Large signal 2nd harmonic on wafer MESFET characterization

An automatic test set which performs a real time harmonic load-pull characterization is proposed. An active load technique is used in order to set the load at the test frequency and its harmonics and a complete set of device parameters useful for power amplifier design purposes can be measured versus the harmonic loads. The calibration procedure, based on substrate and coaxial standards, has been mainly developed for on wafer measurement in order to set the reference planes directly on the DU

Save the "THRU" in the A.N.A. calibration

The conventional network analyzer (NWA) two-port calibration procedures require a standard thru line to be connected between the ports. Unfortunately in many applications, for example when measuring MMIC or on-wafer devices with not aligned ports, a custom thru line must be used. The procedure here applied overcomes the difficulty due to the poor knowledge of this thru element since it is based on a generic reciprocal unknown two port structure, provided that its S21 phase shift is roughly known. Some experimental comparisons with other well sound calibration techniques will be here presented where different reciprocal two-port structures were used as unknown thr

Structural instability of nonlinear plates modelling suspension bridges: mathematical answers to some long-standing questions

We model the roadway of a suspension bridge as a thin rectangular plate and we study in detail its oscillating modes. The plate is assumed to be hinged on its short edges and free on its long edges. Two different kinds of oscillating modes are found: longitudinal modes and torsional modes. Then we analyze a fourth order hyperbolic equation describing the dynamics of the bridge. In order to emphasize the structural behavior we consider an isolated equation with no forcing and damping. Due to the nonlinear behavior of the cables and hangers, a structural instability appears. With a finite dimensional approximation we prove that the system remains stable at low energies while numerical results show that for larger energies the system becomes unstable. We analyze the energy thresholds of instability and we show that the model allows to give answers to several questions left open by the Tacoma collapse in 1940.Comment: 33 page

Saddle Points Stability in the Replica Approach Off Equilibrium

We study the replica free energy surface for a spin glass model near the glassy temperature. In this model the simplicity of the equilibrium solution hides non trivial metastable saddle points. By means of the stability analysis performed for one and two real replicas constrained, an interpretation for some of them is achieved.Comment: 10 pages and 3 figures upon request, Univerista` di Roma I preprint 94/100

Partial symmetry and existence of least energy solutions to some nonlinear elliptic equations on Riemannian models

We consider least energy solutions to the nonlinear equation $-\Delta_g u=f(r,u)$ posed on a class of Riemannian models $(M,g)$ of dimension $n\ge 2$ which include the classical hyperbolic space $\mathbb H^n$ as well as manifolds with unbounded sectional geometry. Partial symmetry and existence of least energy solutions is proved for quite general nonlinearities $f(r,u)$, where $r$ denotes the geodesic distance from the pole of $M$

Accurate on-wafer power and harmonic measurements of mm-wave amplifiers and devices

A novel integrated test system that accurately measures on-wafer S-parameters, power levels, load-pull contours and harmonics over 1 to 50 GHz is presented. The system measures power and S-parameters with single contact measurements and integrated hardware. There are two keys to this system: first, the network analyzer samplers are used as frequency-selective power meters with large dynamic ranges; second, all measurements are vector-corrected to the device under test reference planes. The capabilities and accuracy were demonstrated by measuring the power at the fundamental frequency and four harmonic frequencies of a 50-GHz traveling wave amplifier and the load-pull contours of a MODFET at 30 GH

Regulation of transforming growth factor-beta 1 gene expression by glucocorticoids in normal human T lymphocytes.

Glucocorticoids (GC) modulate immune function in a number of ways, including suppression of T cell proliferation and other IL-2-mediated T cell functions. These inhibitory effects are similar to those induced by transforming growth factor-beta 1 (TGF-beta 1), a cytokine with potent T cell inhibiting activities. We examined the hypothesis that GC effects may be at least partially achieved through modulation of the expression of the TGF-beta 1 gene in activated T cells. Normal T cells were cultured with or without purified phytohemagglutinin (PHA-p) and 4 beta-phorbol 12-myristate 13-acetate (PMA) in the presence or absence of the synthetic GC, dexamethasone (100-200 micrograms/ml). The production of latent and active forms of TGF beta by these cells were analyzed by immunoblotting and bioassays. The steady-state levels of TGF-beta 1 mRNA were analyzed in total RNA from these cells by Northern hybridizations using a human TGF-beta 1 cDNA. The results showed that dexamethasone caused an increase in TGF beta production and a dose-dependent two to fourfold increase in TGF-beta 1 mRNA in activated as well as in unstimulated T cells, 1 h after exposure of the cultures to the steroid. The increase in TGF-beta 1 mRNA levels by dexamethasone was further potentiated two to threefold by cycloheximide, suggesting that the steroid effect may be due to inhibition of the synthesis of proteins that decrease TGF-beta 1 gene transcription or the stability of its transcripts. Finally, in vitro nuclear transcription studies indicated the dexamethasone effects on TGF-beta 1 gene expression to be largely transcriptional