A transmission-line model is developed for predicting the response of a twisted-wire pair (TWP) circuit in the presence of a ground plane, illuminated by a plane-wave electromagnetic field. The twisted pair is modeled as an ideal bifilar helix, the total coupling is separated into differential- (DM) and common-mode (CM) contributions, and closed-form expressions are derived for the equivalent induced sources. Approximate upper bounds to the terminal response of electrically long lines are obtained, and a simplified low-frequency circuit model is used to explain the mechanism of field-to-wire coupling in a TWP above ground, as well as the role of load balancing on the DM and CM electromagnetic noise induced in the terminal loads.

In this letter, the phenomenon of mode conversion as the basis of radiated emissions (RE) and radiated susceptibility (RS) of differential-line circuits terminated with not-perfectly balanced loads is investigated. To this end, the concept of weak imbalance of line terminations is introduced, and its fundamental role in RE and RS is evidenced by resorting to modal analysis and to the concept of the dominant mode. This leads to simple equivalent circuits in which mode conversion is represented by dependent sources. Although here it is applied to RE and RS prediction, the proposed approach has general value and the potential to readily explain and quantify the role of imbalance in systems involving differential lines.

The influence of twist-pitch nonuniformity on the radiated susceptibility of a twisted-wire pair (TWP) running above ground and illuminated by a plane-wave field is investigated via a transmission-line model. The geometrical representation of the TWP involves a bifilar helix with a variable pitch function, which readily allows for the description of any unwanted/unknown deviations from the ideal twist geometry. Basic examples are used to show that such a deformation plays a fundamental role in the pickup of differential-mode (DM) noise, thus substantiating the need for the proposed prediction model. By describing the pitch random variations as a spatial stochastic process, a realistic representation of the twisting features is obtained (both for TWPs with imperfect uniform twists and for TWPs intentionally manufactured with random nonuniform twists). Repeated runs are used to characterize the sensitivity of the induced DM noise to random nonuniform twisting. Especially, it is shown that the twist-pitch nonuniformity, sometimes intentionally used to reduce pair-to-pair crosstalk in TWP cables, may however lead to a severe decrease of the TWP immunity to external fields.

An approximate multiconductor transmission-line (MTL) model is developed for predicting the radiated susceptibility of bundles composed of twisted-wire pairs (TWPs) running above a ground plane, and illuminated by a nonuniform electromagnetic field. Through the use of averaged per-unit-length parameters and sampling of the incident electromagnetic field, the nonuniform MTL is modeled as an equivalent uniform MTL including common-mode (CM) voltage sources accounting for field-to-wire coupling effects. The proposed model is able to predict CM currents/voltages induced in the terminal loads of every TWP in the bundle. In particular, it is shown that the CM noise significantly differs depending on the position of the considered TWP in the bundle cross section. Comparison of results versus full-wave simulations validates accuracy and computational efficiency of the proposed model.

In this paper, mode conversion occurring in differential lines with cross section affected by geometric imbalance is investigated by recognizing a strict analogy with crosstalk, and by rephrasing in the modal domain the assumption of weak coupling (usually exploited for crosstalk analysis) in terms of weak imbalance. The derivation focuses on the conversion of the differential mode signal into common mode (CM) noise responsible for undesired radiated emissions, and leads to an approximate circuit model for CM-current prediction, in which the effects due to mode conversion are included by distributed voltage and current sources. Model accuracy is assessed by experimental and numerical validations involving unbalanced microstrip lines.

A multiconductor-transmission-line (MTL) model of field coupling to a random bundle composed of twisted-wire pairs (TWPs) running above ground is proposed for the computation of the induced common-mode (CM) noise. The nonuniform bundle is effectively represented by cascading uniform MTL sections, whose models are derived from a single reference solution through permutations of rows and columns in matrices and vectors. This is achieved by exploiting the concepts of: 1) averaged per-unit-length parameters, 2) reference cross-section, 3) minimum-distance shifts of TWP positions in the reference cross-section. Repeated-run analysis is used to outline the high sensitivity (tens of dBs) of the induced CM noise to random configurations of electrically long bundles, and to derive statistical estimates of CM voltages induced by a plane-wave field at each TWP terminal. Accuracy and computational efficiency of the proposed approach are proved by comparison versus full-wave simulations.