39 research outputs found

    Precise Characterization and Multiobjective Optimization of Low Noise Amplifiers

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    Although practically all function blocks of the satellite navigation receivers are realized using the CMOS digital integrated circuits, it is appropriate to create a separate low noise antenna preamplifier based on a low noise pHEMT. Such an RF front end can be strongly optimized to attain a suitable tradeoff between the noise figure and transducer power gain. Further, as all the four principal navigation systems (GPS, GLONASS, Galileo, and COMPASS) work in similar frequency bands (roughly from 1.1 to 1.7 GHz), it is reasonable to create the low noise preamplifier for all of them. In the paper, a sophisticated method of the amplifier design is suggested based on multiobjective optimization. A substantial improvement of a standard optimization method is also outlined to satisfy a uniform coverage of Pareto front. Moreover, for enhancing efficiency of many times repeated solutions of large linear systems during the optimization, a new modification of the Markowitz criterion is suggested compatible with fast modes of the LU factorization. Extraordinary attention was also given to the accuracy of modeling. First, an extraction of pHEMT model parameters was performed including its noise part, and several models were compared. The extraction was carried out by an original identification procedure based on a combination of metaheuristic and direct methods. Second, the equations of the passive elements (including transmission lines and T-splitters) were carefully defined using frequency dispersion of their parameters as Q, ESR, etc. Third, an optimal selection of the operating point and essential passive elements was performed using the improved optimization method. Finally, the s-parameters and noise figure of the amplifier were measured, and stability and third-order intermodulation products were also checked

    Oscillations of bubbles attached to a capillary: case of pure liquid

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    An oscillating bubble attached to a tip of a capillary is used for probing interfacial properties of liquids containing surface-active agents. Nevertheless, available theories even for the case of pure liquid are not satisfactory. In this contribution, we therefore present results of a linear inviscid theory for shape oscillations of a spherical bubble, which is in contact with a solid support. The theory allows determining eigenmodes (i.e. eigenfrequencies, eigenmode shapes and damping of eigenmode oscillations), but also response of the bubble shape to a motion of its support or to volume variations. Present theory covers also the cases previously analyzed by Strani and Sabetta (J. Fluid Mech., 1984) and Bostwick and Steen (Phys. Fluids, 2009), and it can be applied to both bubbles and drops. The theory has been compared to experiments. Good agreement is found for the case of small bubbles, which have spherical static shape. Experimental results for larger bubbles and drops deviate from the theory, if a neck is formed. It is shown that this deviation correlates well with a ratio of bubble volume to the maximum volume, when a detachment occurs

    Flow Visualization of an Impinging Jet Subjected to Bimodal Forcing

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    Influence of bubble approach velocity on coalescence in α-terpineol and n-octanol solutions

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    This work presents results of an experimental study of the influence of the approach velocity on the coalescence of bubbles in aqueous solutions of n-octanol and α-terpineol. Experiments were performed in a coalescence cell allowing synchronized growth of a pair of bubbles in a liquid. High speed camera imaging was used to characterize the growth of bubbles and their interaction in aqueous solution of different concentrations of surfactants. The coalescence efficiency and contact time till coalescence were determined as a function of the approach velocity between bubbles and the concentration of surfactant. It was found that, for both surfactants, when the approach velocity between bubbles was higher than ~1 mm/s, the coalescence efficiency was independent of the approach velocity and that the contact time was independent of the concentration of surfactant. Below ~1 mm/s, both the coalescence efficiency and the contact time were the function of surfactant concentration. For the higher velocities, the suppression of coalescence occurred at concentrations similar to the concentration of immobilization of the surface of free rising bubbles

    The Effective Upgrading of Raw Biogas to Methane by Selective Membranes

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    The aim of this work was to develop innovative membranes able to separate carbon dioxide and eventually other undesirable compounds from raw biogas. Such membranes should be stable in an aggressive environmemt and resistant to humidity present in biogas. Therefore, three completely different types of membranes were investigated in this work, namely a supported ionic liquid membrance, a water condensing membrane and water-swollen thin film composite membrane. This work deals mainly with a model mixture, but raw biogas taken from a sewage plant was used to complete the results of the work
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