Vol 62, No 3 (2017)

The paper presents the results of studies of the possible influence of electromagnetic radiation from wireless communication systems on a man in natural conditions. The studies were performed by the method of auxiliary sources (MAS). On the basis of computer simulation, the field distributions in rooms containing a model of a man and a radiator are investigated. The dependence of the radiation pattern on the position of the model and the radiator with allowance for the open part in the form of a window is studied. As a rule, the walls of a room have a certain transparency, depending on their design and the incident frequency. The simulation by the MAS method shows how this transparency can be regulated. Due to the resonance phenomena, the amplitude of the resonant field at certain frequencies and the transparency of the walls become extremely large, which is the reason for large values of absorbed electromagnetic energy and its undesirable effect on a man.

On the basis of a phenomenological model of the Earth surface in the form of a set of statistically independent elementary scatterers, signals scattered by an extended random surface in spatially multichannel MIMO systems on moving carriers have been described. Generalizations of the clutter distribution function and the Westerfield formula for calculation of the clutter power in the considered case of linear motion of transmitting and receiving system elements for signals with large time–frequency product have been obtained.

With the use of a linear approximation, the quantizer characteristics are defined by the stability conditions of the discrete models of the first- and second-order sigma‒delta analog-to-digital converters. The processes occurring in the nonlinear converter models are discussed at an arbitrary amount of digit positions in the representation of numbers under periodic external actions. An investigation method whereby stationary oscillations are represented as an invariant set of nonlinear discrete point mappings is employed. The spectral composition of the converter response and nonlinear signal distortions with an arbitrary period are calculated using the classical and modified discrete Fourier transformations. The dependences of the nonlinear- distortion coefficient of harmonic signals in the first- and second-order converters on the oversampling coefficient, the approximation method, the amount of digits in the representation of numbers, and the time constant of an integrating filter are presented.

The problem of refinement of the quality of filtering of noisy audio signals with the help of the methods based on a discrete wavelet transform with real bases and a dual-tree (complex) wavelet transform using analytical wavelets as basis functions is considered. Test examples and processing of experimental data have shown that, in the case of the optimum selection of the threshold level, the approach using the dual-tree wavelet transform ensures the minimum signal reconstruction error after correction of wavelet coefficients.

The problem of interval estimation of the time delay of the coupling between oscillatory systems from observed time series is considered. It is shown that the known asymptotic estimates based on the empirical model in the form of a system of first-order phase oscillators and the maximum likelihood formalism can lead to false inferences of the value of the time delay in two typical situations: 1) nonlinear low-dimensional systems whose phases are well-determined but, as a result of significant amplitude fluctuations, the phase approximation is insufficient for describing the dynamics and 2) systems whose phases are defined not quite well because of too large amplitude fluctuations. A method for empirical diagnostics of problematic situations and its modification (coarse estimation) providing a low probability of false inferences in these situation are proposed. The efficiency of the diagnostic criterion and coarse estimation suggested is demonstrated on reference systems with different dynamic properties (linear stochastic oscillators, van der Pol oscillators, and Ressler and Lorenz chaotic systems).

Isofrequency surfaces and dependences of various (spin, nonresonant, and postresonant) electromagnetic waves that propagate in infinite ferromagnetic space under saturated magnetization are calculated. In a frequency interval of ω_H < ω < ω_⊥, the propagation of spin waves is characterized by open isofrequency dependences and the cutoff angles that depend on the frequency and material parameters. When the conditions are not satisfied, closed dependences are obtained. The properties of the isofrequency dependences that determine specific features of the reflection and diffraction divergence of waves are analyzed. The spin wave that propagates perpendicularly to the uniform magnetic field may exhibit zero angular width of the beam in the plane that is parallel to the magnetic field vector. In the same plane, the nonresonant wave with linear dispersion may have the diffraction divergence that is significantly less than that of the wave in isotropic media. In the presence of a plane interface, the reflection of the wave to the ferromagnetic material may give rise to two reflected beams.

The results of a theoretical study of the clinotron effect in a clinotron model, which takes into account static electron trajectories in the focusing magnetic field and the initial spacing between the lower beam boundary and the surface of the slow-wave structure, are presented. Physical processes of interaction between a thick electron beam with transverse dimensions comparable with the length of the slow wave and the RF field at its oblique incidence onto the structure are considered. It is demonstrated that, as the current is increased from the starting to the working value, a beam tilt is necessary to reduce the nonlinear effects of interaction between the beam and the RF field and prevent the complex self-oscillation dynamics in the clinotron; in the small amplitude mode (start), the beam tilt is related to optimization of the beam supply into the interaction space, which ensures the minimum starting current. In addition, it is shown that, in the presence of the initial spacing between the beam and structure, the reduced clinotron efficiency η/С increases and, at the optimum tilt angle, is no less than the normalized efficiency of classical backward-wave tubes with a thin electron beam.

A resonator method for measuring the dielectric properties of strongly absorbing materials, which enables one to detect small variations in the permittivity, is developed. By this method, the permittivity of fresh (without anticoagulants) venous and capillary blood and its variation in the process of clotting with time is determined. The dependence of the reflection coefficient of the resonator at the resonance frequencies of the permittivity of venous blood in the course of reading of the glycaemic profile is studied, and a certain correlation between the variation in the glucose content in blood and these quantities is found. The dependence of the reflection coefficient of the resonator loaded to the human hand in the process of reading of the glycaemic profile is studied, and it is shown that the correlation between the glucose content in human blood and the reflection coefficient of the resonator is observed only in the initial section of the increase in the glucose content in blood after glucose intake.

Temperature dependences of the central frequency and intrinsic loss of delay line based on surface acoustic waves (SAWs) are measured in a temperature interval of 10–300°C for langasite acoustic line and two-input SAW cavity with the acoustic line made of black lithium niobate that have central frequencies of about 342 and 427 MHz, respectively, at room temperature. The application of such SAW devices in temperature sensors at temperatures of no greater than 300°C with the frequency-domain input signal is considered. A method for monitoring of minor temperature variations at a sensitivity of 0.001°C using SAW cavities with the acoustic line made of black lithium niobate is proposed and experimentally tested.

Methods for measuring the current-voltage characteristics (I–V curves) of photodiodes in a 6 × 576 mercury-cadmium-tellurium (MCT) multirow photodetector designed for operation in the longwave part of the infrared (IR) spectral range are analyzed. The I–V curve is plotted using the resultes of measurements of output signals of a large-scale readout integrated circuit (ROIC) hybridized with a row of IR photodiodes. The method of independent current measurement at each point of the I–V curve is compared to the method of additive current measurements. A method of determining optimum working points of photodiodes by plotting and analyzing the dependence of the differential resistance of photodiode on the bias voltage is proposed. Distributions of photodiode currents for a sample of a 6 × 576-element focal plane array (FPA) based on MCT photodiodes with a p-type conductivity substrate having the cutoff wavelength of λ_0.5 = 10.5 μm are considered.

The influence of parameters of the MOS hydride epitaxy on structural and electrophysical characteristics of InGaAs/InP heterostructures is studied experimentally. The chosen parameters are used to grow device structures and fabricate planar avalanche photodiodes based on them. The results of measuring of their photoelectrical properties suggest that the developed structures are suitable for fabrication of commercial planar avalanche photodiodes.

Modern trends in the technology of Cd_xHg_1–xTe-based photosensitive barrier structures for the middle and far infrared bands, which can operate at near-room temperatures, are analyzed. Main approaches to solving the problem of increasing the photodiode-detector operating temperature have been considered and analyzed.

Experimental results of the investigation and analysis of structural properties of cadmium–zinc–tellurium (CZT) substrates intended for the cadmium–mercury–tellurium (MCT) epitaxy by the methods of X-ray diffraction, selective etching, and infrared (IR) microscopy are considered. A relation between the shape and the full width at half-maximum (FWHM) of the rocking curve with structural defects, which are present in the material, is demonstrated. Precipitates and inclusions of the second phase, which are present in the substrate material in the amount of 10²–10⁴ cm^–2, do not affect values of the FWHM of the rocking curve. Broadening of the rocking curve is caused by either high density of dislocations (>8 × 10⁵) or the cellular property of their distribution. Maps of the FWHM distribution of the rocking curve for determination of structural perfection throughout the entire areas of the samples allowing evaluation of the suitability of wafers for subsequent technological process have been made.

Values of the absorption coefficient of InGaAs structures grown by means of gaseous-phase epitaxy from metaloranic compounds have been studied and calculated. Experimental data have been compared to the theoretical model of the absorption spectrum based on the phenomenon of fundamental absorption and the general theory of direct interband optical transitions. The energy gap width has been graphically calculated from the slope of the experimental absorption characteristic.