclosure is first determined. Then, based on the current distribution, the penetrating fields are computed. The equations are developed in such a way as to preserve a lumped circuit analogue for the low-frequency region. The basic circuit equations for magnetic field penetration are rederived from a rigorous solution. Rules to estimate the rise-time, fall-time, and peak magnitudes of transient penetrating fields are developed. The electric shielding effectiveness is developed in a similar manner. In both cases the results of the circuit approach agree well with those based on rigorous solutions of the electromagnetic boundary conditions. The results also agree with published experimental data on both large and small enclosures.

Low-Frequency Shielding Effectiveness of Nonuniform Enclosures

Abstract: This paper presents a quasi-static approximate solution to the magnetic shielding of several nonuniform enclosures using the integral form of Maxwell's equations and insight gained from other approaches. The solution is called quasi-static as the assumptions made are from physical arguments based on low-frequency cases where the enclosure size is much less than a wavelength. The integral form of Maxwell's equations is used to obtain a first order correction to the static solution to obtain induced currents in the time-varying case. A cylindrical shell immersed in an axial magnetic field is used to illustrate the method, which is then extended to derive a formula for a similarly excited rectangular enclosure. These shields are seen to behave like a low-pass filter. Although the enclosure dimensions are small compared to the wavelength, the skin depth effects in the walls cannot be neglected even for relatively thin material as usually encountered in an enclosure. These skin effects are included in the analysis and experimental checks performed on a variety of enclosure sizes and materials, excited by a Helmholtz coil show agreement within two decibels over the 4-octave frequency range examined. No one can say whether this method offers a better solution to the shielding problem, as all solutions are approximate, but the author attempts to present an alternative formulation that aids in understanding the physical processes involved in the shielding effectiveness of an enclosure and fills some of the gaps between the plane-wave analysis and circuit approaches presently used.

Lossy Transmission Line Filters

Abstract: Reactive bandpass and low-pass filters often exhibit spurious responses in the frequency region above the design passband. The spurious responses limit the effectiveness of the filter in preventing interference by out-of-band signals. A simple and inexpensive technique for reducing or eliminating these passbands is to add short sections of an appropriate lossy transmission line in series with the conventional filter. A significant size reduction of conventional transmission line reactive filters can also be realized along with the reduction or elimination of spurious passbands by combining a lossy dielectric with the conventional reactive elements. This paper discusses the application of lossy line filters in cascade with and integrated into conventional reactive filters. The design of lossy transmission line sections and wide-band impedance matching techniques are described.

Low-Frequency Shielding Resonance

Abstract: A low-frequency resonance in the 5- to 200-kHz range has been discovered in typical shielding enclosures. It is due to unequal phase shifts in wave transmission to a receiving point via two or more parallel paths: one through the shielding material itself and others through leakage paths such as seams. Theoretical and experimental results are shown to be in close agreement and indicate a new approach to the design of shielding enclosures.

Sequency Filters Based on Walsh Functions

Abstract: The theory of filters based on sine and cosine functions leads to frequency filters. They can be implemented almost ideally by coils and capacitors. The arrival of integrated circuits imposes new requirements that are hard to meet by frequency filters: coils are undesirable, and no tuning and no temperature compensation should be required. It is reasonable to look for a theory of filters based on other functions that are more suitable to the demands of integrated circuits, rather than to try to adapt frequency filters to the new technological requirements. Sequency filters based on Walsh functions can be implemented by integrated circuits about as ideally as frequency filters by coils and capacitors.

Shielding of Cylindrical Tubes

Abstract: Shielding attenuation values at low frequencies have been computed for magnetic and nonmagnetic cylindrical tubes based on formulas developed by King.[2] The attenuation in decibels was computed on a digital computer for a range of ratios of wall thickness to depth of penetration from 0.1 to 10 as a function of a nondimensional parameter as used by Kaden.[4] Two graphs are presented that summarize the computer results.

Thin-Film Shielding for Microcircuit Applications and a Useful Laboratory Tool for Plane-Wave Shielding Evaluations

Abstract: The results of plane-wave shielding effectiveness tests on vacuum deposited metallic thin films are discussed. The test data are presented in graphical form for thin films of copper, aluminum, silver, gold, and nickel. Shielding effectiveness as high as 90 dB was obtained. In addition, a novel test instrument and measurement technique were devised for this study. The test procedure is described, design information for the construction of the test device is given, and a theoretical basis for its use is presented.

RF Shielding Design

Abstract: An RF shielding design procedure based on a nonuniform transmission-line analogy of shielding is presented. The theory treats the propagation of an EM wave through the walls of a shielding enclosure as a path through the material itself in parallel with one or more paths through defects, seams, or other discontinuities in the shielding structure. Data necessary for the design of a shield are presented in the form of material factors, seam factors, size factors, and others. Factor dependency on frequency is discussed. Two typical design examples are given.

The Modern Network Theory Approach to Microwave Filter Design

Abstract: A review of the modern network theory approach to microwave filter design based on Richards' transformation is given. Basic concepts are discussed, including the use of Kuroda's identities and the specialized theory of optimum filter forms. Applications of the theory presented to elliptic-function filters, special rejectiontype filters, and multiplexers are given. Throughout the presentation, emphasis is placed on basic methods and techniques rather than on detailed design procedures.

Shielding Theory of Coaxial Cylindrical Structures

Abstract: A theory on the shielding effectiveness of n coaxial metallic tubings which shield a coaxial cable has been developed based on electromagnetic (EM) field theory. The theory holds for all homogeneous, linear, and isotropic shields, magnetic or nonmagnetic, and covers essentially the entire frequency range. When a cable carries an evenly distributed axial current, the dominant mode of propagation is transverse magnetic (TM) and has only three field components, i.e., Ez, H?, and Er. The fields of the dominant mode leaking from the cable, with and without shields, have been determined rigorously from the solutions of Maxwell's equations and boundary conditions. The shielding effectiveness of the tubings, defined as the insertion loss, has thus been readily obtained. To simplify the obtained expressions to a certain degree such that numerical calculations are manageable, various approximations have been introduced and precisely justified. The limitations imposed on the simplified expression due to the approximations have been clearly listed. It has been shown that Schelkunoff's shielding theory is merely a special case of the present work. As an example, the shielding effectiveness of a single copper tubing surrounding an RG-8/U cable has been considered. The data measured from a carefully designed experimental setup show that at high frequencies, i.e., above 10 kHz, the curve predicted by the present work is about 1 dB above the empirical curve, while the curve due to Schelkunoff is about 5 dB below the empirical curve. At low frequencies, i.e.

New Dimensions in Shielding

Abstract: The problem first confronting shielded enclosure designers is that of determining the correct type of metal to be used and its required thickness. The shield effectiveness equations that must be solved to answer this problem are at best complex and cumbersome to handle. Graphical techniques developed to date are inaccurate, time consuming, and, in general, very difficult to use. The shielding charts developed for this presentation offer a simple, rapid method for solving the shielding problem in minutes. The examples herein demonstrate the great power and scope of the charts.

Diode Mixer Coefficients for Spurious Response Prediction

Abstract: The precise prediction of spurious response levels in a superheterodyne receiver would greatly aid the receiver designer besides being necessary for the representation of a radio frequency interference environment. The power series is used to represent the nonlinear receiver mixer in order to obtain an expression for the receiver spurious response sensitivity. This sensitivity expression contains the power series coefficients and shows that the coefficient magnitudes must be known before the spurious response levels can be computed. Therefore, a method is developed to provide a solution for the power series coefficients of a typical mixer circuit configuration. In the analysis to determine the coefficients, the input voltage-output current loop equation is written for a crystal mixer equivalent circuit. This expression is expanded in a power series and then the reversion of a series technique is implemented to obtain the desired output-input expression in series form. The effects of the nonlinear element (diode) and the total circuit resistance are included in the results. Coefficient expressions are determined up to the ninth order in the manner described above and the level of each expression is computed versus the product ?( IR + Is) RT where ? and IR are diode parameters, RT is the equivalent circuit resistance, and Is, is the dc bias current. Also, the effect of diode bias upon spurious response sensitivity is considered and the results are plotted. The results show the mixer dc operating conditions which must exist in order to reduce spurious response sensitivity.

Electromagnetic Interference Reduction Filters

Abstract: This paper presents the theory and application of lumped-image parameter reactive filter networks, absorptive-type filter networks, and combined absorptive-reactive filter networks as conducted electromagnetic interference reduction circuits. A comparison of the passband characteristics by specific examples is given for filter configurations of the pi, T, absorptive, and absorptive-reactive type under implementation conditions of a fixed source impedance and a varying load impedance as encountered when filtering prime power sources feeding ac to dc converters. This paper illustrates the problem the design engineer faces when applying the usual data given in filter catalogs. The filter characteristics are presented for 50-ohm resistive source and load conditions because of MIL-STD-220A quality control testr equirements. A test configuration is recommended to determine the admittance transfer of a filter network, indicative of the networks performance characteristics under application conditions.

Introduction to Digital Filters

Abstract: The purpose of this paper is to provide an introduction to the use of digital filters. Their capabilities and limitations are discussed, as well as application requirements as direct replacements for conventional active or passive filters in real-time situations. Basic operating principles are described, and amplitude and phase characteristics are illustrated. The technique offers many advantages among which are very accurate drift-free operation, ease in changing filter characteristics, and small physical size. In addition, a linear phase characteristic can readily be obtained with some types of digital filters. A design example of a linear phase low-pass filter is included.

USAECOM Contributions to Shielding Theory

Abstract: To its three shielding study contracts over the past four years, ECOM contributed the analytical advances required to replace concepts found to be unsatisfactory at the start of the studies, yet still prevalent today. The treatment of oblique incidence was contributed by ECOM; the previous restriction to normal incidence is now rejected. The treatment of shields as circuit elements was originated by ECOM after rejecting, through consideration of Poynting vectors, the concept of high- and low-impedance fields. Thin films have required new techniques to which ECOM has contributed. This paper is written to bring these advances to all specialists in the field so that their talents can be brought to bear on the application or furtherance of these concepts.

Compatible EMI Filters

Abstract: A vexing problem in electromagnetic compatibility is the effective filtering of conducted interference from power-supply lines. Because of unavoidable and severe mismatch, conventional suppression filters operate only conditionally; such filters are often so large that they are omitted from the system. Three new classes of power-line filters without these limitations are described. They are active, truly lossy, and ruggedized ceramic filters, covering the frequency range from direct current to microwaves. Present filter test methods are shown to be misleading and are replaced by a rather simple realistic test. There is a set of filter classes that is rapidly growing in importance and, by necessity, is characterized by the absence of impedance matching. Without matching, all the elegant filter theories developed invalidate the very premise upon which they were based, and the theories are wholly inadequate and misleading. Such conditions exist for filters inserted into power lines; power wiring is contrasted with impedance-matched cabling that interconnects subsystems for irformation handling and for which conventional filtering is fully adequate. Suppression of conducted EMI (electromagnetic interference) constitutes an essential part of EMC (electromagnetic compatibility) and has, thus far, been frustrating and inadequate. Conventional EMI or RFI (radio-frequency interference) filters, because of interface mismatching are, in general, undependable. Another basic difference of power-line filters, as contrasted with information handling filters, is that they are strongly biased by the very power they have to pass. Three generic types of filters are described and discussed in some detail.

Proposed Recommended Practices for the Measurement of Shielding Effectiveness of High-Performance Shielding Enclosures

Abstract: Uniform measurement procedures are proposed for determining the relative effectiveness of room-size high-performance shielding enclosures. Test procedures are designated to measure the effectiveness of a rectangular parallelepiped with edge dimensions ranging from 1.5 to 15 meters. They are formulated to enable use of commercially available equipment for conducting tests under nonideal conditions. The standard does not, however, specify construction details for shielding enclosures nor does it suggest methods for calculating their effectiveness.

Electrostatic Potential Generated by Rockets on Vehicles in Space

Abstract: Investigators have reported the existence of electric current in the exhaust plumes of rocket engines and have speculated that this current could generate a sufficiently high electrostatic potential on vehicles in space to interfere with or damage electronic subsystems or instrumentation. This paper identifies the source of the observed current, calculates its magnitude as a function of the engine parameters, and demonstrates that the rocket plume itself can discharge a vehicle in space and prevent buildup of dangerous potentials.

Digital Filter Design Techniques in the Frequency Domain

Abstract: In digital filtering, the spectrum is shaped using digital components as the basic elements. Although the physical realization is different, the aims of digital filtering are, thus, the same as those for continuous filtering. It is likely that digital filtering, already in extensive use for computer simulation of analog filters, will find increasing real-time application. Real-time digital filters have several advantages over their analog counterparts: a greater degree of accuracy can be attained in their realization; a larger variety can be built since certain realization problems (akin to negative elements) do not arise; no special components are needed to realize filters with time-varying coefficients; and they are of particular utility at very low frequencies where analog components become large and unwieldy. In contrast to the linear differential equations of continuous filter theory, linear digital filter theory is based on the mathematics of linear difference equations. Using the z-transform calculus, a number of digital filter design techniques are discussed. One technique is useful in designing a digital filter whose impulse response is like that of a given analog filter, whie other techniques are suitable for designing digital filters meeting specified frequency response criteria. Another yields filters with linear phase, specified frequency response, and controlled impulse response duration. The effect of digital arithmetic on digital filter behavior is considered.

Survey of Techniques for Measuring RF Shielding Enclosures

Abstract: An intensive literature survey has been conducted to review existing techniques for measuring the effectiveness of RF shielding enclosures. Prevalent methods of measuring E- and H-field components and for sensing the total field are discussed and evaluated. The procedures described in the literature are tabulated into groups determined by their similarities in method or theory and are compared by listing the advantages and disadvantages inherent in each. (A weighting system is devised to aid in evaluating each of the principal methods discussed.) Also discussed are the theoretical considerations underlying each of the methods listed, as applicable to the measurement of shielding effectiveness. Among these are the concepts of the resultant wave impedance of the E-M field at a point inside a shielded enclosure and the relationship of low-impedance and high-impedance fields in the near field regions of antenna systems. Techniques for extending the frequency and intensity of illuminating fields are also discussed, as well as the insertion loss method of determining leakage from a shield. Limitations and deficiencies of this latter procedure are noted.

FM "Chirp" Communications: Multiple Access to Dispersive Channels

Abstract: The white Gaussian noise-limited channel so often assumed by system designers is encountered less and less in practice as the density and sophistication of modem communications increase. Long-range radio communications in particular tend to be limited by interference rather than by receiver noise; for that reason, the design of the signal waveform has great impact on system performance. A waveform that shows particular promise for channels such as satellite relay and HF radio is swept FM or "chirp" modulation. The chirp waveform is characterized in Fig. 1. A carrier is swept in frequency over a band W during a sweep interval T. The sweeps shown are linear and assume a square amplitude envelope of unity value, although nonlinear sweeps and shaped envelopes may be used. The sweep slope W/T may be used to convey information in a variety of ways, the simplest being +W/T for binary 1 and -W/T for binary 0 (slope-shift keying SSK). Higher order alphabets can be represented by different slopes, and amplitude can be added if desired. SSK will usually be most efficient, reserving the distinguishability of different slopes for the purpose of defining channels or addresses in multiple-access applications. The value of chirp modulation derives from its efficient use of available bandwidth and transmitter power in combating multipath and other forms of interference, including the "clutter" of other transmissions in multiple-access systems.

Performance of Electrically Small Array Receiving Systems in the Presence of Noise

Abstract: The performance of broad band electrically small beamed receiving arrays operating in the high-frequency band (3 to 30 MHz) and in a random noise environment is studied. The concepts of system noise figure (SNF) and signal-to-noise ratio improvement factor (SNIF) are proposed as criteria for comparing one array antenna receiving system to any reference antenna system when the conditions of operation are defined. Graphical representations of SNIF for the two- and four-element antenna systems versus a single-element system are presented. SNIF is shown to depend on the geometry of the array, on the phasing network, and on the ratio of atmospheric noise power to internal system noise power.

Two Worst-Case Insertion Loss Test Methods for Passive Power-Line Interference Filters

Abstract: The feasibility of two methods for determining the worst-case insertion loss of power-line interference filters is demonstrated. The first method makes use of injection and detection probes (transformers) which introduce an extremely small impedance (L ?50 nH, R ? 3 m?) into the test circuit. The second method does not use injection and detection probes, thus eliminating their problems. Both methods maintain the high Q of the test circuit and permit interfacial resonances to occur between the power-line and filter interface impedances. A conventional filter and a lossy filter were measured by both methods. Large resonances, including a negative insertion loss at 150 kHz, were measured in the stopband of the conventional filter. No resonances were detected in the stopband of the lossy filter. Measurements with the two test methods differed by only a few dB.

Magnetic Field Pickup by Flexible Braid Coaxial Cables

Abstract: Cabling systems have long been known to be a source of unwanted pickup of radio-frequency interference. A typical source of pickup occurs when the outer sheath of a coaxial cable system forms part of a ground return for potentially interfering electromagnetic energy. Another source of penetration arises from a magnetic field which is perpendicular to the axis of the coaxial cable. A previous study demonstrated that the field emanating from a coaxial cable could be characterized as an equivalent open-wire pair. This particular effect is caused by the eccentricity between the centers of the outer and the inner conductors or because of other asymmetrical characteristics. The susceptibility of a coaxial cabling system to perpendicular magnetic fields can also be characterized in terms of an equivalent open-wire pair for short runs. The purpose of this investigation was to measure the shielding effectiveness of a typical flexible braid coaxial cable to magnetic fields perpendicular to its axis and to demonstrate that a coaxial cable may be considered as an equivalent open-wire pair in a cylindrical shield. A test setup employed a parallel plane transmission line to generate a uniform magnetic field perpendicular to the axis of the coaxial cable. The shielding effectiveness of the braid was measured for several classes of braid conditions. Then, using the same braid, a coaxial cable with an experimentally induced known eccentricity was employed and the overall pickup measured. The results confirmed the objectives of the investigation.

Passive Nonreciprocal HF Helicon Devices

Abstract: Ferrite isolators and circulators are well known at microwave frequencies but are low-frequency limited and cannot be used in the HF region (below 100 MHz). A different approach to the basic nonreciprocal mechanism is reported here. This approach makes use of solid-state plasma waves, so-called helicon waves, that are nonreciprocal by their nature. Isolators using these helicon waves have been built in the 5-to 15-MHz range with great success and a helicon circulator has been demonstrated for the first time. This paper describes the mechanism by which these devices work and discusses their results. It is pointed out that in addition to helicon isolators and circulators, many other interesting and useful devices using helicon waves are possible.