11,512 research outputs found

    Measurements of noise in Josephson-effect mixers

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    We present new heterodyne receiver results obtained at 100 GHz using resistively-shunted Nb and NbN tunnel junctions. In addition, we have carried out accurate measurements of the available noise power of these devices at the L-band (1.5 GHz) IF frequency. Both the heterodyne and the output noise measurements show that the noise of these devices can be a factor of five or more higher than that predicted by the simple current-biased RSJ model. The noise approaches the appropriate thermal or thermal and shot noise limits for bias voltages where the nonlinearity is not strong (i.e., V>ICRN), but as expected from the RSJ model, can be significantly higher at the low voltages where the mixers are typically biased. The bias voltage dependence of the noise shows structure which is associated with resonances in the RF embedding circuit. Surprisingly, we find that changes in the high-frequency (100 GHz) impedance presented to the junction can dramatically affect the magnitude and voltage dependence of the low-frequency (1.5 GHz) noise. This emphasizes the necessity of very closely matching the junction to free space over a wide frequency range

    Inactivation of cloned Na channels expressed in Xenopus oocytes

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    This study investigates the inactivation properties of Na channels expressed in Xenopus oocytes from two rat IIA Na channel cDNA clones differing by a single amino acid residue. Although the two cDNAs encode Na channels with substantially different activation properties (Auld, V. J., A. L. Goldin, D. S. Krafte, J. Marshall, J. M. Dunn, W. A. Catterall, H. A. Lester, N. Davidson, and R. J. Dunn. 1988. Neuron. 1:449-461), their inactivation properties resemble each other strongly but differ markedly from channels induced by poly(A+) rat brain RNA. Rat IIA currents inactivate more slowly, recover from inactivation more slowly, and display a steady-state voltage dependence that is shifted to more positive potentials. The macroscopic inactivation process for poly(A+) Na channels is defined by a single exponential time course; that for rat IIA channels displays two exponential components. At the single-channel level these differences in inactivation occur because rat IIA channels reopen several times during a depolarizing pulse; poly(A+) channels do not. Repetitive stimulation (greater than 1 Hz) produces a marked decrement in the rat IIA peak current and changes the waveform of the currents. When low molecular weight RNA is coinjected with rat IIA RNA, these inactivation properties are restored to those that characterize poly(A+) channels. Slow inactivation is similar for rat IIA and poly(A+) channels, however. The data suggest that activation and inactivation involve at least partially distinct regions of the channel protein

    Cottontail Rabbit Habitat Use on Delaware Wildlife Area, Ohio

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    Author Institution: Ohio Department of Natural Resources, Division of WildlifeA radio telemetry study was performed on cottontail rabbits (Sylvilagus floridanus mearnsii) to quantify cover type use and infer selections with respect to cover type availability, year, sex- and age-class, time of day, and season. This study recorded 16,785 locations for 62 rabbits from 1981-1983. Briars (Rubus spp., Rosa multiflord) and old field were the two most used types of cover, together comprising more than 50% of locations. Relative to cover type availability, the briars type was the most preferred (P < 0.05). Both types were used heavily during day and night throughout the year, although use of briars was higher during day than night (P < 0.05). Rabbits that had survived a winter and spring spent more time in briars (P < 0.05) and less time in old field OP < 0.05) than those that had not. Implications are that wildlife managers must increase their emphasis on providing briars or other dense woody vegetation to maintain rabbit populations. Based on use percentages, rabbit cover type needs would be better met if percentages of area were: briars, herb-shrub, and cropland — each 25-35%; woodland — 0-10%

    Results of pasture fertilization at Lafayette, Louisiana

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    Alignment-insensitive coupling for PLC-based surface mount photonics

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    A flip-chip waveguide coupler with an order of magnitude greater alignment tolerance than competing approaches is presented for the first time. Experimental data for an "optical jumper" agree with simple design considerations. Application to a planar lightwave circuit-based surface mount photonics platform is outlined
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