Wide-band balanced active HEMT mixer

The design and characteristics of a balanced active high electron-mobility transistor (HEMT) mixer operating in the 4.5-10-GHz frequency band are described in this paper. It consists of two parts implemented as independent hybrid circuits, namely, an microwave part fabricated by using a uniplanar technology and comprising a 180° hybrid ring coupler, HEMTs, and input-output matching circuits, and a low-frequency part consisting of an L-C balun and a low-pass filter built of discrete elements. The design of the microwave part of the mixer ensures a high degree of isolation between the signal and local-oscillator (LO) inputs within a wide frequency band at low IF. The measurements show a conversion gain of 5-7 dB, noise figure of 5-7.5 dB, and isolation between the signal and LO ports greater than 20 dB within the 4.5-10-GHz range     

Representing a scientific field: A bibliometric approach

A new bibliometric method is proposed for representing links between subfields as defined by a classification scheme. The frequency of co-occurrence of articles from different subfields in selected journals is used for measuring the degree of relatedness between these subfields. The results of such quantitative analysis could be compared to the tree topology of the classification network established in a qualitative analysis. The method is applied to describe the internal links within the field of condensed matter physics using the 1984 Physics Abstracts database. A distinction is made between experimental and theoretical links on the basis of treatment codes assigned to journal articles. The links described by cluster analysis are matched against the cross-reference network of the International Classification for Physics.     

Optical characterization of thin chalcogenide films by multiple-angle-of-incidence ellipsometry

Optical properties of thin chalcogenide films from the systems As-S(Se) and As-S-Se were investigated as a function of the film composition, film thickness and conditions of illumination by light using multiple-angle-of-incidence ellipsometry. Thin films were deposited by thermal evaporation and exposed to white light (halogen lamp) and to monochromatic light from Ar⁺ — (λ = 488, 514 nm) and He-Ne- (λ = 632.8 nm) lasers. The ellipsometric measurements were carried out at three different angles of light incidence in the interval 45-55°, at λ = 632.8 nm. An isotropic absorbing layer model was applied for calculation of the optical constants (refractive index, n and extinction coefficient, k) and film thickness, d. The homogeneity of the films was checked and verified by applying single-angle calculations at different angles. It was shown that the refractive index, n of As-S-Se films is independent of film thickness in the range of 50 to 1000 nm and its values varied from 2.45 to 3.05 for thin layers with composition As₂S₃ and As₂Se₃, respectively. The effect of increasing in the refractive index was observed after exposure to light which is related to the process of photodarkening in arsenic containing layers. The viability of the method for determining the optical constants of very thin chalcogenide films with a high accuracy was confirmed.     

Predicting Reaching Targets from Human EEG

Extracting reach information from brain signals is of great interest to the fields of brain-computer interfaces (BCIs) and human motor control. To date, most work in this area has focused on invasive intracranial recordings; however, successful decoding of reach targets from noninvasive electroencephalogram (EEG) signals would be of great interest. In this article, we show that EEG signals contain sufficient information to decode target location during a reach (Experiment 1) and during the planning period before a reach (Experiment 2). We discuss the application of independent component analysis and dipole fitting for removing movement artifacts. With this technique we get similar classification accuracy for classifying EEG signals during a reach (Experiment 1) and during the planning period before a reach (Experiment 2). To the best of our knowledge, this is the first demonstration of decoding (planned) reach targets from EEG. These results lay the foundation for future EEG-based BCIs based on decoding of planned reaches.     

Cosine tuning minimizes motor errors

Cosine tuning is ubiquitous in the motor system, yet a satisfying explanation of its origin is lacking. Here we argue that cosine tuning minimizes expected errors in force production, which makes it a natural choice for activating muscles and neurons in the final stages of motor processing. Our results are based on the empirically observed scaling of neuromotor noise, whose standard deviation is a linear function of the mean. Such scaling predicts a reduction of net force errors when redundant actuators pull in the same direction. We confirm this prediction by comparing forces produced with one versus two hands and generalize it across directions. Under the resulting neuromotor noise model, we prove that the optimal activation profile is a (possibly truncated) cosine--for arbitrary dimensionality of the workspace, distribution of force directions, correlated or uncorrelated noise, with or without a separate cocontraction command. The model predicts a negative force bias, truncated cosine tuning at low muscle cocontraction levels, and misalignment of preferred directions and lines of action for nonuniform muscle distributions. All predictions are supported by experimental data.     

Effect of urea on the polymer buffer solutions used for the electrophoretic separations of nucleic acids

The present study describes the effect of urea on the properties of hydroxyethyl cellulose (HEC) polymer solutions used for the separation of nucleic acids. Solution properties were investigated by viscosity measurements and supplemented by Raman spectroscopy of the solution components. By using viscosimetry, it was possible to identify that borate, urea, and HEC participate in an interaction causing an increase in the viscosity of dilute solutions. In addition, short polymer chains exhibited a 4-fold decrease in the entanglement threshold in the presence of 4 M urea and 0.356 M total borate concentration. These interactions were found to be specific to HEC. Raman spectroscopy monitored serial addition of 8 M urea to HEC sieving solutions combined with factor analysis indicated formation of minor urea species. Lack of change in the Raman spectrum and relative amount of borate suggested that there is no direct interaction between borate and urea. These effects on HEC sieving solution properties lead to the use of low HEC concentrations that are beneficial for the separation of nucleic acids under denaturing conditions.     

Stable, soluble T-cell receptor molecules for crystallization and therapeutics

Antibody and T-cell receptors (TCRs) are the primary recognitionmolecules of the adaptive immune system. Antibodies have beenextensively characterized and are being developed for a largenumber of therapeutic applications. This has been possible becauseof the ability to manufacture stable, soluble, monoclonal antibodieswhich retain the antigen specificity of B cells. Unlike antibodies,TCRs are not expressed in a soluble form, but are anchored tothe T-cell surface by an insoluble trans-membrane domain. Characterizationand development of TCRs has been hampered by the lack of suitablemethods for producing them as soluble and stable proteins. Herewe report the engineering of soluble human TCRs suitable forcrystallization studies and potentially for in vivo therapeuticuse. Received March 11, 2003; revised July 1, 2003; accepted July 30, 2003.     

Biochemical,antimicrobial and molecular characterization of a noncytotoxic bacteriocin produced by Lactobacillus plantarum ST71KS

Lactobacillus (Lb.) plantarum ST71KS was isolated from homemade goat feta cheese and identified using biochemical and molecular biology techniques. As shown by Tricine-SDS-PAGE, this lactic acid bacterium produces a bacteriocin (ST71KS) with an estimated molecular weight of 5.0 kDa. Bacteriocin ST71KS was not affected by the presence of α-amylase, catalase and remained stable in a wide range of pH and after treatment with Triton X-100, Triton X-114, Tween 20, Tween 80, NaCl, SDS, urea and EDTA. This bacteriocin also remained active after being heated at 100 °C for 2 h and even after 20 min at 121 °C; however, it was inactivated by proteolitic enzymes. Production of bacteriocin ST71KS reached 6400 AU/mL during stationary growth phase of Lb. plantarum cultivated in MRS at 30 °C and 37 °C. Bacteriocin ST71KS displayed a bactericidal effect against Listeria monocytogenes strains 603 and 607 and did not adsorb to the producer cells. Lb. plantarum ST71KS harbors two bacteriocin genes with homology to plantaricin S and pediocin PA-1. These characteristics indicate that bacteriocin ST71KS is a class IIa bacteriocin. The peptide presented no toxic effect when tested in vitro with kidney Vero cells, indicating safe technological application to control L. monocytogenes in foods.