Signal Formation Process Research Articles

Analysis of formation processes of informative features in eddy current probes with pulsed excitation mode

The modern pulsed eddy current (EC) technique for flaw detection in structure inspections of aircraft and automotive elements, or other responsible constructions is typically carried out in aperiodic mode. At the same time, the unstable characteristic points of the EC signal usually used as informative parameters can restrict the potential of this excitation mode due to influence measurement errors. The article considers an application of the pulsed EC method of NDT based on the oscillatory mode. To obtain the conditions concerned with different modes of EC probe response oscillations, an equivalent scheme of the “testing object – EC probe” system was developed. The conditions represent the signal formation process and allow analyzing it for impedance and differential probes. The obtained mathematical model of the probe signals allowed for the dependence of proposed signal parameters on the characteristics of the testing object to be evaluated due to simulation. According to this, the frequency and attenuation coefficient of natural oscillations are proposed as the informative parameters. Also, the simulation results are used for developing enhanced algorithmic software for determining and analyzing the EC signals. The proposed informative parameters are experimentally investigated using a set of specimens. The obtained experimental and simulation results are correlated and it confirms the possibility of the proposed methodology to enhance the inspection procedures related to the specimen’s parameters measurements as well as the detected defect sizing.

Валідація MCNP-моделі формування струму фонової жили детектора прямого заряду у ВВЕР-1000

The article presents the results of the numerical simulation of the signal formation process of the background wires of self-powered neutron detectors (SPND) under the action of gamma radiation in the VVER-1000 core using MCNP code. The validation of the MCNP model was carried out on the results of experimental determination of the current of the background wires of the SPND, obtained at three different power units with VVER-1000 during the fuel campaign. The article also proposes a new gamma-ray method for determining the thermal power of the VVER-1000 reactor (TPR) based on the signals from the background wires of the SPND. TPR is an important safety parameter of VVER-1000, therefore, increasing the accuracy of determining TPR with the introduction of an additional gamma method for its determination is an urgent task, given the plans to increase the TPR of VVER-1000. The results of the experimental determination of the VVER-1000 TPR by the traditional neutron method based on the SPND signals are presented, and problematic issues regarding the error in determining the TPR by the neutron method are pointed out. The article presents the results of modeling to study the influence of the main factors affecting the change in the proportionality coefficient Kgm between the actual TPR and the TPR determined by the gamma method. To improve the accuracy of determining the TPR by the gamma method, a correction model for Kgm is proposed, which takes into account the effect of nuclear fuel burnup on the change in the signal of the background wires of the SPND. Taking into account that the signal of the background wires of the SPND is inertialess with respect to the change in the neutron power of the reactor, the introduction of the method for determining the TPR by the gamma method is promising for the implementation of an additional alternative channel for generating an emergency protection signal in terms of both power and the period of the reactor.

Formation and informative content of photoplethysmographic signal under the conditions of controllable external compression

The physical nature and quantitative characteristics of the photoplethysmographic (PPG) signal under conditions of external compression of the studied tissue area are theoretically investigated. Dependences of the constant and variable components of the PPG signal on the applied pressure are revealed, which have maxima at certain compression values. The physical model of the PPG signal formation process takes into account the optical and mechanical properties of biological tissues. The model is based on the consideration of the diffuse propagation of light in a two-component scattering medium (blood + surrounding tissues) and employs the Kubelka – Munk two-stream theory; in this case, the amount of blood in the tissues is determined by the conditions of mechanical equilibrium and the laws of tissue deformation.
 It is shown that the developed model correctly describes the observed qualitative features of the PPG signal, and the condition for the best agreement between theoretical and experimental curves allows real-time determination of the peripheral microcirculation quantitative characteristics (mean value and amplitude of the blood pressure fluctuations in precapillaries, erythrocyte concentration, average degree of blood oxygenation, the vessel walls elasticity, etc.). The developed methods for determining the blood circulation parameters can be used as the basis for inexpensive and effective express methods of research and medical diagnostics, which are especially useful in mass surveys or in environmental monitoring.

Advanced spectral reconstruction (ASR) for setup-independent universal Raman spectroscopy models

Raman spectroscopy is a versatile tool for the non-invasive determination of system or material properties. The desired information about a property or state must always be extracted from the acquired spectra. To that end, a variety of techniques ranging from data-driven machine learning methods to physics-based modeling of spectra are applied. However, most of these techniques suffer from setup dependency, i.e. even minor changes in the experimental setup can lead to major prediction errors when applying previously generated models. Consequently, laborious retraining, recalibration, or even complete rebuilding of a method are required.To weaken this constraint and therefore enable model transfer we propose a novel physics-based method for the advanced spectral reconstruction (ASR) of Raman spectra. It is established on the basic understanding and corresponding modeling of the signal formation process covering all relevant steps from excitation to detection. In contrast to common methods, error-prone preprocessing or other signal manipulation steps are omitted. Instead, experimental influences are explicitly incorporated into the reconstruction model. Hence, this model consists of two independent modules: one modeling the setup-independent fundamental Raman signal and one representing the setup influences. The setup model is determined a priori based on a straightforward setup characterization.To demonstrate the model transfer capabilities of the ASR routine we analyzed multiple liquid samples (water, toluene, and ethanol), polymers (polyethylene and polyamide 6), and a protein (lysozyme) with four different setups. For every pure substance, a parametrized fundamental Raman model could be established from high-quality reference spectra and utilized subsequently in conjunction with the respective setup influence model to reconstruct all experimentally acquired spectra very accurately. Furthermore, an ASR mixture model for the composition determination of binary ethanol/toluene mixtures was set up and calibrated. Again, this model (including the calibration factor) was transferred to evaluate spectra from the same mixtures recorded with different Raman sensors. Without any recalibration (just by implementing the respective instrument model) the evaluation yields an average root mean square error of prediction (RMSEP) of 1.19% highlighting the ability of ASR to transfer or share models for spectral quantification among sensors and/or researchers.

SAR Signal Formation and Image Reconstruction of a Moving Sea Target

Maritime application of synthetic aperture radar (SAR) technology for sea-target surveillance and imaging is considered in this study. A SAR scenario, including the kinematics of a SAR satellite and a ship moving on the sea, along with the geometry of the target, are analytically described. A linear frequency modulation (LFM) waveform is applied for the target’s illumination. Based on the target’s geometry, SAR and target kinematics and the LFM waveform, a SAR signal model is synthesized. It is proven that the process of signal formation is a transformation of the three-dimensional (3D) image into a two-dimensional (2D) signal, whereas the target’s 2D imaging is an inverse transformation of the 2D signal into the target’s 2D image. SAR signal components, linear Fourier terms and higher-order phase terms are analytically derived and discussed in detail. Moreover, it is proven that SAR image reconstruction is a motion-compensation procedure, i.e., it removes all phases induced by first- and higher-order motion. Based on the SAR signal analysis, an illustrative iterative image-reconstruction algorithm is derived. The quality of the imaging is evaluated by an entropy cost function. Simulation experiments are carried out to verify the correctness of the theoretical statements in respect of SAR signal formation and image reconstruction.

Optically smooth and optically rough surface in the view of optical 3D sensors based on imaging

An optical 3D sensor is a device intended to measure the geometric shape of objects using optical methods. When designing an optical 3D sensor, it is necessary to consider whether the surface to be measured is smooth or rough. This is because the signal formation process is different for each of the two cases. Moreover, it turns out that the signal formation process is not only influenced by the roughness of the measured surface but also by other parameters of both the measured surface and the optical measuring system. The additional parameter of the measured surface is the surface correlation length. The parameters of the optical measuring system are the wavelength of the light used and the radius of the resolution cell. Based on the knowledge of the parameter values of the surface to be measured and the measuring system, the surface to be measured can be classified as optically smooth or optically rough. We present an analysis to determine whether a given surface measured by a given optical measuring system will behave as an optically smooth or optically rough surface.

Behavior and process of background signal formation of hydrogen, carbon, nitrogen, and oxygen in silicon wafers during depth profiling using dual‐beam TOF‐SIMS

The behavior and mechanism of background signals during depth profiling of atmospheric elements using dual‐beam time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) have been experimentally investigated for silicon wafers. The background signals of atmospheric elements were found to be inversely proportional to the sputtering rate. Most of the background signals are largely attributable to the accumulation of components through adsorption and ion bombardment in the pre‐equilibrium state. On the other hand, the contribution of real‐time adsorption during the instant after the last sputtering in the equilibrium state is negligible under the present experimental conditions. H2O is dominant in the background formation process of hydrogen and oxygen, which is supported by the higher adsorption coefficients. The background levels of carbon and nitrogen are lower than those of hydrogen and oxygen. Furthermore, the background signal of carbon with respect to the sputtering rate shows a different trend than the other elements. This could be attributed to accumulation in the pre‐equilibrium state. These results indicate that the background levels can be lowered close to those of dynamic‐SIMS by using an extremely high sputtering rate in dual‐beam TOF‐SIMS.

Signal Retrieval from Non-Sinusoidal Intensity Modulations in X-ray and Neutron Interferometry Using Piecewise-Defined Polynomial Function.

Grating-based phase-contrast and dark-field imaging systems create intensity modulations that are usually modeled with sinusoidal functions to extract transmission, differential-phase shift, and scatter information. Under certain system-related conditions, the modulations become non-sinusoidal and cause artifacts in conventional processing. To account for that, we introduce a piecewise-defined periodic polynomial function that resembles the physical signal formation process, modeling convolutions of binary periodic functions. Additionally, we extend the model with an iterative expectation-maximization algorithm that can account for imprecise grating positions during phase-stepping. We show that this approach can process a higher variety of simulated and experimentally acquired data, avoiding most artifacts.

RHAPSODIE: Reconstruction of High-contrAst Polarized SOurces and Deconvolution for cIrcumstellar Environments

Context. Polarimetric imaging is one of the most effective techniques for the high-contrast imaging and characterization of circumstellar environments. These environments can be characterized through direct-imaging polarimetry at near-infrared wavelengths. The Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE)/IRDIS instrument, installed on the Very Large Telescope (VLT) in its dual-beam polarimetric imaging mode, offers the capability to acquire polarimetric images at high contrast and high angular resolution. However, dedicated image processing is needed to eliminate the contamination from the stellar light, instrumental polarization effects, and blurring from the instrumental point spread function. Aims. We aim to reconstruct and deconvolve the near-infrared polarization signal from circumstellar environments. Methods. We used observations of these environments obtained with the high-contrast imaging infrared polarimeter SPHERE-IRDIS at the VLT. We developed a new way to extract the polarimetric signal using an inverse approach method that benefits from the additional knowledge of the detected signal formation process. The method includes a weighted data fidelity term and smooth penalization, and it takes the instrumental polarization into account. Results. This method enables us to accurately measure the polarized intensity and angle of linear polarization of circumstellar disks by taking into account the noise statistics and the convolution by the instrumental point spread function. It has the capacity to use incomplete polarimetry cycles, which enhance the sensitivity of the observations. The method improves the overall performances in particular for instances of both low signal-to-noise (S/N) and small polarized flux compared to standard methods. Conclusions. By increasing the sensitivity and including deconvolution, our method will allow for more accurate studies of these disks morphology, especially in the innermost regions. It also will enable more accurate measurements of the angle of linear polarization at low S/N, which would lead to in-depth studies of dust properties. Finally, the method will enable more accurate measurements of the polarized intensity, which is critical for the construction of scattering phase functions.

ANALYTICAL MODEL OF PACKET TRANSMISSION OF SPEECH AND DATA IN HF RADIO CHANNELS IN A COMPLEX SIGNAL-INTERFERENCE ENVIRONMENT

The author has carried out an analytical simulation of the process of signal formation and processing in the HF channel during packet transmission of speech and data in a complex signal-interference environment. The totality of the result obtained can be the basis for the development of complex mathematical models of HF channels.

Signal Origin of Electrochemical Strain Microscopy and Link to Local Chemical Distribution in Solid State Electrolytes.

Electrochemical strain microscopy (ESM) is a distinguished method to characterize Li-ion mobility in energy materials with extremely high spatial resolution. The exact origin of the cantilever deflection when the technique is applied on solid state electrolytes (SSEs) is currently discussed in the literature. Understanding local properties and influences on ion mobility in SSEs is of utmost importance to improve such materials for next generation batteries. Here, the exact signal formation process of ESM when applied on sodium super ionic conductor (NASICON)-type SSE containing Na- and Li-ions is investigated. Changes in the dielectric properties, which are linked to the local chemical composition, are found to be responsible for the observed contrast in the deflection of the cantilever instead of a physical volume change as a result of Vegard´s Law. The cantilever response is strongly reduced in areas of high sodium content which is attributed to a reduction of the tip-sample capacitance in comparison to areas with high lithium content. This is the first time a direct link between electrostatic forces in contact mode and local chemical information is demonstrated on SSEs. The results open up new possibilities in information gain since dielectric properties are sensitive to subtle changes in local chemical composition.

Analysis of Amplitude-Frequency Response of Acousto-Optic Delay Line

A mathematical model of the signal formation process at the output of acousto-optic delay line (AODL) with direct detection has been proposed. This model is used to derive an equation of the AODL transient response. This equation is used to derive a formula for the pulse response characteristic that is utilized to determine the AODL voltage frequency transfer factor. The exponential notation of the frequency transfer factor is discussed. Analytical expression for the amplitude-frequency response of AODL with direct detection is obtained, and its numerical analysis is carried out. The analysis results have been experimentally tested on an appropriate mock-up. The results of experimental investigations confirmed the validity of the proposed mathematical model.

Experimental validation of the ANTS2 code for modelling optical photon transport in monolithic LYSO crystals.

In this work the scintillation energy spectra originating from the background radioactivity from polished monolithic lutetium yttrium oxyorthosilicate coupled to position-sensitive silicon photomultipliers (SiPM) was studied using the open source Monte Carlo simulation package ANTS2. Two crystal sizes, fully and partially covering the photosensor area, three surface crystal wrappings (black, specular or diffuse) and the full signal formation process in the photosensor were considered. The simulation results were validated with experimental data acquired under the same geometric and detector operating conditions. In all cases ANTS2 simulated spectra have very good agreement with experimental results, reproducing the expected shape, with correct onset and end at 88 and 1190keV, respectively, as well as sharp edges at the reference energies of 88, 88+202, 88+307 and 88+202+307keV. The normalized root-mean square error between simulated and measured spectra varied between 4.3% and 10.4%.

Recognition of radar images generated by synthetic aperture radar systems

In the field of radar remote sensing of the Earth, the problem of detecting and / or identifying spatially distributed targets against the background of a homogeneous surface is becoming increasingly important, for example, the tasks of the coast guard, monitoring of unauthorized forest logging, assessing the consequences of natural disasters, and others. This study is devoted to solving the problem of developing the optimal algorithm for making a decision on the class of a spatially distributed target based on data from side-scan radar systems with a synthetic aperture. A detailed description of the signal formation process in the fixed range channel is given, taking into account possible interference factors. Based on the statistical criterion by the method of maximum likelihood, the recognition algorithm is proposed, expressions for the formation of a feature vector are obtained, and the nonparametric decision rule is proposed. The algorithm has been tested on the example of recognizing three classes of spatially distributed targets that differ in size.

Influence of a signal description model on the calculations of the efficiency indicators of optoelectronic systems

The work is aimed at establishing the boundaries of the use of models for describing signals in optoelectronic systems in calculating efficiency. A description of the signal formation process is proposed, taking into account the corpuscular and wave properties when registering signals in a wide range of intensities. A description of the statistical features of the output signals depending on the energy properties of the signal and noise components is proposed. It is shown that when describing the output signals of optoelectronic systems that register signals with different properties, Poisson and Gaussian distributions are used. The invariance of Poisson flows determines the description of an additive mixture of signal and background flows using Poisson flow. The efficiency of optoelectronic systems is calculated by the signal-to-noise ratio criterion based on the corpuscular and wave description of signals. Efficiency calculations have shown the expedience of using this criterion, provided that the statistical properties of signal and background flows are stabilized. It is shown that under the condition of changes in the energy characteristics of signals, from the point of view of the wave and corpuscular models, the statistical characteristics of the signals have different descriptions. The analysis of theoretical methods of signal analysis in optoelectronic systems is carried out, which is aimed at an adequate characteristic of the system operation, depending on the conditions of its operation. Taking into account the method of describing the process of receiving and processing signals will take into account additional statistical characteristics of signals, for example, an increase of the variance of the output signal. The use of adaptive methods for describing signals will make it possible to increase the efficiency of systems when receiving strong signals in a difficult interference environment, as well as when receiving weak signals

Technical Note: Detective quantum efficiency simulation of a-Se imaging detectors using ARTEMIS.

This work studies the detective quantum efficiency (DQE) of a-Se-based solid state x-ray detectors for medical imaging applications using ARTEMIS, a Monte Carlo simulation tool for modeling x-ray photon, electron and charged carrier transport in semiconductors with the presence of applied electric field. ARTEMIS is used to model the signal formation process in a-Se. The simulation model includes x-ray photon and high-energy electron interactions, and detailed electron-hole pair transport with applied detector bias taking into account drift, diffusion, Coulomb interactions, recombination and trapping. For experimental validation, the DQE performance of prototype a-Se detectors is measured following IEC Testing Standard 62220-1-3. Comparison of simulated and experimental DQE results show reasonable agreement for RQA beam qualities. Experimental validation demonstrated within 5% percentage difference between simulation and experimental DQE results for spatial frequency above 0.25 cycles/mm using uniform applied electric field for RQA beam qualities (RQA5, RQA7 and RQA9). Results include two different prototype detectors with thicknesses of 240 μm and 1 mm. ARTEMIS can be used to model the DQE of a-Se detectors as a function of x-ray energy, detector thickness, and spatial frequency. The ARTEMIS model can be used to improve understanding of the physics of x-ray interactions in a-Se and in optimization studies for the development of novel medical imaging applications.

Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications.

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modelling of the energy- and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

Light scattering in a medium with fluctuating gyrotropy: Application to spin-noise spectroscopy

The spin noise signal in the Faraday-rotation-based detection technique can be considered equally correctly either as a manifestation of the spin-flip Raman effect or as a result of light scattering in the medium with fluctuating gyrotropy. In this paper, we present rigorous description of the signal formation process upon heterodyning of the field scattered due to fluctuating gyrotropy. Along with conventional single-beam experimental arrangement, we consider here a more complicated, but more informative, two-beam configuration that implies the use of an auxiliary light beam passing through the same scattering volume and delivering additional scattered field to the detector. We show that the signal in the spin noise spectroscopy arising due to heterodyning of the scattered field is formed only by the scattered field components whose wave vectors coincide with those of the probe beam. Therefore, in principle, the detected signal in spin noise spectroscopy can be increased by increasing overlap of the two fields in the momentum space. We also show that, in the two-beam geometry, contribution of the auxiliary (tilted) beam to the detected signal is represented by Fourier transform of the gyrotropy relief at the difference of two wave vectors. This effect can be used to study spin correlations by means of noise spectroscopy.

RF system calibration for global Q matrix determination

The use of multiple transmission channels (known as Parallel Transmission, or PTx) provides increased control of the MRI signal formation process. This extra flexibility comes at a cost of uncertainty of the power deposited in the patient under examination: the electric fields produced by each transmitter can interfere in such a way to lead to excessively high heating. Although it is not possible to determine local heating, the global Q matrix (which allows the whole-body Specific Absorption Rate (SAR) to be known for any PTx pulse) can be measured in-situ by monitoring the power incident upon and reflected by each transmit element during transmission. Recent observations have shown that measured global Q matrices can be corrupted by losses between the coil array and location of power measurement. In this work we demonstrate that these losses can be accounted for, allowing accurate global Q matrix measurement independent of the location of the power measurement devices.

Simulation of Generation of Peripheral Arterial Blood Pulsation

A mathematical model for the signal formation process of peripheral arterial blood pulsation in vessels of upper limbs is discussed. Formation of peripheral arterial blood pulsations is represented as a composition of forward and reflected pulse waves propagating through the arterial system. The study proposes a system of analogies between the propagation of pulse wave in the human arterial system and propagation of electrical oscillations in lines with distributed parameters. Dependences of pulse wave propagation in the arterial system were obtained by solving the one-dimensional Navier–Stokes equations for several special cases.