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It is well known that a Vandermonde matrix generates an ill-conditioned system matrix when applied with finite numerical precision. This deficiency affects the Cauchy method by restricting its application to only lower order systems. This paper presents innovative, accurate, and robust formulations of the Cauchy method to rectify this limitation and make the Cauchy method suitable for the extraction of a high-order microwave duplexer polynomial model. The techniques employed are: the change of polynomial basis into the Krylov subspace and the precondition technique, both acting on the system matrix of the classic Cauchy method formulation. A novel formulation using the QR algorithm on the two characteristic functions of the duplexer and a suitable combination of the QR method and the precondition technique are then presented. Each of these procedures has been successfully verified by numerical application examples. Index Terms--Cauchy method, condition number, microwave duplexer, numerical analysis, robust modeling.

A procedure for synthesizing microwave diplexers is presented. It is based on the evaluation of the characteristic polynomials of the diplexer including the three-port junction connecting the TX and RX filters (two types of junctions are considered, suitable for waveguide and coaxial diplexers modeling). The novel method is particularly suited for small separation between the two diplexer channels: as known, this is the most difficult case in the diplexers design, due to the strong interaction between the TX and RX filters. The proposed synthesis approach offers noticeable design flexibility, as it allows the synthesis of the two composing filters independently of their connections to the diplexer, taking into account at the same time the interaction produced in the diplexer junction. Moreover the choice of the filters topology is absolutely arbitrary (the number of poles and the transmission zeros of the two filters have to be specified). The proposed method has been validated by the design and fabrication of a diplexer for global system for mobile communications base stations. Index Terms--Circuit synthesis, diplexers, microwave filters.

This paper introduces two possible approaches to the design of microwave filters presenting two passbands separated by one rejection band, which is created by suitably placing transmission zeros in the stopband. In the first method, the two passbands have almost equal extension and the transmission zeros are symmetrically placed in the stopband; the second method instead allows the synthesis of filters with different passbands width and all the transmission zeros in the stopband placed at the same frequency. Both design methods are based on suitably defined frequency transformations and employ well-established prototype synthesis techniques; the practical implementation is performed through the classical multiple-coupled cavity topology. Two test filters operating in the global system for mobile communication 800-MHz band, designed with the described procedures and actually fabricated, have validated the novel design methods. Index Terms--Chebyshev filters, circuit synthesis, elliptic filters, microwave filters.

A general and direct synthesis technique of pseudoelliptic inline filters with arbitrarily placed attenuation poles (APs) (transmission zeros) at real frequencies is presented. The APs are brought about and independently controlled by dedicated resonators, which are coupled to nonresonating nodes. Simple rules to properly determine the phases of the reflection coefficients at the input and output are given. To reduce the effect of roundoff errors, especially for higher order filters, the extraction of the elements of the network is performed from the input and output simultaneously. Multiplicity and scaling properties of the solutions are discussed. Synthesis examples are presented to demonstrate the soundness of the procedure. Theoretical results are compared with measurement to demonstrate the validity of the presented theory. Index Terms--Elliptic filters, extracted poles, inline filters, nonresonating node (NRN), resonator filters, synthesis.

In this paper, a general simple technique for reducing any canonical n + 2 coupling matrix to the more useful modular (i.e., cascaded N-tuplets) form is presented. This is accomplished by performing a suitable sequence of matrix rotations whose angles are not derived through optimization, but are analytically computed from the starting coupling matrix elements and the transmission zeros frequencies. The proposed technique allows the association of a transmission zero to a specific block (i.e., a triplet or a quadruplet) increasing the degree of freedom in the filter design process. Index Terms--Circuit synthesis, elliptic filters, microwave filters.

A linearizer architecture, based on predistortion and well suited for monolithic-microwave integrated-circuit (MMIC) implementation is presented in this paper. A mathematical model for the linearizer is described, which allows an optimum choice of the various design parameters (taking into account several constraints due to the specific implementation environment). The power amplifier to be linearized is described by nonlinear functions experimentally derivable from large signal S-parameters; the mathematical model is based on the complex power series expansion of these functions and on the definition of a suitable scaling rule illustrated in this paper. The overall linearizer model has been implemented in a system simulator (working with complex signals), also allowing the investigation of second-order distortion effects (not taken into account during the dimensioning of the structure). A preliminary prototype of the linearizer operating in the 12-16-GHz band has been implemented as a MMIC device (GaAs 0.25-[micro]m technology); the measured performances have confirmed the promising potential of the novel architecture introduced. Index Terms--Intermodulation distortion, linearization, predistortion, RF power amplifier (PA).

This paper presents a review of the recent advances in the field of substrate integrated waveguide (SIW) filters, with particular emphasis on the techniques adopted to increase the out-of-band rejection of the filters and on the miniaturization strategies. After an overview of classical approaches to introduce transmission zeros in SIW filters, a number of recently introduced techniques are presented. In particular, the use of specially implemented extracted-poles topologies and of pre-filters is described, discussed, and compared. For each of these techniques, numerical and experimental results are reported for validation. [ABSTRACT FROM AUTHOR]

A general method for designing low-pass prototype networks with cascade inline topology is presented. The generalized Chebyshev characteristic is imposed, allowing an equiripple response in the passband and transmission zeros arbitrarily displaced in the complex plane (symmetry with respect to the imaginary axis is, however, required). A computationally efficient method is employed for evaluating reflection and transmission polynomials, given the transmission zeros; then, the synthesis of a starting coupling matrix is performed using one of the methods available in the literature. The desired inline topology is then assembled by cascading elementary building blocks (triplets and/or quadruplets), following specific rules which assure the feasibility of the structure; finally, the synthesis is performed by applying a multiple similarity transform to the starting coupling matrix (the set of rotation angles is determined by means of an efficient and fast optimization procedure). Examples are reported which validate the proposed synthesis procedure. Index Terms--Circuit synthesis methods, coupling matrix, microwave filters.

Broad-band stripline circulators are studied by means of a mixed numerical technique which employs both Boundary Integral and Segmentation methods; this technique allows the analysis of planar circuits where the substrate is constituted by several regions with arbitrary shapes and different electrical properties. It is known that tracking circulators require matching structures because they present a low-gyrator impedance (real and almost constant in an octave frequency band). The matching structures (generally tapers or multisection's transmission line transformers) must be realized on a reciprocal substrate. The overall device (circular disk on ferrite substrate and matching structure on dielectric substrate) constitutes a planar circuit with an inhomogeneous medium. The method of study presented here allows the determination of the overall impedance matrix of the planar circuit constituted by the nonreciprocal disk with sections of striplines connected to each port; in this way, the discontinuities between reciprocal and nonreciprocal medium are included in characterization of the overall device. Moreover the accuracy of the representation is increased. In fact, the coupling ports of the overall device may be located at a suitable distance from the disk boundary where higher-order modes excited by the discontinuities have been sufficiently attenuated and only the TEM mode is present on the striplines (which is the only one considered in the design of the matching structures). Index Terms - Boundary elements, planar circulators, stripline devices.

A novel approach to the design of general cavity filters with each cavity coupled to an arbitrary number of other cavities is presented. This approach is based on a suitable characterization of the filter structure which does not require to model separately the cavities (resonators) and the coupling elements. Suitably defined equivalent admittances are associated with each cavity allowing to design the filter structure once the parameters of a suitable low-pass prototype are given; an efficient procedure for the synthesis of such a prototype with equiripple passband response is also presented which allows to arbitrarily prescribe transmission zeros placed in the complex plane (even asymmetrically). The described design approach is particularly convenient when the filter structure does not allow a simple modeling of the resonators and coupling elements separately. This is the case of slot-coupled cavity filters and of filter structures based on arrays of coupled transmission lines. It is also shown that the simplified design approach often adopted in the past, where only two coupled cavities at a time are considered, can produce large errors even in the case of filters with all attenuation poles at infinity (i.e., two couplings per cavity). Index Terms - Bandpass filters, cavity resonator filters, microwave filters.

A CAD procedure is described for designing bandpass filters realized with circular cylindrical dielectric resonators placed in circular waveguide below cutoff. The dielectric resonators operate on [TM.sub.01[delta]] mode, in order to allow both low loss achievement and reduced spurious passbands. The described CAD procedure employs a monomodal equivalent circuit of the filter structure in order to allow a synthesis approach for designing small or moderate percentage bandwidth filters; however the computation of equivalent circuit parameters is based on an accurate multimodal characterization of the discontinuity between the empty circular waveguide and the dielectric loaded waveguide, which makes use of mode matching. The CAD procedure requires a preliminary choice of some geometrical parameters of the structure. Guidelines for this choice are also given, in order to show how different design targets may be achieved, such as low losses, miniaturization of the structure, and a sufficiently large frequency range free from spurious passbands. To verify the procedure, some numerical simulations, performed employing the generalized scattering matrix techniques, are also presented and compared with measurements on a prototype filter designed through said procedure and actually fabricated.

The application of a new boundary-integral technique to the study of planar circulators in stripline/microstrip technology is described. The technique employs Green's second identity with plane waves as weighting functions, and leads to a very efficient algorithm for the analysis of planar devices in anisotropic media. Extensive comparison is made with results from the standard Contour-Integral formulations with very good agreement between the two techniques. The new technique is more stable in the critical region ([[mu].sub.eff] close to 0) and is somewhat faster.

A new method to analyze shielded planar transmission lines with finite metallization thickness is described. It is based on a mixed spectral-space domain representation of Green's function. The method allows the accurate determination of the static parameters of the transmission line with great efficiency as compared to methods available in the literature. Several numerical examples are shown on single and coupled planar transmission lines.