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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

STAR FORMATION IN THE RHO OPHIUCHI DARK CLOUD

Wilking, Bruce Alan January 1981 (has links)
New and sensitive millimeter-wave and near-infrared observation have been performed in the central regions of the ρ Ophiuchi dark cloud. High spatial resolution observations of the optically thin C¹⁸O emission lines are shown to be free of self-absorption. They permit the first accurate determination of the distribution of molecular gas and allow us to estimate the mass and visual extinction in the cloud. A completely sampled near-infrared survey of a 105 sq. arcmin area which encloses the region of highest visual extinction has revealed twenty objects (sixteen were previously unknown). We show that these objects are young stars embedded in the cloud. Synthesizing our new observations with existing radio and infrared data, we have made a detailed study of the energetics and star formation process within the ρ Oph cloud. This dissertation concludes that the high temperatures of the molecular gas cannot be due to collisions with warm dust. The feasibility of alternate heat sources such as cosmic-ray ionization, shocks, and the distortion of magnetic fields are discussed. We derive high star formation efficiencies (32-46%) in the centrally condensed core of the ρ Oph cloud which suggests that it is forming a bound open cluster. The most consistent interpretation of our data requires that an efficient burst of star formation has produced a relatively large (with respect to the initial mass function) population of low luminosity stars in ρ Oph within the last 3 million years. At this present rate of star formation, most of the molecular gas will be converted into stars in about 10⁷ yr. yielding a bound cluster. Unless this rate rapidly decreases with time, the duration of star formation in the (rho) Oph cluster (10 million years) will be considerably shorter than that suggested for the Pleiades cluster (175 m. y). The lack of stars in the 3-10 M(⊙) range indicates that subsequent star formation must be dominated by more massive stars if the stellar mass spectrum of the ρ Oph cluster is to resemble that of the conventional initial mass function. The youth of the ρ Oph cluster suggests that it may be the most recent episode of star formation in the Sco-Cen OB association.
22

Capacity and coverage of mmWave ad hoc networks

Thornburg, Andrew Scott 07 October 2014 (has links)
Ad hoc networks provide a flexible, infrastructure-free means to communicate between soldiers in war zones, aid workers in disaster areas, or consumers in device-to-device (D2D) applications. Ad hoc networks, however, are stilled plagued by interference. Communication with millimeter-wave (mmWave) devices offers hope to ad hoc networks through higher bandwidth, reduced interference due to directional antennas, and a lighter interference field due to blockage. This report uses a stochastic geometry approach to characterize the one-way and two-way coverage probability of a mmWave ad hoc network with directional antennas and random blockages. The coverage probability in the presence of noise and both line-of-sight and non-line-of-sight interference is analyzed and used to derive the transmission capacity. Several reasonable simplifications are used to derive the transmission capacity. Performance of mmWave is then analyzed in terms of area spectral efficiency and rate coverage. The results show that mmWave networks support larger densities, higher area spectral efficiencies, and better rate coverage compared to microwave ad hoc networks. / text
23

Millimeter-Wave Concurrent Dual-Band BiCMOS RFIC Transmitter for Radar and Communication Systems

Huynh, Cuong Phu Minh 1976- 14 March 2013 (has links)
This dissertation presents new circuit architectures and techniques for improving the performance of several key BiCMOS RFIC building blocks used in radar and wireless communication systems operating up to millimeter-wave frequencies, and the development of an advanced, low-cost and miniature millimeter-wave concurrent dual-band transmitter for short-range, high-resolution radar and high-rate communication systems. A new type of low-power active balun consisting of a common emitter amplifier with degenerative inductor and a common collector amplifier is proposed. The parasitic neutralization and compensation techniques are used to keep the balun well balanced at very high frequencies and across an ultra-wide bandwidth. A novel RF switch architecture with ultra-high isolation and possible gain is proposed, analyzed and demonstrated. The new RF switch architecture achieves an ultra-high isolation through implementation of a new RF leaking cancellation technique. A new class of concurrent dual-band impedance matching networks and technique for synthesizing them are presented together with a 25.5/37-GHz concurrent dual-band PA. These matching networks enable simultaneous matching of two arbitrary loads to two arbitrary sources at two different frequencies, utilizing the impedance-equivalence properties of LC networks that any LC network can be equivalent to an inductor, capacitor, open or short at different frequencies. K- and Ka-band ultra-low-leakage RF-pulse formers capable of producing very narrow RF pulses in the order of 200 ps with small rising and falling time for short-range high-resolution radar and high-data-rate communication systems are also developed. The complete transmitter exhibiting unique characteristics obtained from capabilities of producing very narrow and tunable RF pulses with extremely RF leakage and working concurrently in dual bands at 24.5 and 35 GHz was designed. Capability of generating narrow and tunable RF pulses allows the radar system to flexibly work at high and multiple range resolutions. The extremely low RF leakage allows the transmitter to share one antenna system with receiver, turn on the PA at all time, comply the transmitting spectrum requirements, increase the system dynamic range, avoid harming to other systems; hence improving system size, cost and performance. High data-rate in communication systems is achieved as the consequence of transmitting very narrow RF pulses at high rates. In addition, the dissertation demonstrates a design approach for low chip-area, cost and power consumption systems in which a single dual-band component (power amplifier) is designed to operate with two RF signals simultaneously.
24

Far-infrared & sub-millimeter studies of circumstellar disks

Bulger, Joanna Mary January 2013 (has links)
Circumstellar disks are critical structures in the star and planet formation processes, as they provide a conduit to channel material onto the central object and supply a reservoir of dust and gas to form planets. This thesis focuses on the far-infrared, and sub-millimeter observations of circumstellar disks at two key evolutionary phases; pri- mordial proto-planetary disks, and evolved debris disks – remnants of a system that has undergone a degree of planet formation. Four individual studies of circumstellar disks are presented in this thesis. The results of a 97% complete census of far-infrared emission measured with the Herschel Space Observatory, targeting stars of spectral types M4 and later, in the Taurus molecular cloud are presented. This census is the first large-scale survey sensitive to emission across the stellar and sub-stellar boundary. Results from an initial test grid of model spectral energy distributions, generated with the radiative transfer code MCFOST, show that 73% of the observed Class II population are constrained by canonical disks that are viewed from face-on to edge-on inclinations. Sub-millimeter observations with the Caltech Sub-millimeter Observatory are presented for an association of young T-Tauri stars in the Aquila star-forming region. The results of disk frequency and disk mass of this complete census are investigated in this extremely low stellar density environment. A sub-millimeter investigation for two populations of candidate debris disk; warm and cold excess disks is presented. None of the candidate disks were detected in the sub-millimeter despite upper-limits below that expected, based on blackbody model fits to excesses at shorter wavelengths. Several scenarios are investigated in order to identify the null detection rate, such as stellar multiplicity and background-source contamination. Finally, a partially resolved sub-millimeter map of the debris disk around the HR 8799 multiple planetary system is presented. The planet formation history of the system is investigated through the witnessed morphology of the emission.
25

Multiple Channel, Multiple Data Type, Rugged 8mm Recorder

Harris, Kevin E. 10 1900 (has links)
International Telemetering Conference Proceedings / October 25-28, 1993 / Riviera Hotel and Convention Center, Las Vegas, Nevada / Low cost recording devices for telemetry and other data acquisition applications are of vital importance in light of today's shrinking budgets and project cut-backs. The desire to replace large, expensive, multi-channel recorders with smaller, less expensive recorders is becoming commonplace in government and industry. Many of these small recorders in the past have been limited to a single recording channel, and to one particular data type, due to recorder architectures. The 8 millimeter (8mm) tape cartridge recorder has been looked at in the past as a low cost recording device, however products utilizing this technology have been strictly limited in the number of channels, and data types. In response to this need, Veda has developed a new data acquisition recorder utilizing an 8mm recorder packaged in a small, flight qualified rugged enclosure with modular, and interchangeable, input channels. These microprocessor controlled inputs are capable of accepting PCM telemetry, MIL-STD-1553, voice, IRIG time, and ARINC 429/629 data. The new architecture allows for multiplexing of multiple channels onto the single channel tape medium as well as direct playback from the recorder for certain data types. This paper will discuss the recorder's architecture, design problems solved during development, and general capabilities.
26

Millimetre wave radar for monitoring of railway ballast and surrounding area /

Tran, Quoc Dong. January 2001 (has links) (PDF)
Thesis (M. Eng. Sc.)--University of Queensland, 2001. / Includes bibliographical references.
27

A dual-mode wide-band CMOS oscillator for millimeter-wave applications

Agarwal, Shatam 10 November 2010 (has links)
Broadband voltage-controlled oscillators are critical to the design of millimeter wave frequency synthesizers. This thesis proposes a design technique that can be used to significantly extend the achievable frequency span of an oscillator. A dual-band oscillator topology is described that can be configured to operate in one of two modes, by an electrical reconfiguration of the negative resistance core around the resonant tank, without switching passive elements within the tank itself. The configuration helps to minimize the difference in phase noise performance between the two modes, while achieving a wide tuning range. To verify the concept, a mm-wave VCO that operates at 30-GHz is designed in a commercial 0.18-um CMOS technology, with an approximate simulated tuning range of 20%. A dual-mode oscillator is also designed in a 0.13-um technology at 60-GHz. / text
28

Study of thru-reflect-line calibration : applications to microwave/millimeter wave characterization

Pham, Anh-Vu Huynh 12 1900 (has links)
No description available.
29

Millimeter-wave spectra of the jovian planets

Joiner, Joanna 05 1900 (has links)
No description available.
30

Study and interpretation of the millimeter-wave spectrum of venus

Fahd, Antoine K. 08 1900 (has links)
No description available.

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