Dissertations / Theses: 'Down and Up conversions'

1

Smara, Zakarya. "Etudes des propriétés optiques de nanoparticules de fluorures Na(Y,Bi)F4 dopées par des ions de terres rares et synthétisées par coprécipitation." Electronic Thesis or Diss., Université Clermont Auvergne (2021-...), 2021. http://www.theses.fr/2021UCFAC030.

Full text

Abstract:

Ce travail est consacré à l’élaboration de nanoluminophores de formulations β-NaBiF4 et α-NaYF4 monodopés Eu3+, Tb3+, Pr3+, codopés avec Yb3+ et tridopés avec Ce3+. Ces matériaux ont été préparés par coprécipitation, et les paramètres de synthèse ont été optimisés afin de produire des nanocubes (NCs) avec des longueurs d’arête entre 35 et 65 nm, de façon reproductible et avec une distribution de taille étroite. Les NCs de fluorures obtenus ont été caractérisés d’un point de vue structural (DRX, IR), morphologique (MEB et TEM) et optique (photoluminescence) permettant de confirmer leurs puretés cristallines d’une part et d’enregistrer des répartitions spectrales de luminescence conformes à celles attendues d’autre part. L’analyse des spectres d’émission et des déclins de la fluorescence, sous excitations UV et/ou proche infrarouge, a permis de démontrer que des processus de conversion Stokes et anti-Stokes efficaces se produisent dans ces NCs. Les résultats sont discutés en considérant divers chemins de désexcitations radiatives et de transfert d’énergie, permettant de conclure que l’on peut générer sur un même NC les deux processus, même si ces derniers sont en concurrence dans certains cas
This work is devoted to the development of nanophosphors of formulations β-NaBiF4 and α-NaYF4 monodoped Eu3+, Tb3+, Pr3+, codoped with Yb3+ and tridoped with Ce3+. These materials were prepared by coprecipitation, and the synthesis parameters were optimized to produce nanocubes (NCs) with edge lengths between 35 and 65 nm, reproducibly and with a narrow size distribution. The obtained NCs were characterized from a structural (XRD, IR), morphological (SEM and TEM) and optical (photoluminescence) point of view, making it possible to confirm their crystalline purities on the one hand and to record spectral distributions of luminescence in accordance with those expected on the other hand. Analysis of emission spectra and fluorescence decays, under UV and / or near infrared excitations, has demonstrated that efficient Stokes and anti-Stokes conversion processes occur in these NCs. The results are discussed by considering various paths of radiative de-excitations and energy transfer, making it possible to conclude that the two processes can be generated on the same NC, even if the latter are in competition in certain cases

APA, Harvard, Vancouver, ISO, and other styles

2

Termos, Hassan. "Study of up & down conversion technique by all-optical sampling based on SOA-MZI." Thesis, Brest, 2017. http://www.theses.fr/2017BRES0021/document.

Full text

Abstract:

La conversion de fréquence est une fonction clef présente dans divers contextes, particulièrement dans les systèmes mixtes photoniques-hyperfréquences. Aujourd’hui, la suprématie des réseaux optiques pour le transport de données à haut débit sur de grandes distances incite à l’intégration de telles fonctions dans le domaine optique afin de bénéficier des faibles pertes, larges bandes passantes, faibles poids et tailles propres aux technologies optiques. Dans ce travail, nous étudions un mélangeur tout-optique utilisant un composant SOA-MZI (Semiconductor Optical Amplifier Mach-Zehnder Interferometer) et une technique d’échantillonnage permettant la conversion vers les hautes et les basses fréquences. Le principe du mélange exploite les caractéristiques spectrales d’un signal échantillonné pour lequel des répliques du signal d’origine existent à différentes autres fréquences. Utiliser une telle technique pour la conversion de fréquences offre deux avantages : la conversion vers les hautes et les basses fréquences utilise la même configuration du mélangeur et la fréquence de l’oscillateur local peut être inférieure à la gamme des fréquences visées.L’implémentation d’une telle technique d’échantillonnage nécessite un interrupteur optique contrôlé optiquement.Comme cela est montré dans ce travail, un SOA-MZI peut jouer ce rôle. Selon la phase relative entre ses bras, un interféromètre Mach-Zehnder (MZI) peut transmettre ou non un signal optique d’entrée. En plaçant un SOA dans chaque bras de la structure MZI, la modulation croisée de la phase qui existe au sein d’un SOA est mise à profit pour contrôler l’état de l’interféromètre. Contrôlé par une source impulsionnelle optique, cet interrupteur optique permet d’échantillonner un signal optique porteur de données à modulation complexe. La conversion de fréquence de signaux mono et multi-porteuses dans le domaine 0,5-39,5 GHz a été obtenue avec succès. Par utilisation d’une configuration différentielle du SOA-MZI, des conversions vers les hautes et les basses fréquences jusqu’à un débit de 1 Gb/s ont pu être réalisées
Frequency mixing is a key function existing in different systems, especially in mixed photonic-microwave ones. Today, the supremacy of optical networks to carry high bitrate data over large distances motivates the optical implementation of such functions to benefit from the low loss, high bandwidth, low size and weight of optical technologies. In this work, we study a photonic mixer based on a SOA-MZI (Semiconductor Optical Amplifier Mach-Zehnder Interferometer) device and a sampling technique allowing both conversion towards high and low frequencies.The involved mixing principle exploits the spectral characteristics of a sampled signal in which replicas of the original spectrum exist at different other frequencies. Basing the frequency conversion on a sampling technique gives two advantages: the photonic mixer configuration is the same for up and down conversions, and the frequency of the local oscillator can be less than the addressed frequency range.The implementation of such a sampling technique needs an optically-controlled high-frequency optical switch. As shown in this work, a SOA-MZI can play this role. Depending on the relative phase between its arms, an interferometric structure (MZI) can transmit or cancel an optical input signal. By locating one SOA in each arm of the MZI structure, the cross-phase modulation that exists inside an SOA is exploited to optically control the optical switch state of the MZI.Controlled by an optical pulse source, this optical switch is able to sample an optical input signal carrying complexmodulated data. Frequency conversions of mono and multi-carrier signals in the range 0.5-39.5 GHz have been successfully achieved. By using a differential configuration of the SOA-MZI, both up and down conversions at bitrates up to 1 Gb/s are reached

APA, Harvard, Vancouver, ISO, and other styles

3

Cheroura, Youcef. "Photoconversion et exaltation plasmonique de nanoparticules de β-NaGdF4 pour potentielles applications photovoltaïques sur silicium." Electronic Thesis or Diss., Université Clermont Auvergne (2021-...), 2021. http://www.theses.fr/2021UCFAC023.

Full text

Abstract:

Un système multifonctionnel a été élaboré pour être utilisé comme double capteur de lumière ultraviolette (UV) et proche infrarouge (NIR) et permettant de convertir ces rayonnements par des mécanismes Stokes (DS-DC) et anti-Stokes (UC) en lumière visible susceptible d’accroître le rendement des cellules photovoltaïques. Des nano-bâtonnets ( 140 nm x 60 nm) de β-NaGdF4 monodopés par Eu3+, Pr3+, Er3+ et codopés par Yb3+ ont été synthétisés de façon reproductible par co-précipitation (conditions optimisées : 150°C, 60 min). Les propriétés structurales et morphologiques ont été caractérisées par diffraction des rayons X, spectroscopie infrarouge, Raman et microscopie électronique (MET) confirmant la qualité des matériaux synthétisés. L’utilisation des propriétés de sonde structurale de l’ion Eu3+ a permis d’identifier et de discuter de l’ordre local en s’appuyant sur la théorie de Judd-Ofelt. L’analyse des propriétés de fluorescence a été menée sous excitations impulsionnelle et continue. Les canaux de désexcitation radiative et de transfert d’énergie pour les deux processus DC et UC, pour chaque système, ont été identifiés et discutés en s’appuyant sur l’évolution des intensités de fluorescence en fonction de la puissance de pompe en régime continu et sur les cinétiques de décroissances temporelles sous excitations UV et NIR. Une attention particulière a été accordée au système Er3+/Yb3+ pour lequel un couplage par effet plasmonique a été envisagé avec des nanoparticules métalliques (NPMs) de métaux nobles or et argent. Selon la nature, la forme et la taille des NPMs, des exaltations ou des atténuations des intensités de fluorescence, s’accompagnant dans certains cas d’un accroissement des taux de décroissance temporelle ont été observées. Elles sont discutées en considérant deux processus principaux, la diffusion et l’effet Purcell
A multifunctional system was developed to be used as a dual ultraviolet (UV) and near infrared (NIR) light harvester and photo-converters by Down (DS-DC) and Up Conversions (UC) in visible light to enhance the light trapping properties of thin-film solar cells. Nanorods ( 140 nm x 60 nm) of β-NaGdF4 singly doped by Eu3+, Pr3+, Er3+ and co-doped by Yb3+ were synthesized by co-precipitation method (optimized conditions: 150°C, 60 min). The structural and morphological properties were characterized by X-ray diffraction, infrared spectroscopy, Raman and electron microscopy (TEM) confirming the quality of the elaborated materials. Using the structural probe properties of the Eu3+ ion, local order was identified and discussed on the basis of Judd-Ofelt theory. The analysis of the fluorescence properties was carried out under pulsed and cw excitations. The radiative de-excitation and energy transfer channels for both DC and UC processes, occurring in each system, have been identified and discussed based on the evolution of fluorescence intensities as a function of continuous pump power and on the kinetics of luminescence decays under UV and NIR excitations. Particular attention was paid to the Er3+/Yb3+ system for which a coupling by plasmonic effect was considered with metallic nanoparticles (NPMs) of noble metals gold and silver. Depending on the nature, shape and size of the NPMs, enhancements or attenuations of fluorescence intensities, in addition in some cases to an increase in decaying rates have been observed. They are discussed considering two main processes, scattering and the Purcell effects

APA, Harvard, Vancouver, ISO, and other styles

4

Dieudonné, Belto. "Guides d’onde en verres et vitrocéramiques fluorés dopés terre rare élaborés par PVD pour l’émission dans le visible et la conversion de fréquence." Thesis, Le Mans, 2012. http://www.theses.fr/2012LEMA1023/document.

Full text

Abstract:

Le projet s’inscrit dans le développement de sources lasers RGB miniaturisées pour l’affichage et la vidéoprojection, la conversion de fréquence dans les cellules solaires.Les verres fluorés ZLAG (ZrF4-LaF3-AlF3-GaF3) codopés terres rares ont été considérés. Ce verre possède une faible énergie de phonon, une forte solubilité des terres rares et peut être fabriqué en couche mince par la technique PVD. Il est de plus le précurseur de vitrocéramiques transparentes. On a observé dans les verres massifs et les guides d’onde des émissions bleue, orange et rouge avec un codopage Pr3+-Yb3+, bleue et rouge avec un co-dopage Tm3+-Yb3+. L’émission RGB dans les verres tri-dopés Tm3+-Er3+-Yb3+ semble prometteuse. Par ailleurs, la vitro-céramisation a permis d’augmenter de plus de 30% les sections efficaces d’absorption des ions Pr3+ et Yb3+.Une efficacité de transfert de 92% a été obtenue dans les verres co-dopés 0,5Pr3+-10Yb3+ pour le processus de conversion d’un photon bleu en deux photons IR
The project joins in the development of miniaturized laser sources RGB for display and videoprojection, frequency conversion in solar cells.Fluoride glasses ZLAG ( ZrF4-LaF3-AlF3-GaF3) co-doped with rare earths were studied. This glass has a low phonon energy, a strong solubility of the rare earth ions and can be fabricated as thin films by PVD. It is also the precursor of transparent glass-ceramics. Similar emissions in both co-doped bulk and waveguides have been observed ; blue, orange, red emission for Pr3+-Yb3+ and blue, red emission for Tm3+-Yb3+. The RGB emission in bulk Tm3+-Er3+-Yb3+ tri-doped glass seems promising. Furthermore, the absorption cross section of Pr3+ and Yb3+ ions has been increased by 30% with the ceraming process.An energy transfer efficiency (ETE) of 92% has been obtained for co-doped glass with 0,5Pr3+-10Yb3+ for the conversion process of a blue photon into two infrared one

APA, Harvard, Vancouver, ISO, and other styles

5

Tian, Hai, Tom Trojak, and Charles H. Jones. "COMMUNICATIONS OVER AIRCRAFT POWER LINES: A PRACTICAL IMPLEMENTATION." International Foundation for Telemetering, 2006. http://hdl.handle.net/10150/603939.

Full text

Abstract:

ITC/USA 2006 Conference Proceedings / The Forty-Second Annual International Telemetering Conference and Technical Exhibition / October 23-26, 2006 / Town and Country Resort & Convention Center, San Diego, California
This paper presents a practical implementation of a hardware design for transmission of data over aircraft power lines. The intent of such hardware is to significantly reduce the wiring in the aircraft instrumentation system. The potential usages of this technology include pulse code modulation (PCM), Ethernet and other forms data communications. Details of the fieldprogrammable gate array (FPGA) and printed circuit board (PCB) designs of the digital and analog front end will be discussed. The power line is not designed for data transmission. It contains considerable noise, multipath effects, and time varying impedance. Spectral analysis data of an aircraft is presented to indicate the difficulty of the problem at hand. A robust modulation is required to overcome the harsh environment and to provide reliable transmission. Orthogonal frequency division multiplexing (OFDM) has been used in power line communication industry with a great deal of success. OFDM has been deemed the most appropriate technology for high-speed data transmission on aircraft power lines. Additionally, forward error correction (FEC) techniques are discussed.

APA, Harvard, Vancouver, ISO, and other styles

6

Dieudonné, Belto. "Guides d'onde en verres et vitrocéramiques fluorés dopés terre rare élaborés par PVD pour l'émission dans le visible et la conversion de fréquence." Phd thesis, Université du Maine, 2012. http://tel.archives-ouvertes.fr/tel-00808593.

Full text

Abstract:

Le projet s'inscrit dans le développement de sources lasers RGB miniaturisées pour l'affichage et la vidéoprojection, la conversion de fréquence dans les cellules solaires.Les verres fluorés ZLAG (ZrF4-LaF3-AlF3-GaF3) codopés terres rares ont été considérés. Ce verre possède une faible énergie de phonon, une forte solubilité des terres rares et peut être fabriqué en couche mince par la technique PVD. Il est de plus le précurseur de vitrocéramiques transparentes. On a observé dans les verres massifs et les guides d'onde des émissions bleue, orange et rouge avec un codopage Pr3+-Yb3+, bleue et rouge avec un co-dopage Tm3+-Yb3+. L'émission RGB dans les verres tri-dopés Tm3+-Er3+-Yb3+ semble prometteuse. Par ailleurs, la vitro-céramisation a permis d'augmenter de plus de 30% les sections efficaces d'absorption des ions Pr3+ et Yb3+.Une efficacité de transfert de 92% a été obtenue dans les verres co-dopés 0,5Pr3+-10Yb3+ pour le processus de conversion d'un photon bleu en deux photons IR.

APA, Harvard, Vancouver, ISO, and other styles

7

Björklund, Daniel. "Implementation of a Software-Defined Radio Transceiver on High-Speed Digitizer/Generator SDR14." Thesis, Linköpings universitet, Elektroniksystem, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-78213.

Full text

Abstract:

This thesis describes the specification, design and implementation of a software-defined radio system on a two-channel 14-bit digitizer/generator. The multi-stage interpolations and decimations which are required to operate two analog-to-digital converters at 800 megasamples per second (MSps) and two digital-to-analog converters at 1600 MSps from a 25 MSps software-side interface, were designed and implemented. Quadrature processing was used throughout the system, and a combination of fine-tunable low-rate mixers and coarse high-rate mixers were implemented to allow frequency translation across the entire first Nyquist band of the converters. Various reconstruction filter designs for the transmitter side were investigated and a cheap implementation was done through the use of programmable base-band filters and polynomial approximation.

APA, Harvard, Vancouver, ISO, and other styles

8

Ball, Justin. "Up-down asymmetric tokamaks." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:b7932b07-1388-497e-8d95-4bdffabc52d9.

Full text

Abstract:

Bulk toroidal rotation has proven capable of stabilising both dangerous MHD modes and turbulence. This has allowed existing tokamaks to generate extra fusion power at a fixed size and magnetic field. However, most methods of inducing the plasma to spin do not appear to scale well to larger devices such as ITER or a future power plant. In this thesis, we explore a notable exception: up-down asymmetry in the tokamak magnetic equilibrium. When tokamak flux surfaces are not mirror symmetric about the midplane, turbulence can transport momentum from one surface to the next, creating spontaneous rotation that is "intrinsic" to the geometry. We seek to maximise this intrinsic rotation by finding optimal up-down asymmetric flux surface shapes. First, we use the ideal MHD model to show that low order external shaping (e.g. elongation) is best for creating up-down asymmetric flux surfaces throughout the device. Then, we calculate realistic up-down asymmetric equilibria for input into nonlinear gyrokinetic turbulence analysis. Analytic gyrokinetics shows that, in the limit of fast shaping effects, a poloidal tilt of the flux surface shaping has little effect on turbulent transport. Since up-down symmetric surfaces do not transport momentum, this invariance to tilt implies that devices with mirror symmetry about any line in the poloidal plane will drive minimal rotation. Accordingly, further analytic investigation suggests that non-mirror symmetric flux surfaces with envelopes created by the beating of fast shaping effects may create significantly stronger momentum transport. Guided by these analytic results, we carry out local nonlinear gyrokinetic simulations of non-mirror symmetric flux surfaces created with the lowest possible shaping effects. First, we consider tilted elliptical flux surfaces with a Shafranov shift and find little increase in the momentum transport when the effect of the pressure profile on the equilibrium is included. We then simulate flux surfaces with independently-tilted elongation and triangularity. These two-mode configurations show a 60% increase over configurations with just elongation or triangularity. A rough analytic estimate indicates that the optimal two-mode configuration can drive rotation with an on-axis Alfven Mach number of 1.5% in an ITER-like machine.

APA, Harvard, Vancouver, ISO, and other styles

9

Solway, David. "Teaching down or learning up." Mémoire, Université de Sherbrooke, 1995. http://hdl.handle.net/11143/9212.

Full text

Abstract:

Résumé : L'élément important que cette thèse sous-tend est que l'enseignement efficace n'est pas seulement constitué de techniques et de méthodologie, mais plutôt d'attitude et d'approche envers l'enseignement. Ceci ne veut pas nécessairement dire que plusieurs méthodes d'enseignement reçues dans un cours avec l'intention d'optimaliser les mécanismes de transmission et d'assimilation de la matière sont inappropriées. Cependant, l'absence de ce que nous pourrions définir comme un ton pédagogique est essentiel, c'est-à-dire, qu'une attitude positive à la productivité autant vis-à-vis de la matière à transmettre que vis-à-vis de l'individu impliqué dans "l'acte" de réception versus la découverte, aura davantage de succès. Toute autre méthode sera complètement inefficace, inaccessible, voire même inutile. D'emblée, dans l'hypothèse de départ, l'argument principal présente une attitude générale d'enseignement à divers échelons ; soit au niveau secondaire ou collégial qui est inappropriée, incomplète ou négative. En d'autres mots, cette approche thérapeutise l'éducation. Dans l'exercice de cette approche, l'enseignant ou l'enseignante adopte plutôt le rôle d'un thérapeute que celui d'un éducateur. De ce fait, le professeur en situation a une approche plutôt de thérapeute que celle de maître-précepteur et que la matière présentée est souvent diluée, et réduite à des niveaux d'apprentissage accompagnés de carences notoires et d'échecs académiques. Les attentes d'une performance dans le milieu académique sont souvent des plus modestes. Cette même tendance d'une éducation à la baisse est évidente aussi dans le processus d'évaluation. Il est certain que dans les disciplines non scientifiques, l'évaluation formative a grandement suivi l'évaluation normative conduisant le précepteur, tour à tour, dans une évaluation dormative dans laquelle l'effort et l'intention remplacent les aptitudes et les habilitées réelles. Si l'approche pédagogique est vraiment l'élément crucial de l'éducation, il Importe que l'approche générale influence le climat de l'éducation contemporaine, de fait, devienne un palliatif contre-productif souvent réhabilitant. De plus, cette pseudo-thérapie d'où d'écoule une attitude exigeante envers l'enseignant et l'apprenant dont le fondement est la reconnaissance des impératifs culturels qui en sont le reflet et le corps doit-être affirmé et transposé dans la réalité. Cette dernière comprend des attentes très poussées en ce qui concerne la performance en classe et aussi le respect de la matière qui contient la présentation routinière et fondamentale; renouveau intense du processus d'évaluation qui fournira des standards communs et des objectifs externes dans l'évaluation du travail de l'étudiant. Cette connaissance et domestication empirique que nous présente Vygotsky dans un climat contemporain qu'il a expliqué ces termes comme "des zones de développement proximales" basées sur la doctrine suivante que le bon apprentissage précède le développement et que conséquemment s'ensuit une pédagogie d'apprentissage plutôt qu'une pédagogie centrée sur l'apprenant. L'application significative de ces derniers principes ou de ces épistémologiques s'imbriquent dans une situation d'apprentissage ascentionnel dont la structure est détaillée et considérée par différentes perspectives de la recherche qui suit.||Abstract : The central tenet of this thesis is that effective teaching is not only and perhaps not primarily a matter of technique and methodology but of attitude and approach. This is not to say that diverse methods of classroom instruction intended to optimize the mechanics of transmission and the assimilation of data are inappropriate but that in the absence of what we might denominate as a certain pedagogical tone. that is, a productive attitude toward both the material to be conveyed and the individuel engaged in the 'act' of reception-and-discovery, even the most powerful methods will be differentially unavailing or, at best, inefficient. Given this initial assumption, the argument proceeds that the general attitude toward instruction currently in place at the secondary echelons, that is, on the high school and college levels, may be popularly represented as a 'teaching down' approach, in other words, as one which seeks to therapeuticize education. In practice this means that the teacher tends to manifest in situ more as a therapist than as a preceptor, that the material to be presented is frequently diluted or scaled down to perceived levels of cognitive (dis)ability (as is also the case with the rate of instruction), and that performance expectations in the current pedagogical milieu are commonly quite modest. The same downward trend is evident in assessment protocols as well. Certainly in the nonscientific disciplines, normative evaluation has been widely succeeded by formative evaluation, leading in turn to a peculiar kind of dormative evaluation in which intangibles such as effort and intention may deputize for realized ability. If pedagogical approach is indeed the crucial element in instruction, and if the general approach that pervades the contemporary climate of instruction is indeed counter-productively remedial or rehabilitory, that is, therapeutic, then it should follow that a more demanding attitude toward teaching and learning founded on the recognition of the culturel imperative which teaching both reflects and embodies needs to be re-affirmed and translated into practice. This latter would entail the maintenance of high expectations with regard to classroom performance, a respect for the material which precludes its routine mitigation or debasement, a renewed insistance on grading protocols that provide an external, 'objective' or communal standard against which the student's work can be measured, the empirical acknowledgment or domestication of what Vygotsky has termed "the zone of proximal development," based on the doctrine that good learning proceeds in advance of development, and conséquently, a learning-centered rather than learner-centered pedagogy. The meaningful application of this latter set of principles or epistemological gradients comprises the 'learning up' situation whose structure is excunined in some détail and considered from various perspectives in the ensuing.

APA, Harvard, Vancouver, ISO, and other styles

10

Karlsson, Rikard. "Hybrid start up/shut down vibrations." Thesis, KTH, Farkost och flyg, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-39688.

Full text

Abstract:

A vehicle start/stop engine system is investigated with focus on human perception of discomfort. The FRF between engine rotational acceleration around the neutral torque axis (NTA) and seat rail acceleration with the specific interest in start up and shut down events is estimated from measurements, using a shaker torque excitation. The inverted torque sensitivity formulates a frequency dependent target of powertrain torque related to seat rail acceleration. The main rotation direction for the transversally mounted engine is the pitch-direction, yielding high acceleration levels in both longitudinal x-, and vertical z-direction compared to the lateral y-direction. Specific weighting functions are used to normalize these accelerations to human perception. Acceleration levels in x-, and z-direction at the seat rail are similar after weighting curves have been applied. The amount of discomfort experienced by a person depends on vibration amplitude, frequency, direction and time duration. However, there is no evidence on in what way a specific time dependence affects the perceived comfort. The NTA around which the engine as a rigid body rotates is experimentally estimated and verified by comparing to the reference CAD NTA. The powertrain mounting system center of gravity is assumed to be known from the CAD model. The seat rail is found to be a more convenient measurement position to use in order to achieve reproducible results rather than the seat cushion. This, since the acceleration at the cushion also depends on several other factors such as person size, weight and body posture and also on the non-linear properties of the seat foam padding.

APA, Harvard, Vancouver, ISO, and other styles

11

Giacaglia, Giuliano Pezzolo. "Integrating bottom-up and top-down information." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/91813.

Full text

Abstract:

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 69-70).
In this thesis I present a framework for integrating bottom-up and top-down computer vision algorithms. I developed this framework, which I call the Map-Dictionary Pixel framework, because my intuition is that there is a need for tools that make it easier to build computer vision systems that mimic the way human visual systems process information. In particular, we humans humans create models of objects around us, and we use these models, top-down, to interpret, analyze and discern objects in the information that comes bottom-up from the visual world. After introducing my Map-Dictionary Pixel framework, I demonstrate how it empowers computer vision algorithms. I implement two different systems that extract the pixels of the image that correspond to a human. Even though each system uses different sets of algorithms, both use Map-Dictionary Pixel framework as the connecting pipeline. The two implementations demonstrate the utility of the Map-Dictionary Pixel framework and provide an example of how it can be used.
by Giuliano Pezzolo Giacaglia.
M. Eng.

APA, Harvard, Vancouver, ISO, and other styles

12

Bennett, Robert. "Fibonomial Tilings and Other Up-Down Tilings." Scholarship @ Claremont, 2016. https://scholarship.claremont.edu/hmc_theses/84.

Full text

Abstract:

The Fibonomial coefficients are a generalization of the binomial coefficients with a rather nice combinatorial interpretation. While the ordinary binomial coefficients count lattice paths in a grid, the Fibonomial coefficients count the number of ways to draw a lattice path in a grid and then Fibonacci-tile the regions above and below the path in a particular way. We may forgo a literal tiling interpretation and, instead of the Fibonacci numbers, use an arbitrary function to count the number of ways to "tile" the regions of the grid delineated by the lattice path. When the function is a combinatorial sequence such as the Lucas numbers or the q-numbers, the total number of "tilings" is some multiple of a generalized binomial coefficient corresponding to the sequence chosen.

APA, Harvard, Vancouver, ISO, and other styles

13

Herdevall, Alva. "Dropping Down Pop Up : Redefining Pop-Up Urbanism as a Kickstarter for Urban Development." Thesis, KTH, Stadsbyggnad, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-186161.

Full text

Abstract:

This master project adresses the challenge of transforming the former Bergs Oil Terminal in Nacka into an adaptive, collaborative, and socially just environment supporting the development of a self-sufficient post petroleum society. As tools, it experiments with temporary "pop ups" with various life cycles. The project critically examines urban development under the viewpoint of bottom-up activities where future development must be adapted to current socio-economical conditions and ecological concerns.

APA, Harvard, Vancouver, ISO, and other styles

14

Burke, Madeline Jane. "Top-down and bottom-up approaches to tissue engineering." Thesis, University of Bristol, 2017. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720837.

Full text

APA, Harvard, Vancouver, ISO, and other styles

15

Best, Maisy Jane. "Top-down and bottom-up influences on response inhibition." Thesis, University of Exeter, 2016. http://hdl.handle.net/10871/24650.

Full text

Abstract:

Following exposure to consistent stimulus–stop mappings, response inhibition can become automatised with practice. What is learned is less clear, even though this has important theoretical and practical implications. The main contribution of this thesis is to investigate how stimulus-stop associations are acquired and the conditions under which they influence behaviour. To this end, this thesis addressed several outstanding issues concerning the associative architecture of stop learning, the role of expectancies, and the specificity of learning in inhibition tasks. Experiments 1-4 provide evidence that participants can acquire direct associations between specific stimuli and the stop goal without mediation via a single representation of the stop signal. However, these experiments also suggest that the influence of stimulus-stop associations on behaviour depends on top-down attentional settings: if participants begin to ignore the stop-associated stimuli, the effects of stop learning are diminished or eliminated entirely. Across eight experiments, this thesis provides evidence that participants generate expectancies during stop learning that are consistent with the stimulus-stop contingencies in play. However, Experiments 5-6 indicate that there may be some differences in the relationships between stimulus-stop expectancies and task performance under instructed and uninstructed conditions; stimulus-stop associations that are acquired via task instructions or via task practice have similar effects on behaviour, but seem to differ in how they trigger response slowing for the stop-associated items. Experiments 7-8 investigated the role of signal detection processes during the acquisition of stimulus-stop associations. To distinguish between stimulus-stop learning and stimulus-signal learning, the contingencies between specific stimuli and the stop goal and the contingencies between specific stimuli and the spatial location of the stop signal were independently manipulated. Although these experiments showed evidence of stop/go (goal) learning, there was no evidence that participants acquired the stimulus-signal associations. Across four experiments, this thesis investigated the specificity of stop learning. Experiments 9-10 compared the effects of training on behavioural performance in inhibition (go/no-go) and non-inhibition (two-choice) tasks. The results of these experiments revealed that learning in inhibition and non-inhibition tasks could arise through similar associative mechanisms, but suggest that the effects of training in these tasks could also depend on top-down response settings and general non-associative processes. Experiments 11-12 investigated the neural specificity of stop learning. These experiments also revealed similar effects of training across the go/no-go and two-choice tasks adding weight to the claim that training in inhibition tasks primarily influences task-general processes. Combined, the overall conclusion of this thesis is that bottom-up control can influence response inhibition but what is learned depends on top-down factors. It is therefore important to consider bottom-up factors and top-down factors as dependent, rather than independent, influences on response inhibition.

APA, Harvard, Vancouver, ISO, and other styles

16

Bartram, Julian. "Synaptic plasticity during cortical up-down state oscillatory activity." Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:4c86eb98-6b51-42bd-a818-756922df480d.

Full text

Abstract:

The functions of sleep are diverse and still poorly understood, but a strong effect on cognition is evident. An impressive number of studies suggest that particularly deep sleep, with characteristic cortical slow-wave activity, mediates some important beneficial effects on learning and memory, such as memory consolidation and integration. In the light of this, it is surprising how little we know about the specific rules of synaptic plasticity associated with characteristic activity patterns of different sleep stages. Therefore, using whole-cell recordings from single or synaptically coupled principal cells and two-photon Ca2+ imaging of dendritic spines, I explored how the ongoing network state might promote activity-dependent synaptic plasticity in an in vitro model of the medial entorhinal cortex. This experimental setup allowed precise control over Up-Down state oscillations (cellular membrane potential fluctuations associated with slow-wave activity) - a methodological advantage that is difficult to achieve in vivo. I found that evoking subthreshold synaptic inputs during the Up state phase of cortical slow-wave activity induced N-methyl-D-aspartate receptor-dependent synaptic weakening. In fact, the spontaneous, intrinsically generated recurrent network activity that underlies cortical Up states was able to depress the very inputs that help maintain it. These findings are in agreement with the synaptic homeostasis hypothesis of sleep, a proposal for which descriptions of clear molecular and cellular mechanisms have been missing. Next, I investigated spike-timing dependent plasticity during Up state periods. I found that input-correlated postsynaptic spiking can prevent synaptic weakening. This suggests that while subthreshold synaptic inputs become continuously weaker during slow-wave activity, correlated inputs become relatively more dominant - a process that could be related to memory consolidation. Finally, I investigated Ca2+ signalling in dendritic spines during Up- Down state oscillations using a novel multi-photon microscope based on the remotefocusing technology. These experiments identified a biochemical signature that could drive the observed plasticity rules.

APA, Harvard, Vancouver, ISO, and other styles

17

Johansson, Anton. "Vertical Metaphors : English down and up in Swedish contrast." Thesis, Linnéuniversitetet, Institutionen för språk (SPR), 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-101957.

Full text

Abstract:

This thesis aims to compare the English metaphorical expressions up and down in relation to Swedish. For this purpose, English original texts with Swedish translations and Swedish original texts and English translations were analyzed by using the Linnaeus English-German-Swedish corpus. Thus, the study will show that the metaphorical expressions up and down are used more often in English translated texts as well as what the most common Swedish translation or original of the metaphorical expressions are. Furthermore, the paper will sort the metaphorical expressions based on target domains and compare this with the Swedish translations.

APA, Harvard, Vancouver, ISO, and other styles

18

Itti, Laurent Koch Christof. "Models of bottom-up and top-down visual attention /." Diss., Pasadena, Calif. : California Institute of Technology, 2000. http://resolver.caltech.edu/CaltechETD:etd-12022005-103530.

Full text

APA, Harvard, Vancouver, ISO, and other styles

19

Chua, Hwee Chuan. "Turn-up/turn-down of throughput in membrane bioreactors." Thesis, University of Bath, 2005. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.419220.

Full text

APA, Harvard, Vancouver, ISO, and other styles

20

Metelues, Francis Gabriel. "The Knee Response during Squats with Heels Up and Down." University of Toledo / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1388574269.

Full text

APA, Harvard, Vancouver, ISO, and other styles

21

Terra, Idelma Aparecida Alves. "Investigação espectroscópica e estudo dos processos de conversão de energia em vidros e nano-cristais co-dopados com íons Tb3+ e Yb3+." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/18/18158/tde-26082013-172114/.

Full text

Abstract:

A busca por alternativas viáveis de produção de energia limpa e renovável, utilizando recursos naturais, tem sido um grande desafio. Em especial, o interesse no uso da energia solar para obter energia elétrica tem aumentado. Todavia, as células solares convencionais que são confeccionadas a partir de silício cristalino (Si-c) apresentam uma eficiência de conversão limitada, principalmente, devido às perdas por incompatibilidade espectral. Sendo assim, um dos objetivos dos pesquisadores na área de fotovoltaicos tem sido converter eficientemente fótons da região do visível do espectro solar para a região do infravermelho, onde a célula solar de Si-c possui maior eficiência. A eficiência desta conversão poderia ser melhorada de 28% até 40% usando conversores de energia compostos por íons terras raras. Neste trabalho foram estudadas as propriedades espectroscópicas dos íons terras raras Térbio (Tb3+) e Itérbio (Yb3+), embebidos em três diferentes materiais: vidro aluminosilicato de cálcio com baixa concentração de sílica (7%) (LSCAS), vidro tetraborato de cálcio-lítio (Calibo) e nano-cristais de óxido de zircônia (ZrO2). Sendo assim, nossos estudos visam elucidar os mecanismos geradores dos processos de conversão ascendente e descendente de energia. No processo de conversão ascendente de energia há conversão dos fótons de excitação de baixa energia na região do infravermelho em fótons de alta energia na região do visível. Por outro lado, no processo de conversão descendente de energia ocorre a conversão dos fótons de excitação de alta energia na região do ultravioleta/visível em um ou mais fótons de baixa energia na região do infravermelho. As propriedades espectroscópicas dos materiais estudados foram discutidas a partir das medidas de absorção, luminescência, excitação, evolução temporal da luminescência, curva de potência, difração de raios-X, X-Ray Absorption Near Edge Structure e ressonância paramagnética eletrônica, em função da concentração dos íons Tb3+ e Yb3+. Os resultados mostraram que todas as amostras apresentam conversão ascendente de energia. Assim como, em todas as amostras ocorre o processo de conversão descendente de energia. Em ambos os processos foram identificados os mecanismos geradores dos mesmos. Estes resultados mostram um avanço nos estudos dos processos de conversão ascendente e descendente de energia em sistemas co-dopados com íons Tb3+ e Yb3+. Os resultados sugerem que os materiais estudados podem ser empregados para aumentar a eficiência de conversão da célula solar de Si-c via conversão descendente de energia.
The search for possible alternatives to produce clean and renewable energy using the natural resources has been a great challenge. In particular, the interest to use the solar energy to produce electricity has been increased. However, crystalline silicon-based (c-Si) conventional solar cells have limited conversion efficiency, mainly due to spectral mismatch losses. Thus, one of the goals of some researchers in the photovoltaic field has been to efficiently convert photons in the visible region of the solar spectrum to the infrared region, where the c-Si solar cell has its higher efficiency. The efficiency of this conversion could be enhanced from 28% up to 40% using energy converters based on rare-earths ions. In this work, the spectroscopic properties of rare earth ions, such as Terbium (Tb3+) and Ytterbium (Yb3+) embedded in three different materials were studied: low- silica calcium aluminosilicate glass (7%) (LSCAS), lithium calcium tetraborate glass (Calibo) and zirconium oxide nano-crystals (ZrO2). Thus, our studies aimed to elucidate the mechanisms that generate the Up-conversion and Down-conversion processes. In the Up-conversion process, there is the conversion of the low-energy excitation photons in the near-infrared to high-energy photons in the visible. On the other hand, in the Down-conversion process there is the conversion of the high-energy excitation photons in the ultraviolet/visible region to low-energy photons in the near-infrared region. The optical properties of the studied materials were discussed through absorption, luminescence, excitation, temporal evolution of the luminescence, power curve, X-ray diffraction, X-Ray Absorption Near Edge Structure and electron paramagnetic resonance measurements, as a function of the concentration of Tb3+ and Yb3+ ions. The results showed that all samples exhibit Up-conversion process. In addition, in all samples occurs the Down-conversion process. In both processes were identified the mechanisms that produce them. These results show a breakthrough in the studies of the Up-conversion and Down-conversion processes in Tb3+ and Yb3+ co-doped systems. The results suggest that the materials are applicable in enhancing the conversion efficiency of the Si-c solar cell via NIR Down-conversion.

APA, Harvard, Vancouver, ISO, and other styles

22

Kondo, Michio. "TOP-DOWN AND BOTTOM-UP FORCES STRUCTURING BIOLOGICAL COMMUNITIES : THEORETICAL CONSIDERATIONS." 京都大学 (Kyoto University), 2001. http://hdl.handle.net/2433/150859.

Full text

APA, Harvard, Vancouver, ISO, and other styles

23

Hicks, Jeremy David. "A combined top-down/bottom-up route to fabricating graphene devices." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49100.

Full text

Abstract:

The purpose of this work is to explore a method that combines both top-down and bottom-up elements to fabricate electronic devices made from graphene, a single sheet of carbon atoms related to carbon nanotubes and graphite. This material has garnered interest in the semiconductor industry for many reasons, including its potential for ballistic conduction, natural ambipolar (both n- and p-type) carrier transport, and impermeability to nearly all elements. However, its lack of a band gap, and a lack of viable options for creating one in the material, suggests a limited future as a silicon replacement material. A solution to this problem is presented that uses a recently-reported technique of creating pre-patterned graphene features from the thermal decomposition of specially-structured silicon carbide (SiC) surfaces. We employ a combination of direct band structure measurements and electrical results to suggest that a semiconducting bent graphene nanostructure exists in this structured SiC system, creating a possible route toward a broad class of future graphene electronics.

APA, Harvard, Vancouver, ISO, and other styles

24

Abdussalam, S. S. "Bottom-up and top-down approaches to low energy supersymmetry breaking." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.595324.

Full text

Abstract:

In this thesis we address the phenomenology of SUSY breaking from the superstring scale to the electroweak scale in order to produce a guide map for discovery at the LHC. From the top-down approach we study SUSY breaking at low-energies from a well defined superstring theory scenario and extract its phenomenological implications. From the bottom-up approach (independent of string theory), using Bayesian inference techniques, we make the first complete model-independent scan of the low energy phenomenological MSSM (pMSSM) parameter space. We perform the first statistically convergent pMSSM global fit to all current collider and DM data. We demonstrate the application of Bayesian model selection methodology to high energy SUSY breaking models with modest number of free parameters. We found that SUSY breaking from superstring theory should eventually help test its reality and that direct data is required in order to assign unambiguous preferences between different high energy SUSY breaking models. We predict that the Higgs boson mass lies between 117 GeV and 128 GeV at 95% confidence level. We believe this is a robust prediction that should be confirmed once SUSY is discovered at the LHC. Our pMSSM parameters fit provides an appropriate arena for the LHC studies of the MSSM which we wish to pursue further in future work.

APA, Harvard, Vancouver, ISO, and other styles

25

Grellmann, Doris. "Top-down and bottom-up effects in a Fennoscandian tundra community." Doctoral thesis, Umeå universitet, Institutionen för ekologi, miljö och geovetenskap, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-96883.

Full text

Abstract:

The objective of this thesis was to investigate the effects of mammalian grazers, such as microtine rodents and reindeer, (top-down effects) and nutrient availability (bottom- up effects) on the plant community of a tundra heath. I conducted a large-scale fertilization experiment and studied the impact of grazers using exclosures. I measured the effects of fertilization and grazing on soil microbial activity and nutrient cycling. I investigated the responses to fertilization of the invertebrate community, I studied the effects on the quality of bilberry as food for mammalian herbivores, and I looked at how concentrations of nutrients and carbon-based secondary defences against herbivory fluctuated between seasons in unfertilized and fertilized treatments. The results of my thesis show that the plant community investigated is exposed to a strong top-down control by mammalian herbivores. On the fertilized and grazed areas the aboveground biomass of the vascular plant community did not increase compared to unfertilized areas. However, the productivity of the plant community was clearly nutrient- limited. During the eight years of the experiment, on the fertilized areas plant biomass was significantly increased inside the herbivore exclosures In my study mammalian herbivores at comparatively low densities and grazing outside the growing season were sufficient to control the biomass of a heterogeneous plant community. Microtine rodents (Norwegian lemmings and grey-sided voles) preferred the fertilized areas for overwintering. The food plant quality of bilberry for grey-sided voles was improved on the fertilized areas throughout the year. Grazing decreased the nitrogen storage in the aboveground plant biomass. Reindeer and rodents had also important indirect effects on the plant community by decelerating soil nutrient cycling and soil microbial activity. This effect may be accelerated by the impact of herbivore on plant species composition. Graminoids, which contained the highest nitrogen concentrations in their tissues, increased rapidly on the fertilized areas, but their abundance was significantly lower on grazed fertilized areas. The invertebrate community was detritus-based and received their energy indirectly from the litter via soil microbes and detritivores. Fertilization increased the biomass of invertebrate carnivores, but had no effect on the biomass of invertebrate herbivores. Apparent competition between detritivores and invertebrate herbivores, mediated by carnivorous invertebrates predating on both of them, is supposed to keep the densities and grazing pressure of invertebrate herbivores low. Grazing damage by invertebrates was very low and only 0.021 % of the total vascular plant biomass was removed.

Diss. (sammanfattning) Umeå : Umeå universitet, 2001, härtill 6 uppsstser.

digitalisering@umu

APA, Harvard, Vancouver, ISO, and other styles

26

Almabrok, Almabrok Abushanaf. "Gas-Liquid Two-Phase Flow in Up and Down Vertical Pipes." Thesis, Cranfield University, 2013. http://dspace.lib.cranfield.ac.uk/handle/1826/8447.

Full text

Abstract:

Multiphase flows occurring in pipelines with a serpentine configuration is an important phenomenon, which can be encountered in heat exchangers used in a variety of industrial processes. More specifically, in many industrial units such as a large cracking furnace in a refinery, the tubes are arranged in a serpentine manner and are relatively short. As flow negotiates round the 180o bend at the ends of the tubes, the generated centrifugal force could cause flow maldistribution creating local dry spots, where no steady liquid film is formed on the adjacent straight sections of the pipe. As a result, events including coking, cracking and overheating of heat transfer surfaces may occur and lead to frequent shutdown of the facilities. Consequently, this could increase operating costs and reduce production revenue. Thus, it is desirable to know the effect that the bends exert on the flow in the straight part of the pipe. Apart from this, knowledge of the bend effects on the flows in the pipeline could also be important for the design of other pipelines for gas/liquid transport, e.g. offshore gas and oil pipelines. Quite a large number of studies have been found in the literature. The majority of them were for two-phase flow with small diameter pipes (i.d. ≤ 50 mm). However, studies with large diameter pipes (i.d. ≥ 100 mm), have increasingly been considered in recent years as problems related to large diameter vertical pipes are being encountered more and more often in industrial situations. This thesis studies the effect of 180o bends on the characteristics and development of gas-liquid two-phase flows in large diameter downward and upward pipes. The study particularly focuses on the influence of serpentine configuration on flow structure, cross-sectional void distribution and circumferential liquid film profiles and their development along the downward and upward sections. It was found that both the top and bottom bends have considerable impacts on flow behaviour, although to varying degrees. These impacts were highly dependent on the air and water flow rates. For sufficient flow rates, the bends were observed to create flow maldistribution in the adjacent straight section, due to the effects of centrifugal force. The air moved towards the inner zone of the bend and the water towards the outer zone, while a lesser quantity of water was identified on the other surfaces of the pipe. Investigation of the film thickness development in the downward and upward sections showed that, the liquid film behaviour close to the bends was significantly different from those located further away. This can be attributed to the centrifugal force of the bends. Examination of the power spectral density (PSD) along the downward and upward sections showed that, the shape of PSD located in the adjacent section to the bends, was substantially different from those located further away. Furthermore, several flow regime maps were generated which showed that, in addition to bubbly, intermittent and annular flows, unstable flows existed along the upward section, particularly for low gas and water flow rates. In this study it was found that, the lower bend was periodically blocked by the liquid and then blown through by the accumulated air. The data obtained from this study were compared with different theoretical correlations found in the existing literature. Some discrepancy between the results of the current study and those of previous published materials was noted. Updated correlations were presented which provided well results when they applied for the data obtained from the current study and previous studies.

APA, Harvard, Vancouver, ISO, and other styles

27

Goncharov, Alexander. "Magnetization reversal processes in 'bottom-up' and 'top-down' magnetic structures." Thesis, University of Southampton, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.414612.

Full text

APA, Harvard, Vancouver, ISO, and other styles

28

Djebali, Zeineb. "'Starting-up, not slowing down' : social entrepreneurs in an ageing society." Thesis, University of Surrey, 2018. http://epubs.surrey.ac.uk/850016/.

Full text

Abstract:

A growing body of literature examines entrepreneurial intentions of people aged fifty and over who can be referred to as 'later life' entrepreneurs (e.g. Curran and Blackburn, 2001; Singh and DeNoble, 2003; Weber and Schaper, 2003). However, there is a significant gap in our knowledge about entrepreneurial (social) intentions for people in the same age group. This study uses qualitative research incorporating twenty-eight in-depth interviews with social entrepreneurs aged fifty and over as well as seven expert interviews with members of the Age Action Alliance. These were used to explore the factors these social entrepreneurs consider important in their decision to set up their social enterprises, their 'everyday' lives, and the challenges they face and how they might be supported. An interpretive social constructionist approach was adopted to examine and make sense of the participants' daily lived experiences, from their perspectives. Data was analysed using a thematic analysis approach (Braun and Clarke, 2006) and the findings revealed the participants were driven by 'making a difference' to people's lives, with each having a different interpretation of the factors they considered important in influencing their entrepreneurial (social) intentions, affecting their social orientations and their entrepreneurial (social) identity. The participants displayed a strong sense of commitment to the social need they identified. Nevertheless, despite some perceiving income generation to be significant for facilitating their ability to achieve their social objectives, others viewed income generation to be incompatible with their social mission. This study examines the subjective views of social entrepreneurs aged fifty and over and the ways in which they construct their 'everyday lives' as social entrepreneurs. It provides insights about the interplay between the participants' motives for setting up their social enterprises and their age and entrepreneurial (social) identity. In addition, this study provides an in-depth understanding of the benefits the participants gained during the social entrepreneurial process, whilst bringing significant insights into the challenges and barriers they experienced and how they might be supported. As such, this study extends theoretical and empirical research on social entrepreneurship and social enterprise by developing our understanding of the 'everyday' lived experiences of social entrepreneurs in 'later life' from their own perspectives. It is recommended this study be used as a guide for policy makers and organisations that are supporting social entrepreneurs in this age group. Future research should, therefore, be carried out to examine entrepreneurial (social) intentions of different age groups, as more investigation is needed to explain whether the support required by social entrepreneurs is age specific. Furthermore, it is suggested this study could be useful for academic researchers who would like to further their knowledge on the underlying factors that drive those aged fifty and over to become social entrepreneurs and how they view their 'everyday' lives from their perspectives.

APA, Harvard, Vancouver, ISO, and other styles

29

Ball, Justin Richard. "Nonlinear gyrokinetic simulations of intrinsic rotation in up-down asymmetric tokamaks." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/86868.

Full text

Abstract:

Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 131-136).
Experiments and theory show that tokamak plasmas with strong toroidal rotation and rotation shear can suppress turbulent energy transport as well as allow violation of the Troyon [beta] limit. However, using external neutral beams to inject toroidal momentum, as is done in many current experiments, would require a prohibitive amount of energy in larger, reactor-sized devices. The most promising alternative to achieve significant mean plasma flow that scales to large devices is intrinsic rotation, the rotation that is observed in the absence of external momentum injection. Intrinsic rotation is observed in current experiments, but is generated by effects that are formally small in [pi]* =- [pi]i / a, the ratio of the ion gyroradius to the tokamak minor radius. These effects are insufficient in anticipated reactors because [pi]*, will be significantly smaller. Recent theoretical work concludes that up-down asymmetry in the poloidal crosssection of tokamaks can drive intrinsic rotation to lowest order in [pi]*, [1, 2]. In this thesis, we extend GS2, a local [delta] f gyrokinetic code that self-consistently calculates momentum transport, to permit up-down asymmetric configurations. MHD analysis shows that ellipticity is most effective at introducing up-down asymmetry throughout the plasma. Accordingly, tokamaks with tilted elliptical poloidal cross-sections were simulated in GS2 to determine nonlinear momentum transport. The results suggest that the current experimentally measured rotation levels can be generated in reactorsized devices using up-down asymmetry. Surprisingly, linear and nonlinear gyrokinetic simulations also suggest that tilted elliptical flux surfaces may naturally suppress turbulent energy transport. Using cyclone base case parameters [3] (except for an elongation K = 2), a 40% reduction in the linear turbulent growth rate was observed by tilting the flux surface [pi]/4 from vertical. However, this reduction of energy transport was not observed when the background temperature gradient was increased by 50%.
by Justin Richard Ball.
S.M.

APA, Harvard, Vancouver, ISO, and other styles

30

Langfelder, Antonia. "Plasticity associated with Up/Down states in the mouse barrel cortex." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:980a4f39-d60c-4484-a09f-3765ca6063d6.

Full text

Abstract:

Slow wave sleep is characterised by synchronised slow oscillations, which manifest themselves in cortical neurons as depolarised Up states and more hyperpolarised, silent Down states. Up/Down states have been implicated in synaptic plasticity during sleep-associated memory consolidation. However, the underlying mechanism is not well-understood. I used the mouse somatosensory barrel cortex as a model system to investigate whether Up states and Down states differentially facilitate synaptic plasticity. Initial experiments were carried out in vitro in slices of barrel cortex. Here, Up states were evoked by electrical stimulation, making it possible to precisely control onset timing. Perforated patch-clamp recordings were obtained from pyramidal neurons in layer 2/3 to determine whether pairing of inputs with electrically-evoked Up states results in plasticity of the paired input. I found that pairing a train of EPSPs at different frequencies with Up states resulted in synaptic depression. However, there was no significant difference between Up states and Down states in their propensity to mediate plasticity of paired inputs. This lack of state-dependent plasticity could have been due to an insufficient number of pairings, as well as differences between electrically-evoked and spontaneous Up states in their ability to facilitate plasticity of paired inputs in the barrel cortex. To overcome these issues, plasticity was studied using long-term extracellular recordings in vivo under anaesthesia. Here, spontaneous Up states were automatically detected and paired with whisker deflections. While persistently pairing inputs with Up states led to long-term depression of the whisker response in the barrel cortex, Down state pairings did not induce any long-term effects. Taken together, results from experiments in vivo, but not in vitro, suggest that Up states facilitate depression of inputs in an activity-dependent manner.

APA, Harvard, Vancouver, ISO, and other styles

31

Pule, Lebohang. "Constructing efficient multi-asset class portfolios: Top-down or bottom-up?" Master's thesis, University of Cape Town, 2017. http://hdl.handle.net/11427/27753.

Full text

Abstract:

This dissertation concerns itself with the problem of constructing multi asset class portfolios. The investment process is aimed at solving two problems. The first problem is estimating the future returns of individual securities, which is an exercise fraught with uncertainty as the future is fundamentally unpredictable. This uncertainty means that the investor must allocate his portfolio to a number of assets instead of just one, in case his predicted future returns do not materialize. This leads the investor to the second problem of how best to construct the portfolio. It is this part of the investment process which is the subject of this dissertation which examines whether it is best to construct multi-asset class portfolios using a top-down or bottom-up approach. In the top-down approach one begins by creating independent single asset class portfolios which are then combined to create a multi-asset class portfolio. The bottom-up approach constructs the portfolio by considering all the securities available to the investor (irrespective of asset class) at the same time. The Mean-Variance and Black- Litterman models are reviewed in detail. Portfolios are then created using these portfolio construction methods in order to compare the two approaches. In constructing these portfolios, the commonly encountered problem of missing data in financial return series is also examined. The main result is that the top-down and bottom-up approaches create similar efficient frontiers, though the bottom-up approach results in an extended frontier which allows investors to obtain efficient portfolios with either a higher expected return or a lower volatility.

APA, Harvard, Vancouver, ISO, and other styles

32

Green, Jeremy, Nesreen Hasan, Stefan Meinel, Michael Engelhardt, Stefan Krieg, Jesse Laeuchli, John Negele, Kostas Orginos, Andrew Pochinsky, and Sergey Syritsyn. "Up, down, and strange nucleon axial form factors from lattice QCD." AMER PHYSICAL SOC, 2017. http://hdl.handle.net/10150/624681.

Full text

Abstract:

We report a calculation of the nucleon axial form factors G(A)(q)(Q(2)) and G(A)(q)(Q(2)) for all three light quark flavors q is an element of{u, d, s} in the range 0 <= Q(2) less than or similar to 1.2 GeV2 using lattice QCD. This work was done using a single ensemble with pion mass 317 MeVand made use of the hierarchical probing technique to efficiently evaluate the required disconnected loops. We perform nonperturbative renormalization of the axial current, including a nonperturbative treatment of the mixing between light and strange currents due to the singlet-nonsinglet difference caused by the axial anomaly. The form factor shapes are fit using the model-independent z expansion. From G(A)(q)(Q(2)), we determine the quark contributions to the nucleon spin and axial radii. By extrapolating the isovector G(P)(u-d)(Q(2)), we obtain the induced pseudoscalar coupling relevant for ordinary muon capture and the pion-nucleon coupling constant. We find that the disconnected contributions to G(P) form factors are large, and give an interpretation based on the dominant influence of the pseudoscalar poles in these form factors.

APA, Harvard, Vancouver, ISO, and other styles

33

Muhsin, Ali, and Nikola Rusevski. "Kartläggning av Top down och Bottom up i tre fallföretag : Informationsstrategier." Thesis, Tekniska Högskolan, Högskolan i Jönköping, JTH, Logistik och verksamhetsledning, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:hj:diva-46759.

Full text

Abstract:

Purpose – The purpose of the study is to map out the top-down and bottom-up perspectives and investigate the information strategies on how it relates to daily operations in three case companies. To fulfill the purpose, the following three questions have been answered: What approach do the organization have on information strategies and what characterizes them? To what extent do the case organization use the information strategies? What opportunities and difficulties do organizations see with the information strategies? Method – To fulfill the purpose, a case study was carried out on three case companies. The empirical data has been collected through interviews and surveys. The theoretical framework together with the empirical data, has then contributed to achieving the purpose. Findings – First the map out was conducted within three case companies and how the information strategies have been chosen in relation to the daily operations perspective. Based on the mapping, it has then been framed the potential possibilities and difficulties with the respective strategy choices and its implementation. Limitations – The definition of the information strategies within the study is defined as Top-down and Bottom-up and how the choice in how companies choose to coordinate and be responsible for tasks. The authors do not know whether the study can be adapted to the business of other organisations when mapping information strategies. Keywords – information strategies, top-down, bottom-up, information flow, communication.

APA, Harvard, Vancouver, ISO, and other styles

34

Carapezzi, Stefania <1970&gt. "Scaled down physical properties of semiconductor nanowires for nanoelectronics scaling up." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amsdottorato.unibo.it/6222/1/carapezzi_stefania_tesi.pdf.

Full text

Abstract:

Semiconductor nanowires (NWs) are one- or quasi one-dimensional systems whose physical properties are unique as compared to bulk materials because of their nanoscaled sizes. They bring together quantum world and semiconductor devices. NWs-based technologies may achieve an impact comparable to that of current microelectronic devices if new challenges will be faced. This thesis primarily focuses on two different, cutting-edge aspects of research over semiconductor NW arrays as pivotal components of NW-based devices. The first part deals with the characterization of electrically active defects in NWs. It has been elaborated the set-up of a general procedure which enables to employ Deep Level Transient Spectroscopy (DLTS) to probe NW arrays’ defects. This procedure has been applied to perform the characterization of a specific system, i.e. Reactive Ion Etched (RIE) silicon NW arrays-based Schottky barrier diodes. This study has allowed to shed light over how and if growth conditions introduce defects in RIE processed silicon NWs. The second part of this thesis concerns the bowing induced by electron beam and the subsequent clustering of gallium arsenide NWs. After a justified rejection of the mechanisms previously reported in literature, an original interpretation of the electron beam induced bending has been illustrated. Moreover, this thesis has successfully interpreted the formation of NW clusters in the framework of the lateral collapse of fibrillar structures. These latter are both idealized models and actual artificial structures used to study and to mimic the adhesion properties of natural surfaces in lizards and insects (Gecko effect). Our conclusion are that mechanical and surface properties of the NWs, together with the geometry of the NW arrays, play a key role in their post-growth alignment. The same parameters open, then, to the benign possibility of locally engineering NW arrays in micro- and macro-templates.

APA, Harvard, Vancouver, ISO, and other styles

35

Carapezzi, Stefania <1970&gt. "Scaled down physical properties of semiconductor nanowires for nanoelectronics scaling up." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amsdottorato.unibo.it/6222/.

Full text

Abstract:

Semiconductor nanowires (NWs) are one- or quasi one-dimensional systems whose physical properties are unique as compared to bulk materials because of their nanoscaled sizes. They bring together quantum world and semiconductor devices. NWs-based technologies may achieve an impact comparable to that of current microelectronic devices if new challenges will be faced. This thesis primarily focuses on two different, cutting-edge aspects of research over semiconductor NW arrays as pivotal components of NW-based devices. The first part deals with the characterization of electrically active defects in NWs. It has been elaborated the set-up of a general procedure which enables to employ Deep Level Transient Spectroscopy (DLTS) to probe NW arrays’ defects. This procedure has been applied to perform the characterization of a specific system, i.e. Reactive Ion Etched (RIE) silicon NW arrays-based Schottky barrier diodes. This study has allowed to shed light over how and if growth conditions introduce defects in RIE processed silicon NWs. The second part of this thesis concerns the bowing induced by electron beam and the subsequent clustering of gallium arsenide NWs. After a justified rejection of the mechanisms previously reported in literature, an original interpretation of the electron beam induced bending has been illustrated. Moreover, this thesis has successfully interpreted the formation of NW clusters in the framework of the lateral collapse of fibrillar structures. These latter are both idealized models and actual artificial structures used to study and to mimic the adhesion properties of natural surfaces in lizards and insects (Gecko effect). Our conclusion are that mechanical and surface properties of the NWs, together with the geometry of the NW arrays, play a key role in their post-growth alignment. The same parameters open, then, to the benign possibility of locally engineering NW arrays in micro- and macro-templates.

APA, Harvard, Vancouver, ISO, and other styles

36

Larsson, Therese, and Louise Pettersson. "Perspektiv på goodwill : En studie om användningen av teoretiska begrepp i praktiken." Thesis, Karlstads universitet, Avdelningen för företagsekonomi, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-31524.

Full text

Abstract:

Det finns två typer av goodwill, den första typen är internt upparbetad goodwill och den andra typen är goodwill som uppkommer vid förvärv av företag. Fokus i denna uppsats kommer att ligga på den förvärvade goodwill, som kan upplevas som en svart låda då det kan vara oklart vad posten faktiskt består av. Förvärvad goodwill kan definieras på olika sätt av företagen, i den akademiska världen har främst två perspektiv identifierats, top-downperspektivet och bottom-upperspektivet. Ur den första synvinkeln ses goodwill som en restpost och ur den andra synvinkeln kan goodwill ses som en summa av de komponenter som posten består av. Genom att betrakta dessa perspektiv är tanken att få en inblick i hur företagen väljer att redovisa sin goodwill. Det kan vara lättare sagt än gjort, teori och praktik går inte alltid hand i hand. Det finns ett glapp mellan hur praktiker jobbar med goodwill och teoretikers åsikt om posten. Varför det finns meningsskiljaktigheter växer successivt fram under studiens gång. Det finns än så länge bara ett fåtal studier som behandlar själva begreppet goodwill och dess olika perspektiv samt hur företagen ser på goodwill. För att försöka bidra till ökad förståelse på detta område, har vi i denna uppsats undersökt om teorin och praktiken kommit varandra närmare. Det främsta syftet med studien är att ta reda på om företag inom bemanningsbranschen definierar sin goodwill olika i sina årsredovisningar. För att undersöka detta på ett begripligt sätt har tre professionella respondenter intervjuats i ett försök att klargöra komplexiteten av posten. Slutsatsen är att det inte går att identifiera vilket perspektiv som används i en årsredovisning på grund av att dessa är väldigt standardiserade, vilket gör att det blir svårt för intressenten att utläsa särskilt mycket information om goodwillposten. Det alternativ som kändes mest relevant att använda och som skulle hjälpa oss att kunna få klarhet i vår frågeställning, var att vända sig till en urvalsgrupp av representanter på området. Den analys som efterföljde resulterade i att vi kunde konstatera att samtliga svarande på det praktiska området såg goodwillposten mer eller mindre som en restpost. Ingen värdering eller starkare reflektion gjordes kring det andra teoretiska perspektivet. Sammanfattningsvis vågar vi påstå att det inte skett någon märkbar förändring som gjort att teori och praktik närmat sig varandra.
There is two ways to understand goodwill. Number one, internally generated goodwill and number two, acquired purchase goodwill. The main topic for this research is acquired purchase goodwill. Many people believe that goodwill is a sort of black box. It is unclear what goodwill actually is. There is no limit on how you can define acquired purchase goodwill. In academia we talk about two perspectives, top-down perspective and bottom-up perspective. The first perspective sees goodwill as a residual and the second perspective sees goodwill as a sum of its components. When you put on these two glasses, the idea is that you may be able to see how businesses account for their goodwill. It is easier said than done, many times theory and practice does not get along. There is a big space between the theories on how you work with goodwill in the reality. Only a few have studied the definition of goodwill. Because of that we really wanted to investigate this phenomenon. We have analyzed if the gap between theory and reality has been reduced. The purpose of this study is to investigate if the staffing companies define goodwill different in their annual reports. The tool we have used is to interview professional accountants to get their opinion. The conclusion is that the annual report is quite standardized which means it is difficult to understand what goodwill actually is. Everybody we interviewed have the same opinion, that goodwill is a residual. They made no reflections or additions on that subject. In summary, we know now that after our study, no big change has been made between theory and practice.

APA, Harvard, Vancouver, ISO, and other styles

37

Erdem, Mehmet Erkut. "Simultaneous Bottom-up/top-down Processing In Early And Mid Level Vision." Phd thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12610167/index.pdf.

Full text

Abstract:

The prevalent view in computer vision since Marr is that visual perception is a data-driven bottom-up process. In this view, image data is processed in a feed-forward fashion where a sequence of independent visual modules transforms simple low-level cues into more complex abstract perceptual units. Over the years, a variety of techniques has been developed using this paradigm. Yet an important realization is that low-level visual cues are generally so ambiguous that they could make purely bottom-up methods quite unsuccessful. These ambiguities cannot be resolved without taking account of high-level contextual information. In this thesis, we explore different ways of enriching early and mid-level computer vision modules with a capacity to extract and use contextual knowledge. Mainly, we integrate low-level image features with contextual information within uni&
#64257
ed formulations where bottom-up and top-down processing take place simultaneously.

APA, Harvard, Vancouver, ISO, and other styles

38

Sharma, Vinay. "Simultaneous object detection and segmentation using top-down and bottom-up processing." Columbus, Ohio : Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1196372113.

Full text

APA, Harvard, Vancouver, ISO, and other styles

39

Roberts, James W. "The contribution of top-down and bottom-up processes during observational practice." Thesis, Liverpool John Moores University, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582848.

Full text

Abstract:

The primary aim of this thesis was to examine the nature of the representation developed during observational practice. Chapter 2 (Experiment 1-2) investigated the stimulus properties required to learn novel motor skills and the processing mechanisms responsible for learning. Participants attempted a novel relative timing pattern performed on a computer. Following the observation of a novel relative timing pattern via an expert human model (biological motion, BM), constant velocity model (non-biological motion, NBM) or end-state timing information without the trajectory (goal-directed model, GOAL), there was significantly more accurate relative timing error for BM and NBM groups (BM < GOAL, CTL; NBM < CTL (ps < 0.05)) (Experiment 1). Therefore, sequence learning through observation requires the presence of motion information, irrespective of the biological properties. These findings were extended with participants observing biological- or non-biological motion after receiving an instruction that the stimuli were in fact human- or computer-generated (BM-H, BM-C, NBM-H, NBM-C) (Experiment 2). In addition to the test of motor learning, participants completed a crecognition test featuring the detection of previously observed (trained) or novel (new) sequences. The results confirmed significantly more accurate relative timing error for the experimental groups compared to a CTL group. However, the recognition test revealed an attenuation of explicit recognition toward the observation of new sequences for the BM-H group compared to BM-C (p = 0.06), NBM-H and NBM-C groups (ps < 0.05). This indicated implicit motor learning that was specific to BM-H group. Thus, the coding of motion information was subject to top-down processes (i.e., belief), which modulate automatic bottom-up processes sensitive to biological motion. 3 Chapter 2 (Experiment 3-7) focused on the coding of biological kinematics. Thus, the natural movement profile adopted by humans was manipulated by forming an unnatural biological motion model featuring a shift in the proportion of time to peak velocity. The coding of biological kinematics would be indicated by the transition from natural movement kinematics, characterised by an early-to-mid time to peak velocity, toward a novel movement profile featuring a delayed time to peak velocity. Performance was measured by calculating the absolute difference between participant and model pTTPV (imitation error) and pTTPV of segment 1. The results revealed that the observation of unnatural biological motion produced significantly more accurate imitation error and an extended pTTPV, as per the unnatural model (p < 0.05) (Experiment 3). Subsequent experiments were intended to examine the processing mechanisms responsible for the coding of kinematics. Indeed, issuing a secondary movement task during observational practice led to an attenuation of the unnatural kinematics as indicated by no significant differences in imitation error and pTTPV between participants observing natural and unnatural biological motion (p > 0.05) (Experiment 4). This motor interference was effector-independent as indicated by the corresponding attenuation of imitation error and pTTPV after performing secondary movements of an unrelated effector (i.e., foot) (Experiment 5). These results indicate the coding of novel biological kinematics incorporate motor processes typically associated with action-execution. Meanwhile, the coding of biological kinematics was also subject to the direction of attention. That is, a secondary attention demanding counting task attenuated the coding of biological kinematics as there were no significant differences for imitation error and pTTPV (p > 0.05) (Experiment 6). Finally, the instruction to primarily attend to the trajectory compared to a general pre-cue led to a significantly more accurate imitation 4 error score, although significantly attenuated relative timing ~rror (ps < 0.05) (Experiment 7). Together, these results demonstrate the contribution of bottom-up and top-down processes during observational practice.

APA, Harvard, Vancouver, ISO, and other styles

40

Mayne, Elizabeth W. "G-protein coupled modulation of cortical UP and DOWN states in vitro." Thesis, University of Oxford, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543529.

Full text

APA, Harvard, Vancouver, ISO, and other styles

41

Shelley, Elwyn James Bacchus. "Toward positional self-assembly : integration of top-down and bottom-up methodologies." Thesis, University of Birmingham, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.396185.

Full text

APA, Harvard, Vancouver, ISO, and other styles

42

Uygun, David. "Top-down and bottom-up control of drug-induced sleep and anaesthesia." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/43958.

Full text

Abstract:

In recent decades, research has unravelled fascinating detail about the molecular mechanisms underpinning pharmacologic loss of consciousness (LOC). However, the systems-level mechanisms are far less clear. Recent genetic approaches, however, enable unprecedented dissection on neural pathways, and they are paving a way for this line of research. The focus of this thesis is to investigate the neuroanatomical substrates of commonly used drugs which reversibly render us unconscious. Zolpidem is a positive allosteric modulator (PAM) of the GABAA receptor which binds to the benzodiazepine (BZ) site. Because zolpidem binds α1-3,β,γ2 containing GABAA receptors, which are widespread, it acts virtually everywhere. We do not know if zolpidem causes sleep by enhancing GABAergic inhibition throughout the entire brain, or if the therapeutic sleep-inducing property depends upon specific brain circuitry. γ2I77 mice are devoid of zolpidem-sensitivity. But, zolpidem-sensitivity can be restored selectively in brain regions, enabling dissection of the circuitry involved in zolpidem's effect. To isolate the therapeutic effect of zolpidem we deleted GABAA-γ2I77-subunits and replaced them with GABAA-γ2F77-subunits in HDC neurons or frontal-cortex in isolation. We were able to selectively restore zolpidem-sensitivity in target neurons. This conferred zolpidem-enhanced IPSCs locally. Compared with wild-type mice and zolpidem-insensitive γ2I77lox mice, we found that GABAA-γ2F77 receptors in either HDC-neurons or frontal cortex alone were enough to rescue the majority of zolpidem-mediated sleep. The response in HDC-γ2F77 mice was similar to that of an H1-receptor antagonist. By producing a null effect in a negative-control area - the superior colliculus - we show that HDC neurons and the frontal cortex are both substrates involved in zolpidem-mediated sleep. We also investigated the role of synaptic-inhibition onto corticothalamic-neurons in anaesthetic-induced LOC and sleep-wake. To do this, we genetically ablated γ2-subunits from layer-6 corticothalamic-cells by crossing Ntsr1-Cre mice with GABAA-γ2I77lox mice. We found this reduced isoflurane sensitivity, but left sleep-wake behaviours virtually unaffected.

APA, Harvard, Vancouver, ISO, and other styles

43

Löw, Caroline Anna Elisabeth [Verfasser], and Friederike [Akademischer Betreuer] Lang. "Quellenidentifikation der Phosphorernährung von Buchenwäldern mittels Bottom-up- und Top-down-Ansätzen." Freiburg : Universität, 2019. http://d-nb.info/121641727X/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

44

Zhu, You-Xiang, and 朱佑祥. "24 GHz Down-Conversion Mixer and 24 GHz Up-Conversion Mixer." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/85314817588519652923.

Full text

Abstract:

碩士
國立交通大學
電信工程研究所
102
This paper consists of two parts. These proposed circuits are fabricated using a standard TSMC 0.18um CMOS process. Part I present a 24 GHz down-conversion mixer. The Capacitor cross-coupled differential transconductance stage is used to increase effective transconductance and decrease noise figure and the current bleeding is used to make the switching more ideal. We also use the active load to stabilize the bias of the mixer and it can provide a large load. The measurement result of the 24 GHz down-conversion mixer shows the power gain is about 9 dB in 22~26 GHz, noise figure is about 11.5 dB, P1dB is -15 dBm, IIP3 is -5 dBm, isolations are below -25 dB, power consumption is 4.9 mW and the chip size is 0.98x0.96 mm2. Part II proposes a 24 GHz up-conversion mixer. The multi-gate technology is used for transconductance stage to improve the linearity and we also use the negative resistance compensation to improve the conversion gain. The simulation of the mixer shows that the power gain is 5.2 dBm, noise figure is about 11 dB, OP1dB is -8.3 dBm, OIP3 is 2.3 dBm, isolation is under -20 dB, power consumption is 6.4 mW and the chip size is 0.88x1.05 mm2.

APA, Harvard, Vancouver, ISO, and other styles

45

Zhang, Xiu Ming, and 張修銘. "Implementation of 2-30GHz UWB Up Conversion and Down Conversion Mixer." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/45882645846771515651.

Full text

APA, Harvard, Vancouver, ISO, and other styles

46

Zeng, Yu-Shan, and 曾于珊. "Design of Up/Down Conversion Mixer for IEEE 802.11a Application." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/55640032529629305156.

Full text

Abstract:

碩士
國立中山大學
電機工程學系研究所
100
The IEEE 802.11a has become the mainstream protocol used in modern wireless communication system due to its high propagation rate of data (54 Mb/s). To meet high propagation rates, the communication devices used in IEEE 802.11a protocol usually present a high conversion gain and a high linearity (denoted as third order intercept point, IIP3). The IIP3 of conventional up- and down-conversion mixers are only about 0 dBm and -5 dBm, which fail to achieve a high propagation rate of data. This thesis utilizes the TSMC 0.18 μm CMOS technology to design and fabrication up- and down-conversion mixers with very high linearity for IEEE 802.11a application. The proposed high-linearity up-conversion mixer with 1.01 mm × 0.85 mm chip size and its wide bandwidth (5~6 GHz) is well suited for IEEE 802.11a application. To enhance the linearity and bandwidth, a transconductor stage with gm-boosted structure, a switch stgae with LO-body grounded structure and a load stage with shunt peaking structure are adopted in this research. Under 5.2/5.4/5.8 GHz operating frequencies, the implemented up-conversion mixer demonstrates a high conversion gain of 6.8/7.1/6.3 dB and a high linearity of 8.9/9/13.2 dBm, respectivly. In addition, a moderate consuming power (6.86 mW) of such mixer can be achieved at 1.2 V supply voltage. On the other hand, this thesis also designed and fabricated a high-linearity down-conversion mixer with chip size of 1.02 mm × 0.86 mm and 5.2 GHz center frequency. To improve the linearity and isolation and reduce the high-order noise, a transconductor stage with dual-gate structure and a load stage with RC-tank structure are adopted in this research. According to the EM-simulation resutls, the proposed down-conversion mixer presents a moderate conversion gain of 6 dB and a high linearity of 0.8 dBm. Additionly, a moderate consuming power (6.75 mW) of such mixer can be achieved at 1.8 V supply voltage.

APA, Harvard, Vancouver, ISO, and other styles

47

Chi, Chien-Chu, and 紀建竹. "Design and Implementation of 94-GHz CMOS Up-Conversion and Down-Conversion Mixers." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/96147737279994573392.

Full text

Abstract:

碩士
國立暨南國際大學
電機工程學系
103
This thesis mainly utilizes standard TSMC 90nm CMOS process technology to implement up-conversion mixers for W-band. The thesis is composed of four sections: In the first section, a low-power CMOS up-conversion mixer with high CG and excellent LO-RF isolation for 77~81 GHz short range automotive radar is reported. The mixer comprises an enhanced double-balanced Gilbert cell with PMOS Negative Resistance compensation and differential cascode for conversion gain (CG) enhancement, a 79GHz Marchand balun for converting the single LO input signal to differential signal, and another Marchand balun for converting the differential RF output signal to single signal. The measured result indicates that the proposed up-conversion mixer with PMOS Negative Resistance compensation is promising for W-band RFIC applications. In the second section, a low power CMOS up-conversion mixer with high CG for 94 GHz Weather radar is reported. The mixer comprises an enhanced double-balanced Gilbert cell with Active Current Mirror and Source Follower Output Stage for conversion gain enhancement and power consumption reduction (current-reuse). a 94GHz Marchand balun for converting the single LO input signal to differential signal, and another Marchand balun for converting the differential RF output signal to single signal. In the third section, a 94GHz up-conversion mixer, which is utilizes the same technologies in the second section; moreover, it has Folded Cascode for conversion gain enhancement and power consumption reduction. The measured result indicates that the proposed up-conversion mixer with Folded Cascode is promising for 94GHz RFIC applications. 　　In the Fourth section, a low-power CMOS down-conversion mixer with high CG and low Noise for 94 GHz Weather radar is reported. The mixer comprises an enhanced double-balanced Gilbert cell with Differential Cascode and PMOS LC-Oscillator-Based Gm-Stage Load for power consumption reduction, better linearity and conversion gain (CG) enhancement, a Marchand balun for converting the single RF and LO input signal to differential signal. The simulated result indicates that the proposed down-conversion mixer is promising for W-band RFIC applications.

APA, Harvard, Vancouver, ISO, and other styles

48

Si-HongHuang and 黃思弘. "Research on CMOS Millimeter-Wave Low-Power Down-Conversion Mixer and Up-/Down-Conversion Resistive Ring Mixer for Single-Mixer RF Transceiver." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/35385708259727551094.

Full text

Abstract:

碩士
國立成功大學
電腦與通信工程研究所
103
This thesis presents the research on millimeter-wave (MMW) CMOS low-power down- conversion mixer and up-/down-conversion resistive ring mixer for COMS single mixer RF transceiver, which are implemented by 90-nm GUTM CMOS process. A 60-GHz COMS low-power down-conversion mixer uses weak-inversion bias technique to achieve low-power con- sumption and low LO power excitation, the IF buffer chooses trans-impedance amplifier (TIA) to enhance the total conversion gain. Since the mixer core current is so small, that it can be stolen from TIAs via the feedback resistors. Due to the current of mixer core is very small, the operation of TIAs can maintain normal. For up-/down-conversion resistive ring mixer design, we realized two mixers at different RF bands, which include CMOS ring mixers at 60-GHz and the 80-105 GHz bands. The resistive mixer design principle is using a large gate voltage swing of the transistor to change equivalent channel resistance, and the IF output signal has nonlinear effect. The Agilent ADS and Ansoft three-dimensional (3D) EM simulator HFSS were used for design simulation. The measured performances of the designed MMW CMOS mixer ICs were all performed by using on-wafer measurement. Simulation and measurement results are compared and discussed.

APA, Harvard, Vancouver, ISO, and other styles

49

Lai, Cheng-chieh, and 賴政傑. "The Application of Photonic Frequency Up-conversion and Down-Conversion Technique for Radio-on-Fiber System." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/12272441868990184698.

Full text

Abstract:

碩士
明志科技大學
電機工程研究所
97
This paper presents the up-conversion and down-conversion frequency technology in the optical domain. Generating millimeter wave frequency of traditional electronics has been a challenge so far, but we achieved the millimeter-wave signal needed with the four-wave mixing (FWM) of semiconductor optical amplifier (SOA). The millimeter-wave signal generated by this technology has the property of wider linewidth, simpler architecture, and cheaper price. Millimeter wave is suitable for small-scale transmission, broadcast, and broadband Internet signals in the future; down-technology in the optical domain can reduce the problem of fiber dispersion, eliminate the use of millimeter-wave band mixer, and control the transmission distance effectively. We proposed a millimeter wave techology generated by the four-wave mixing effect by the semiconductor optical amplifier. We used the 5.6 GHz microwave drive signal in this technology, and suppressed the carrier by the external modulators with a two-wavelength phase-related intensity. Because of the four-wave mixing of nonlinear effects in the semiconductor optical amplifier, the desired millimeter-wave signals would be produced finally; the frequency of the signal produced can be as great as 30 GHz. Two methods are proposed regarding to the Down-conversion in this paper: Mach-Zehnder modulator and (EAM), using two different signal sources, 2.4 GHzand 5 GHz,respectively;produced 200 MHz differential low-frequency signal by the first order signals from the use of two-tone modulation.

APA, Harvard, Vancouver, ISO, and other styles

50

Mofokeng, Sefako John. "Investigation of up and down-converting rare earth ions doped ZnTiO3 for photovoltaic applications." Thesis, 2019. http://hdl.handle.net/10500/26702.

Full text

Abstract:

We are living in an age where the demand for energy is growing rapidly. This means that supplies to easily accessible oil and natural gas is unlikely to keep up with the demand as times goes on. The world will have to use energy more efficiently and increase its use of other sources of energy. This study is aiming at developing materials that will improve the power conversion efficiency of photovoltaic cells by using up and down-converting phosphor materials. ZnTiO3-Zn2TiO4 composite and ZnTiO3 phosphors doped with Er3+,Yb3+, Eu3+ and Al3+, which display up and down-converted luminescence were synthesized by a simple high temperature conventional solid state reaction method. The structure, particle morphology, absorption, photoluminescent properties and elemental distribution were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis-NIR absorption spectrometer, photoluminescence (PL) spectroscopy and time of flight secondary ion mass spectroscopy (TOF-SIMS), respectively. ZnTiO3-Zn2TiO4 composite doped with different concentration of Er3+ ions was synthesized via solid state chemical reaction method at 1100 ℃. The X-ray diffraction (XRD) confirmed the crystallization of both the hexagonal ZnTiO3 and cubic spinel Zn2TiO4 phases of the composite. The SEM images of ZnTiO3-Zn2TiO4:Er3+ composite showed that the particle morphology was made up of faceted hexagons. Furthermore, the ZnTiO3-Zn2TiO4:Er3+ phosphors were excited in the near-infrared (NIR) region using a laser diode with a wavelength of 980 nm and displayed both green and red up-conversion emission bands in the visible range at 543, 553, 650 – 670 nm. These emission bands correspond to 2H11/2, 4S1/2→ 4 I15/2 and 4F9/2→ 4 I15/2 transitions of Er3+ ions. However, the interaction mechanisms involved in the upconversion process of ZnTiO3-Zn2TiO4:Er3+ phosphor is discussed with the help of an energylevel schematic diagram and the number of the photons involved in the up-conversion luminescence process were of a double photon mechanism. The decay lifetimes were studied by fitting the luminescence decay curve with a single-component exponential decay. Er3+ and Yb3+ incorporated zinc titanate (ZnTiO3) phosphor powders were synthesized using conventional solid-state reaction method at 800 ℃. A ZnTiO3:Er3+,Yb3+ phosphor that resembled an ecandrewsite single phase with space group R-3 (148) was obtained, as proven by X-ray diffraction (XRD). The SEM image showed a surface morphology composed of agglomerated irregular shaped particles. The energy band gap of ZnTiO3 was engineered by incorporating different concentration of the dopant ions. After irradiating ZnTiO3:Er3+with a 980 nm laser beam, the phosphor up-converted the photon energy to display green and red emissions in the visible range that were positioned at 527, 545 and 665 nm. Enhancement of the luminescence intensity of ZnTiO3:Er3+ phosphor was achieved by variation of Er3+ concentration. Co-doping with Yb3+ ions proved to be effective in enhancing the luminescence intensity of the optimized Er3+ ion emission and new emission bands at 410 and 480 nm, through an energy transfer mechanism were observed. The enhancement of the lifetime of the up-conversion luminescence was also achieved by co-doping ZnTiO3:Er3+ phosphor with Yb3+ ion. The energy transfer mechanisms involved in Er3+ - Yb3+ co-doped ZnTiO3 phosphor was illustrated and discussed in detail. The ZnTiO3:Er3+, Yb3+ thin films were successfully deposited by pulsed laser deposition (PLD) by varying the silicon (100) substrate temperature. The distribution of the ions in the films was investigated and the TOF-SIMS showed that the ions were homogeneously distributed throughout the ZnTiO3 host lattice which indicated a successful incorporation of the Er3+ and Yb3+ ions. The optical response of the phosphors revealed that the reflectance percentages of the ZnTiO3:Er3+, Yb3+ vary with the silicon substrate temperature due to the differences in the thickness and morphological roughness of the thin films. The ZnTiO3:Er3+, Yb3+ thin films also exhibited up-conversion emission from Er3+ electronic transitions, with violet, blue, green and red emission lines at 410, 480, 525, 545 and 660 nm from 2H9/2 → 4 I15/2, 4F7/2 → 4 I15/2, 2H11/2 → 4 I15/2, 4S3/2 → 4 I15/2 and 4F9/2 → 4 I15/2 transitions, respectively. These up-conversion emissions were enhanced by increasing the silicon substrate temperature during the deposition. ZnTiO3 host co-doped with Eu3+ and Al3+ was synthesized by solid state reaction to convert the UV photons to visible photons. Charge compensation effects of Al3+ incorporated ZnTiO3:Eu3+ as a co-dopant ion was reported in detail. The structural and morphological characterization show that the addition of Eu3+ and Al3+ does not affect the phase formation and the surface morphology of the host. The visible emission intensity of Eu3+ ions for an optimal concentration of 2 mol% under 395 nm excitation, was enhanced by incorporating Al3+. The energy level diagram showing the charge compensation mechanism was proposed for the co-doped system.
College of Engineering, Science and Technology

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'Down and Up conversions'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles