Pulsed gate bias control of GaN HEMTs to improve pulse-to-pulse stability in radar applications

International audienceA significant improvement is demonstrated in the measured pulse-to-pulse stability of an S-band 6 W GaN high electron mobility transistor (HEMT) power amplifier by generating an appropriate pulse of the gate bias and thus a warm-up drain current just before each radio-frequency (RF) pulse of a periodic and coherent radar burst. The amplitude and the width of this gate bias pulse preceding each periodic RF pulse of the burst are experimentally varied to investigate the trade-off between the improvement of pulse-to-pulse stability and the total power-added efficiency. Finally, this technique of synchronised warm-up gate bias pulse demonstrated a 10 dB improvement of measured amplitude pulse-to-pulse stabilities to meet the critical stability requirement below −55 dB for the RF power amplifier

Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor-sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre-existing low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro-faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor-sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved

Theoretical and Experimental Investigations on Nonlinear Capacitance and Loading Effects on Power PHEMT's Linearity

This paper presents a theoretical and experimental analysis of the phase conversion phenomena in a power PHEMT transistor. Relationships between the linearity of the device and the intrinsic gate-source capacitance CGS, gate-drain capacitance CGD ,transconductance GM as well as the load impedance ZL ,are explained. A judicious choice of the transistor’s operating conditions allows to reduce thephaseconversion AM/PM. In the case of PHEMT devices, the behaviors of CGS,CGD,GM elements and ZL value induce internal compensation phenomena between the intrinsic non-linearities

Morphology, sediments and Quaternary history of the continental shelf between the Straits of Dover and Cape Finisterre.

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Morphologie, sédimentologie et histoire quaternaire du Plateau Continental entre le Pas de Calais et le Cap Finisterre.

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