AC Technique for Eliminating Phase Ambiguity in Clocking Signals

A method involving: distributing two clock signals over a clock signal distribution system; in each of a plurality local clocking regions located along the signal distribution system, detecting the two clock signals and generating therefrom a local clock signal for that local clocking region, wherein the generated local clock signals for at least some of the plurality of local clocking regions are in a first group all of which are aligned in phase with each other and the generated local clock signals for the remainder of the plurality of local clocking regions are in a second group all of which are aligned in phase with each other, and wherein the phase of the first group is out of phase with the phase of the second group by a predetermined amount; and bringing all of the clock signals for the plurality of local clocking regions into phase alignment so that the phase of the first group is in phase with the phase of the second grou

Preliminary characterization of calcium chemical environment in apatitic and non-apatitic calcium phosphates of biological interest by X-ray absorption spectroscopy

Several reports have mentioned the existence of non-apatitic environments of phosphate and carbonate ions in synthetic and biological poorly crystalline apatites. However there were no direct spectroscopic evidences for the existence of non-apatitic environment of calcium ions. Xray Absorption Spectroscopy, at the K-edge of calcium, allows the discrimination between different calcium phosphates of biological interest despite great spectral similarities. A primary analysis of the spectra reveals the existence, in synthetic poorly crystalline apatites, of variable features related to the maturation stage of the sample and corresponding to the existence of nonapatitic environments of calcium ions. Although these features can also be found in several other calcium phosphate salts, and do not allow a clear identification of the ionic environments of calcium ions, they give a possibility to directly determine the maturity of poorly crystalline apatite from calcium X-ray Absorption Near Edge Structure spectra

A System Approach to Integrated Power Amplifier Analysis and Design

Numerous power amplifier circuits are described in the literature, which seem to follow many distinct design approaches. While circuit implementations may be quite different indeed, the underlining system-level signaling inside these circuits follows similar conditions dictated by the common design objective for high efficiency. This paper presents a general theory for analyzing and designing power amplifiers, providing an insight on the fundamental factors limiting the performance, irrespective of the implementation circuits or technology

Mise en forme d'apatites nanocristallines : céramiques et ciments

Les céramiques et les ciments phosphocalciques sont des biomatériaux de plus en plus utilisés en chirurgie osseuse. Ce mémoire traite d'une part, de la mise en forme de ces céramiques par des procédés à basse température et d'autre part, de la conversion du phosphate amorphe en phosphate apatitique. Il comporte trois parties. La première partie est une description des principales caractéristiques physico-chimiques des phosphates de calcium et de certains produits industriels dérivés tels que les céramiques et les ciments phosphocalciques. La deuxième partie est consacrée à la préparation de céramiques à base d'apatite nanocristallines non-stoechiométriques. Différentes voies sont explorées; notamment la compaction simple et le frittage naturel à différentes températures, et une nouvelle méthode est proposée : la compression uni-axiale à basse température. Cette méthode conduit à des céramiques massives formées d'apatite nanocristalline douées de bonnes propriétés mécaniques en compression. Les modifications chimiques de la surface des nanocristaux peuvent altérer les conditions de frittage et les propriétés des céramiques obtenues. La troisième partie porte sur l'influence de plusieurs facteurs sur la transformation du phosphate amorphe en apatite. Cette réaction est à la base de la prise de ciments phosphocalciques commercialisés par la société ETEX. Parmi les différents facteurs étudiés (rapport solide/solution, préchauffage, présence d'inhibiteurs de croissance cristalline, force ionique de la phase aqueuse) seul le rapport solide/solution ne semble pas avoir d'influence notable. ABSTRACT : Calcium phosphate ceramics and cements are increasingly used in bone surgery. This manuscript reports firstly shaping of nanocrystalline Ca-P using low temperature processes and secondly the setting process of a mineral cement based on amorphous calcium phosphate. After a general presentation of the main physicochemical properties of the calcium phosphates and of current ceramics and cements, different preparations of nanocrystalline apatites in aqueous media are described and trials of ceramisation have been carried out : mainly simple compaction and natural sintering at different temperatures and very low temperature uniaxial compression. This last method was found to preserve the nanocrystalline character of the Ca-P apatite and to produce ceramics with a high compressive strength. Optimal sintering conditions and the properties of the resulting ceramics were shown to be affected by the chemical modifications of the nanocrystal surface. Several factors affecting the setting of amorphous calcium phosphate – based cements commercialized by ETEX Corporation have also been investigated. The conversion of the amorphous phase into apatite has been shown to be responsible for the setting of phosphocalcium cements. Among the different factors studied (solid/solution ratio, preheating, presence of crystal growth inhibitors, ionic strength of the aqueous phase) only the solid/solution ratio seems to have no noteworthy influence

Polyphase Filter With Low-Pass Response

A complex low-pass filter that reduces the influence of component mismatch. The filter includes a first filter section for effecting a first single pole transfer function and a second filter section for effecting a second single pole transfer function, where the first and the second single pole transfer functions collectively define a conjugate pair of poles. In higher order low-pass filters, an optimal cascade order follows a shoestring pattern

Nanocrystalline apatites: From powders to biomaterials

Non-stoichiometric nanocrystalline apatite powders are used to elaborate highly-bioactive biomaterials. Their exceptional surface reactivity arises from a structured but rather unstable hydrated layer involving ions in nonapatitic chemical environments, like in bone mineral. The initial powder characteristics can be tailored through precipitation parameters (pH, temperature, maturation time in solution). The drying of nanocrystalline apatite suspensions at very low temperature (4 °C) leads to ceramic-like materials exhibiting average mechanical properties (compressive strength 54 MPa) and a high porosity which could be exploited to entrap active organic compounds (e.g. growth factors). The consolidation at 150–200 °C of nanocrystalline apatite powders has also been studied using uni-axial pressing and spark plasma sintering (SPS). The results indicate only a limited alteration of the initial nanocrystals, and the bioceramics obtained show mechanical properties close to those reached with sintered stoichiometric HA. The high ion mobility in the hydrated layer of the nanocrystals can lead to “crystal fusion” processes. This capability to favor crystal–crystal interactions at low temperature, while preserving the non-stoichiometry and nanometer dimensions of apatite crystals, opens interesting perspectives for the elaboration of new resorbable and highly-bioactive bioceramics

Physico-chemical and thermochemical studies of the hydrolytic conversion of amorphous tricalcium phosphate into apatite

Abstract The conversion of amorphous tricalcium phosphate with different hydration ratio into apatite in water at 25 °C has been studied by microcalorimetry and several physical-chemical methods. The hydrolytic transformation was dominated by two strong exothermic events. A fast, relatively weak, wetting process and a very slow but strong heat release assigned to a slow internal rehydration and the crystallization of the amorphous phase into an apatite. The exothermic phenomenon related to the rehydration exceeded the crystalline transformation enthalpy. Rehydration occurred before the conversion of the amorphous phase into apatite and determined the advancement of the hydrolytic reaction. The apatitic phases formed evolved slightly with time after their formation. The crystallinity increased whereas the amount of HPO 4 2− ion decreased. These data allow a better understanding of the behavior of biomaterials involving amorphous phases such as hydroxyapatite plasma-sprayed coatings. Graphical abstract The hydrolytic evolution of amorphous tricalcium phosphate shows two main exothermic events corresponding to wetting of the particle surface and to a slow inner rehydration associated with crystallization. These data allowed the determination of the rehydration and crystalline transformation enthalpies