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An efficient finite-element reduced-order quasi-TEM model for the frequency-dependent characteristics of lossy multiconductor transmission lines is presented. Conductor losses are evaluated as functions of frequency through a magneto-quasi-static model. Numerically generated problem-matched basis functions reduce the problem size and, therefore, the CPU time required by frequency sweeps without appreciable loss of accuracy. The proposed approach is applied to complex coplanar waveguides and to multiconductor interconnects; its results are compared with quasi-analytical techniques and with the full-wave finite-element method. Index Terms--Coplanar waveguides (CPWs), Damascene multilevel interconnects, electrooptic modulators, finite-element method (FEM), magneto-quasi-static problem, micromachined CPWs, model order reduction, multiconductor transmission lines (MTLs), ohmic losses, skin effect.

A theoretical justification is presented of the partial-capacitance (PC) approach, widely exploited in the modeling of coplanar waveguides on finite-thickness and multilayered substrates. The analysis is based on the static spectral-domain approach to the computation of the capacitance of a set of planar conductors embedded into a multilayered substrate. It is shown that the PC method can be derived from approximating the static Green's function of a multilayered dielectric as the sum of partial contributions; such a decomposition can be applied to the Green's function either in parallel (admittance) or series (impedance) form. The resulting (parallel or series) PC approaches are shown to be accurate with substrates having layers of decreasing or increasing permittivity, respectively. Lines backed by magnetic or electric walls are introduced as limiting cases of multilayered structures. Index Terms--Coplanar lines, partial-capacitance (PC) approach, planar transmission lines, spectral-domain static Green's function techniques.

A new class of coaxial broad-band directional couplers for high-power RF testing is presented. These devices overcome the drawbacks of classical stripline and coaxial couplers, and allow simple low-cost mechanical construction. A design technique for the new longitudinally nonuniform transversally nonsymmetricai couplers is described, providing the synthesis of any coupling shape versus frequency. As a first demonstration of the proposed method, a set of broad-band (2-7.5 GHz) low-coupling (-30, -33, -35 dB) nonsymmetrical coaxial directional couplers in air has been designed, built, and characterized, showing very good agreement between predicted and obtained performances. Index Terms--Broad-band couplers, coaxial cables, coaxial couplers, conformal mapping.

Numerical conformal mapping is exploited as a simple, accurate, and efficient tool for the analysis and design of coaxial waveguides and couplers of complex cross section. An implementation based on the Schwarz-Christoffel Toolbox, a public-domain MATLAB package, is applied to slotted coaxial cables and to symmetrical coaxial couplers, with circular or polygonal inner conductors and external shields. The effect of metallic diaphragms of arbitrary thickness, partially separating the inner conductors, is also easily taken into account. The proposed technique is validated against the results of the finite-element method, showing excellent agreement at a fraction of the computational cost, and is also extended to the case of nonsymmetrical couplers, providing the designer with important additional degrees of freedom. Index Terms--Coaxial cables, coaxial couplers, conformal mapping.

The evaluation of the frequency response of waveguiding structures by means of the full-wave finite-element method requires solving a large generalized eigenvalue problem for each frequency. This paper describes a novel approach, based on the singular-value decomposition, which drastically reduces the order of the eigenvalue problem. By inspection of the singular values, the accuracy level of the procedure may be controlled. The technique is applied to the analysis of open and closed waveguides with arbitrary cross section, lossy conductors, and anisotropic dielectric layers, by means of vector elements of generic order; higher order elements are shown to allow the accurate evaluation of fields inside lossy conductors with fewer unknowns, besides exactly modeling normal field discontinuities at material interfaces. Examples of application of the reduced-order technique are shown concerning both non-TEM and quasi-TEM structures. Index Terms--Finite-element method, higher order vector elements, reduced-order models, singular-value decomposition.

Monte Carlo technique was used to simulate electrical characteristics of single and multiple quantum barrier varactors (QBV). The voltage was modulated at 50 GHz and FFT of the current was applied to also simulate the RF behavior. Results showed that only odd harmonics are generated and the current level of the third harmonic is about ten percent of the injected signal current level. Moreover, it is demonstrated that Monte Carlo simulation is a useful tool to predict nonlinear behavior in QBV.

Closed-form CAD-oriented conformal-mapping approximations are presented for the HF attenuation of two-conductor symmetrical coupled coplanar waveguides (CCPW's). The expressions are compared with the results obtained from a full-wave electromagnetic (EM) simulator and found to yield acceptable agreement. A modified form is also presented according to [1], which partly overcomes the low-frequency limitations of the skin-effect approximation. Index Terms - Attenuation, conformal mapping, coplanar waveguides, couplers, design automation software, skin effect.

In this paper, analytical computer-aided-design (CAD)-oriented conformal-mapping approximations are presented for the high-frequency attenuation of symmetric and asymmetric coplanar waveguides (ACPW's) with finite-extent lateral ground planes. A discussion is presented on the effect of ground-plane width on the losses, and design criteria are derived. Index Terms - Attenuation, conformal mapping, coplanar waveguides, design automation software.

The Schwarz-Christoffel toolbox, a free MATLAB package for the computation of conformal maps, is applied to the quasi-static analysis of coplanar waveguides (CPWs) of arbitrary cross section in order to provide computationally efficient and very accurate estimates of their capacitance, inductance, characteristic impedance, and skin-effect attenuation. A few examples of many-sided polygonal waveguides are discussed, and the trapezoidal CPW, important, for example, for electrooptic modulators, is described in full detail, providing general guidelines for the electrode geometry optimization. The technique is validated through a comparison with the results of a full-wave finite-element method, and excellent agreement is demonstrated both in vacuo and with two-layer dielectric substrates. Index Terms--Coplanar waveguides, electrooptic modulation, integrated opto-electronics, skin effect.

A set of thick coplanar waveguides on lithium niobate substrates, with and without a thin Si[O.sub.2] buffer layer, has been experimentally characterized through on-wafer measurements. The effective refractive index and attenuation were extracted from raw (uncalibrated) measurements up to 40Hz through the thru-reflection-line approach. The characteristic impedance was then obtained from the propagation constant, using an accurate estimate of the in-vacuo capacitance derived from a new conformal mapping approach able to exactly account for large electrode thickness. We observed that the attenuation of lines with or without the oxide buffer layer consistently exhibits a different frequency behavior, thus suggesting that dielectric losses can play a significant role in the upper microwave range. This is confirmed by the results from a full quasi-TEM analytical model, including losses and frequency dispersion, which was derived by extending the model in [1]. The measured and simulated data show good agreement both for the propagation characteristics (attenuation and effective permittivity) and for the line impedance. Index Terms--Coplanar waveguides, dielectric losses, electrooptic modulation, integrated optoelectronics, loss measurements, skin effect.