Amplitude and Frequency Control: Stability of Limit Cycles in Phase-Shift and Twin-T Oscillators

We show a technique for external direct current (DC) control of the amplitudes of limit cycles both in the Phase-shift and Twin-T oscillators. We have found that amplitudes of the oscillator output voltage depend on the DC control voltage. By varying the total impedance of each oscillator oscillatory network, frequencies of oscillations are controlled using potentiometers. The main advantage of the proposed circuits is that both the amplitude and frequency of the waveforms generated can be independently controlled. Analytical, numerical, and experimental methods are used to determine the boundaries of the states of the oscillators. Equilibrium points, stable limit cycles, and divergent states are found. Analytical results are compared with the numerical and experimental solutions, and a good agreement is obtained

RELAXATION MAGNÉTIQUE NUCLÉAIRE ET PROPRIÉTÉS MAGNÉTIQUES DU SYSTÈME (Cr1-xMnx)Be2

La susceptibilité massique des alliages du système Cr1-xMnxBe2 est pratiquement indépendante de la température entre 100 °K et 900 °K et varie légèrement entre 100 °K et 4,2 °K. Le produit T1 T (T1 est le temps de relaxation spin réseau) est constant entre 77 °K et 300 °K pour Be9 et Mn55. Les déplacements de Knight sont négatifs pour des concentrations x > 25 % at de Mn. Ces déplacements de Knight négatifs représentent essentiellement la contribution de la polarisation des électrons du mur par interaction d'échange intra-atomique du type (s-p) ou (s-d).Magnetic properties of Cr1-xMnxBe2 system have been investigated. a) The susceptibilities decrease slowly between 4.2 °K and 100 °K and are constant with temperature above 100 °K. b) T1 T of both Be9 and Mn55 is constant between the temperature range of 77 °K to 300 °K. c) The Knight shift of Be9 is small and negative and that of Mn55 is negative when the Mn concentration x is > 25 % at. The negative Knight shift of Be9 and Mn55 is interpreted as due to the inner core polarization induced by the (s-p) and (s-d) exchange interaction. Finaly, susceptibility, (T1 T) and Knight shift are fonction of Mn concentration

Antiphase phenomena in 2d ising square lattice

Monte Carlo simulations of the two dimensional Ising model were carried out to determine the impact of Antiphase Boundaries (APBs) in some thermodynamic functions of the system. We considered several lattice sizes and sometimes both Ferromagnetic (FM) and Antiferromagnetic (AFM) interactions for the same lattice. The analysis of the curves obtained shows that APBs appears only in AFM interaction and their effect is the creation of an energy gapthat prevents the system from reaching the ground state. This happens only in lattices with odd linear size L (L = 9, 15, 31 …). However, these boundaries do not change the critical temperature of the Antiferromagnetic/Paramagnetic (AFM/PARA) phase transition which remains approximately at 2.26 (in unit of J/kB). Nevertheless, they impose on the curve of the specific heat a jittery behaviour as a function of the temperature and with no clear peak as opposed to the case of even L (L = 16 ,20, 48,…) where there is a clear peak and a smooth curve. There is also a fluctuating magnetization around the value zero at low temperature (AFM phase). We show that this energy gap is proportional to 1/L where L is the linear size of the lattice, thus this gap should vanish if L becomes high. A similarity has been established with the role played by APB in some crystal structures and more especially in the binary alloys that are moreover described by the Ising model

Ab initio study of opto electric properties of the molecules pyrimethamine and sulfadoxine

The optical and electrical properties of the molecules pyrimethamine and sulfadoxine in gas phase and in different mediums (water and ethanol) were calculated using density-functional theory at the B3LYP level and the Restricted Hartree–Fock (RHF) level by employing 6-311++G∗∗ basis set. The dipole moment, polarization density, the electric susceptibility, the refractive index, the dielectric constant and the magnitude of the displacement vector of these molecules have been calculated. Small values of polarization density, dielectric constant, magnitude of the displacement vector, high values of electric susceptibility, refractive index and small HOMO–LUMO energy gaps of these molecules show that, these molecules have very good opto electronic applications

Profil épidémiologique, étiologique et clinique des paralysies oculomotrices en milieu hospitalier

Introduction : La paralysie oculomotrice (POM) est une pathologie retrouvée en consultation neurologique et ophtalmologique. C’est le signe de nombreuses maladies pouvant engager le pronostic vital telles que l’accident vasculaire cérébral (AVC), l’hypertension artérielle (HTA) et les tumeurs intracrâniennes. Le but de cette étude était de déterminer le profil épidémiologique, étiologique et clinique des POM à l’Hôpital Laquintinie de Douala.Méthodologie : Une étude transversale retrospective a été menée de janvier 2016 à décembre 2018 dans les services d’ophtalmologie et de neurologie de l’Hôpital Laquintinie de Douala. Sur 15 490 dossiers colligés, 73 cas de POM acquise ont été retenues pour l’étude. Les variables sociodémographiques, cliniques et paracliniques ainsi que les comorbidités ont été analysées.Résultats : la prévalence des POM acquises est estimée à 0,47%. Les paralysies isolées des nerfs oculomoteurs représentent 78,58% et les atteintes multiples 21,42% des patients. La paralysie isolée du nerf III (45,58%) était la plus fréquente, suivie de l’atteinte isolée du nerf VI (32,88%). Les signes fonctionnels sont dominés par la diplopie (45,21%), la BAV (39,73%) et les céphalées (47,95%). La déviation oculaire (93,15%) occupe le premier rang au niveau des signes physiques. L’étiologie retrouvée la plus fréquente était la pathologie vasculaire (28,77%) représentée ici par l’AVC (19,18%), le diabète (5,58%) et l’HTA (4,11%). Aucun facteur n’était associé à la paralysie du nerf III ni à celle du nerf VI.Conclusion : Dans la présente étude, les paralysies oculomotrices acquises isolées sont la forme la plus fréquente de POM acquises (8/10) et la paralysie isolée du nerf III représente l’atteinte la plus commune. La pathologie vasculaire était l’étiologie la plus fréquemment retrouvée. Mots clés : paralysie oculomotrice, diplopie, Etiologie English Title: Epidemiologogical, etiological and clinical of the oculomotor paralysis in hospitals Background: Oculomotor palsy (OMP) is a pathology found in neurological and ophthalmological consultations. It is a sign of many life-threatening diseases such as cerebrovascular accident (stroke), high blood pressure (hypertension) and intracranial tumours. The aim of this study was to determine the epidemiological, etiological and clinical profile of OMP at the Laquintinie Hospital in Douala.Methodology: A cross-sectional descriptive retrospective study was conducted from January 2016 to December 2018 in the Department ofOphthalmology and Neurology of the Laquintinie Hospital of Douala. Out of 15,490 records collected, 73 cases of acquired POM were retained for the study. Sociodemographic, clinical and paraclinical variables and comorbidities were analysed.Results: The prevalence of acquired OMP is estimated at 0.47%. Isolated ONP accounts for 78.58% and concomitant ones for 21.42% . Isolatedpalsy of nerve III (45.58%) was the most common, followed by isolated palsy of nerve VI (32.88%). Functional signs were dominated by diplopia(45.21%), AVB (39.73%) and headache (47.95%). Ocular deviation (93.15%) ranks first in terms of physical signs. Cases whose etiology was notinvestigated were the most represented with 32.88%. The most frequent etiology found was vascular pathology (28.77%) represented here by stroke(19.18%), diabetes (5.58%) and hypertension (4.11%). We did not find any factors significantly associated with isolated paralysis of nerves III and VI.Conclusion: in the present study, isolated acquired oculomotor palsy is the most common form of acquired POM (8/10) and isolated nerve IIIinvolvement is the most common. Vascular pathology was the most common etiology found. Keywords : oculomotor palsy, diplopia, Etiology&nbsp

Outcomes of small incision cataract surgery in patients with uveitis

Objective: to evaluate the functional outcomes of small incision cataract surgery in patients with uveitis. Patients and methods: We retrospectively reviewed all medical records of patients with uveitis who underwent cataract surgery with intraocular implantation at Manna eye clinic Nkongsamba between January 2010 to December 2018.Results: We recruited 26 patients, 15 males (57.7%) and 11 females (42.3%). The mean age was 45 + 17 years. The types of uveitis found in 26 eyes of the 26 patients included: anterior (80.8%), intermediate (7.7%) and posterior (11.5%). The mean visual acuity (VA) was 1.69 + 0.17 logMAR pre-operatively and 0.50 + 0.53 logMAR post-operatively (P = 0.000). In 09 (34.6%) patients visual acuity did not improved 3 months after surgery (Best VA < 0.6). Causes of poor functional rehabilitation included 01 preexisting posterior segment pathology, 04 capsular complications and 04 post-operative complications (persistent inflammation). Conclusion: This study showed that small incision cataract surgery with implantation resulted in a significant improvement in visual acuity in patients with uveitis. Key-words: Small incision cataract surgery, uveitis, outcome. French Title: Résultats de la chirurgie de la cataracte par petite incision chez des patients avec une uvéite Objectif : évaluer les résultats fonctionnels de la chirurgie de la cataracte manuelle par petite incision chez des patients atteints d’uvéite. Patients et méthodes : Nous avons examiné rétrospectivement tous les dossiers médicaux des patients atteints d'uvéite qui ont bénéficié d’une chirurgie de la cataracte avec implantation intraoculaire à la clinique oculaire de Manna à Nkongsamba entre janvier 2010 et décembre 2018. Résultats : Au total, 26 patients, 15 hommes (57,7 %) et 11 femmes (42,3 %) ont été inclus. L'âge moyen était de 45 + 17 ans. Les uvéites retrouvées dans les 26 yeux des participants étaient antérieures (80,8 %), intermédiaires (7,7 %) et postérieures (11,5 %). L'acuité visuelle (AV) moyenne était de 1,69 + 0,17 logMAR en préopératoire et de 0,50 + 0,53 logMAR en post-opératoire (P = 0,000). L’acuité visuelle ne s'était pas améliorée chez 09 (34,6 %) patients 3 mois en post-opératoire (meilleure AV <0,6). Les causes d'une mauvaise récupération fonctionnelle comprenaient : 01 pathologie préexistante du segment postérieur, 04 complications capsulaires et 04 complications postopératoires (inflammation persistante).Conclusion : La chirurgie de la cataracte par petite incision avec implantation induit une amélioration significative de l'acuité visuelle chez les patients atteints d'uvéite. Mots-clés : chirurgie de la cataracte par petite incision, uvéite, résultats fonctionnels