Design principles of frequency multipliers for digital frequency-measuring instruments

Conclusions It will be seen from this review of various methods for multiplying frequency that, in addition to the generally known multipliers which use functional conversion of the input signal, there can also be designed multiplying devices, based on level quantization, on shifting of signals with respect to their phase or time, and on balancing.It should be noted that the output signal of the first group of multipliers has the same shape as their output signal. The frequency multiplication process in devices of the second and third groups can be accompanied by a transformation of the signal shape.The application of a given type of multiplier in digital frequency-measuring instruments depends basically on the maximum permissible variation of the transducer's output frequency. Thus, in transducers with a small deviation it is, probably, advisable to use frequency multipliers of the first group. It is obvious that balancing multipliers whose comparison element consists of a reversible counter are predominantly suitable for transducers with a large frequency deviation.The problem of obtaining the required frequency multiplier characteristics, consisting of their speed of operation and precision, requires a detailed investigation.Translated from Izmeritel'naya Tekhnika, No. 1, pp. 52–55, January, 1967.     

Material modelling for accurate stresses in involute composites

Material modelling requirements for accurate stress fields in a nonuniform anisotropic composite are established for all six components of stress at a material point. The gauge section from a conical involute structure tested in an earlier investigation provides the case data for this modelling study. Stress results from a tricubic Hermite finite element model are shown to be in agreement with another computational analysis by Pagano using a Reissner energy formulation. Comparisons are made in the interior and at a corner with high interlaminar stress gradients. The small, but critical, matrix dominated stress components are shown to be very sensitive to modelling material orientation changes within each finite element. Uniform stress accuracy was obtained using distribution functions for material orientation of the same degree as the displacement functions, an isoparametric material geometry model.     

Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)

This study proposes a comparison between three identification methods of the mechanical properties of a shape memory polymer (Veriflex®): quasi-static tensile tests, tensile dynamic mechanical analysis (DMA) and modal tests. The Young’s modulus and the Poisson’s ratio are determined at ambient temperature using the first technique. The DMA is used to determine the evolution of the viscoelastic properties versus the temperature and the frequency under harmonic loading. The modal analysis is used to identify the viscoelastic properties of the material at higher frequencies. The purpose of this study is to check the validity of the time–temperature equivalence for the Veriflex® obtained from the DMA measurements. It is shown that the viscoelastic properties predicted through the master curve are consistent with the measurements collected using quasi-static and modal test. The aging effect on SMP properties is also quantified.     

Analysis tools for the accurate evaluation of a small frequency standard

The short, optically pumped cesium beam tube developed at Laboratoire de l'Horloge Atomique has been carefully evaluated. For that purpose, we have developed a digital servo system that controls three parameters: the frequency of the ultra stable oscillator (USO), the microwave power of the signal experienced by the cesium atoms, and the static magnetic field applied to the atoms. The frequency standard shows a very satisfactory level of short- and medium-term frequency stabilities. A relative frequency offset, measured to be 4.10(-12 ), results mainly from the residual phase difference between the oscillatory fields in the two interaction regions, which is due to imperfection in cavity symmetry. We present two different means of analyzing the causes of this spurious frequency offset using theoretical and experimental considerations. First, a numerical simulation of the beam tube response is performed as a function of the microwave field amplitude for different values of the residual phase difference DeltaPhi. Results include the cavity-pulling effect. Compared with the measured frequency offset, the numerical simulation leads to a second-order Doppler shift of -3.3 mHz and a residual phase difference, DeltaPhi, between the fields interacting with the atoms in the second and first regions of the Ramsey cavity, amounting to +150 murad. Second, an experimental method of measurement of DeltaPhi without beam reversal is implemented. The latter yields DeltaPhi=155+/-17 murad. Finally, the clock accuracy is determined. It is equal to +/-14.10(-13).     

变温环境下精密调谐机构频率补偿原理的有限元分析

对精密调谐机构进行了精细的有限元建模,采用等效建模方法对机构的连接弹簧、调谐螺钉、配合关系进行了处理,就精密调谐机构在0～+65 ℃的变温环境下进行了计算,分析了调谐机构在高温环境下的频率补偿原理。实际的测试结果表明了计算分析的正确性。     

Kinetic modelling of quantum effects in laser-beam interaction

We present the results of kinetic modelling of quantum effects in laser-beam interaction. In the developed numerical model, electron-positron pair production by hard photons, hard photon emission and the electromagnetic fields generated by the created charged particles are taken into account. Interaction of a relativistic electron beam with a strong laser pulse is analyzed. It is shown that the quantum effects can be important even for moderately intense laser pulses when the number of emitted photons by single electron is not large. Electron-positron pair plasma production in extremely intense laser field via development of electromagnetic cascades is also studied. The simulation results confirm the prediction of strong laser field absorption in the self-generated electron-positron plasma. It is shown that the self-generated electron-positron plasma can be an efficient source of energetic gamma-quanta.     

Use of GPS ASHTECH Z12T receivers for accurate time and frequency comparisons

The GPS phase measurements described in this paper were obtained using two similar multichannel GPS ASHTECH Z12T receivers belonging to the Bureau International des Poids et Mesures, BIPM, and the Laboratoire Primaire du Temps et des Frequences, BNM-LPTF. These receivers are based on the conventional geodetic ASHTECH Z12 unit, which has been modified to meet the stability requirements of time and frequency comparisons. Comparison of the two receivers operated side by side in different antenna configurations shows typical short-term noise of 1.1 to 3.5 ps. Longer term variations indicate a temperature sensitivity in the equipment, which limits the performance of the GPS phase method. One of the receivers was successfully operated using a temperature-stabilized antenna TSA from 3S Navigation, and the ASHTECH antenna, which feeds the second receiver, was placed in a home-built oven maintained at a constant temperature. These precautions made it possible to reduce a number of systematic effects. A separate study of frequency comparison was carried out between two hydrogen-masers located at the BNM-LPTF (Paris, France) and the PTB (Braunschweig, Germany) using receivers similar to ASHTECH Z12T receivers. The relative frequency stability obtained was about 3.3x10(-15) for an average time of 15 000 s, an interesting result comparable with the outstanding performance of new ultrastable frequency standards.     

Minimisation of inspection and rework cost in a BLU factory considering imperfect inspection

We address an optimisation problem for minimising the cost of both the inspection and the rework incurred throughout an interconnected inspection–rework system. In order to formulate the relevant cost objective function, assuming imperfect inspection, we make a time-based flow analysis between nodes (or shops), and derive the limiting sizes of flows between nodes and the limiting fraction defective by solving a set of nonlinear balance equations. We provide an enumeration method for determining the optimal frequency of inspection cycles at the end of assembly lines which minimises the relevant cost of inspection and rework.     

Inherent homogenous media dispersion compensation in frequency domain optical coherence tomography by accurate k-sampling

Depth dependent broadening of the axial point spread function due to dispersion in the imaged media, and algorithms for postprocess correction, have been previously described for both time domain and frequency domain optical coherence tomography. We show that homogeneous media dispersion artifacts disappear when frequency domain samples are acquired with uniform spacing in circular wavenumber, as opposed to uniform sampling in optical frequency. We further explicate the source of this point spread broadening and simulate its magnitude in aqueous media. We experimentally demonstrate media dispersion compensation in high dispersion glass by choosing sample frequencies at equal intervals of media index of refraction divided by vacuum wavelength, and we recover unbroadened reflections without an additional postprocessing step.     

Fast and accurate modelling of frequencyselective surfaces using a new modular neural network configuration of multilayer perceptrons

The authors consider the analysis and modelling of the scattering from frequency-selective surfaces (FSSs), in the 6-14-GHz band, as a function of its periodic array geometry of thin dipole elements on an anisotropic layer. The accurate full-wave electromagnetic (EM) analysis of each FSS was carried out using the method of moments. From the available EM data, the artificial neural network (ANN) models can be developed. The modelling problem was solved by using a new modular configuration of multilayer perceptrons (MLPs), which is an implementation of the proposal modified from the previous knowledge method of neuromodelling information. Each MLP in the modular configuration was trained separately from the others through the resilient backpropagation algorithm. Within the region of interest studied, the ANN model developed is able to estimate the resonance frequencies and the bandwidths of the FSS band-stop filters, with high accuracy and low computational cost. To verify the advantageous properties of the modular MLP/MLP model, a neural model using a simple MLP was developed in order to analyse the same learning task. A comparative study was done between these models in terms of training the convergence, the accuracy and the computational cost.     

Measurement and modelling of residual stress effects on cracks

Defects that form by mechanisms such as fatigue and stress corrosion cracking are influenced both by external loads on engineering structures and internal, residual stresses that are generated during the manufacture and operation. This paper describes a programme of experimental and analytical work on a high‐strength, low‐toughness aluminium alloy (AL2024‐T351) to assess the influence of residual stress on crack opening displacement (COD) and crack‐driving force (CDF) for a range of fatigue crack lengths in compact tension (CT) specimens containing a mechanically induced residual stress field. Comparison of experimentally measured and numerically predicted CODs, at the mid‐plane and surface of CT specimens, show generally good agreement for cracks introduced into the finite‐element model in a progressive, element‐by‐element manner. Cracks introduced in a simultaneous manner give larger than observed CODs. The CDFs for the progressively introduced crack are always smaller than for simultaneously introduced. These results have implications for the assessment of initiation for slowly growing cracks.     

Shape memory effects in melt-spun Fe-Mn-Si alloys

Humboldt-Fellow, on leave from Centro Atomico Bariloche, 8400 Bariloche, and CRUB, Universidad Nacional del Comahue, Argentina.     

Towards accurate measurement of the frequency dependence of capacitance and resistance standards up to 10 MHz

Progress toward an understanding of the frequency dependence of capacitance and resistance standards at frequencies up to 10 MHz is presented. A qualitative comparison is also made for capacitance and dissipation factor measurements between the National Physical Laboratory (NPL) high-frequency four terminal-pair (4TP) bridge and a commercial impedance analyzer for the first time. A set of novel high-frequency calculable coaxial resistance standards, of nominal 100 /spl Omega/ and 1 k/spl Omega/ values, have been developed and their calculated frequency dependence up to 1 MHz is given.     

Constitutive modelling and numerical simulation of multivariant phase transformation in superelastic shape‐memory alloys

This work concerns the micromechanical constitutive modelling, algorithmic implementation and numerical simulation of polycrystalline superelastic alloys under multiaxial loading. The model is formulated in finite deformations and incorporates the effect of texture. The numerical implementation is based on the constrained minimization of the Helmholtz free energy with dissipation. Simulations are conducted for thin tubes of Nitinol under tension–torsion, as well as for a simplified model of a biomedical stent. Copyright © 2004 John Wiley & Sons, Ltd.     

Low frequency damping properties of a NiMnTi shape memory alloy

The present study investigated the low frequency damping properties of a NiMnTi shape memory alloy (SMA) for the first time. The NiMnTi SMA had a high β?θ′ internal friction peak at approximately 125 °C and a low relaxation peak at approximately − 45 °C in the dynamic mechanical analysis cooling tan δ curve. The relaxation peak possessed an activation energy of 0.64 ± 0.03 eV and its damping capacity gradually decreased with the increase of thermal cycling. The NiMnTi SMA also had a good inherent internal friction with tan δ = 0.009 at approximately 140 °C and is a promising high damping alloy for high temperature applications.     

Micromorphic effects in a modelling of periodic multilayered elastic composites

The paper deals with the problems of modelling of multilayered periodic composites. The proposed models take into account certain micromorphic effects resulting from the periodic structure of the body. The governing equations describing motions and stresses of the nonhomogeneous elastic materials with microperiodic structure in the nonlinear as well as linear cases have been derived. As an example the time-harmonic vibrations of laminated medium consisting of alternating layers of two homogeneous, isotropic, linear-elastic materials have been discussed.