Introduction. Modern methods of stabilizing a frequency of self-oscillations use an improvement of the stability of reactive parameters of the self-oscillators circuit and an increase in the quality factor of an oscillating system. It is also possible to improve the frequency stabilization based on the phenomenon of mutual synchronization of the self-oscillator modes using a multi-loop oscillation system. Previously, a method for reducing a phase noise of an auto-oscillator with synchronization of two modes in a biharmonic auto-oscillator with multiple frequencies was described. The method was developed under the assumption that an active element is inertialess. The idea of the method of synchronizing of the main oscillation with its 2-nd harmonic using an additional loop is based on the consideration that internal fluctuation processes in the active element modulate in-phase all current har-monics. Therefore, it is possible to use this "natural" cross-correlation of noise processes to neutralize their influence.

Aim. Building and analysis of a mathematical model of a biharmonic oscillator in order to analyze the operating modes of such generator and reduction of the phase noise of its output oscillation.

Materials and methods. The mathematical model was developed by the method of slowly changing amplitudes, and the analysis was performed by methods of numerical integration and differentiation.

Results. It was demonstrated that synchronization of two oscillations at multiple frequencies in the active element reduced the phase noise of the main oscillation.

Conclusion. In the paper dynamic modes of a biharmonic Colpitts oscillator operating in the phase synchronization mode of two waves were analyzed. It was shown that with an increase in an inertia of the active element, the synchronous mode was preserved. Shortened differential equations of the system for slowly changing amplitudes and phases of oscillatory modes were obtained. The study of nonlinear dynamics and of stationary synchronous mode of the system was carried out by the method of phase space in coordinates of "mode amplitude – phase difference". The conducted field experiment allows one to conclude that it is possible to reduce the phase noise in a stationary synchronous biharmonic mode. It can be used in the frequency stabilization task.

Introduction. A resonant slotted waveguide antenna allows providing broadside radiation, which coincides with the normal line to the longitudinal axis of the array. Such an antenna can be well-matched in a very narrow frequency band. Even a slight deviation from the operating frequency leads to a significant distortion of the radiation pattern. In this regard, the distortion of the radiation pattern is becoming an urgent task.

Aim. The main objective of this work is to preserve the radiation pattern of the resonant slotted waveguide antenna with longitudinal slots in the broad face in operating bandwidth.

Materials and methods. Subarraying was performed to preserve the undistorted radiation pattern of the slotted waveguide antenna array in the operating bandwidth. The antenna model and its directional properties were analyzed in CAD Ansys HFSS using Visual Basic Scripting Edition macros.

Results. Three slotted waveguide antennas’ models, consisting of two, four, and eight subarrays were developed in Ansys HFSS CAD. The effect of subarraying on the radiation pattern at the center frequency and the upper and the lower operating frequencies is studied. It is shown that the growing number of subarrays leads to a more stable radiation pattern in the operating bandwidth.

Conclusions. Full-wave electromagnetic simulations have shown that subarraying of a resonant slotted waveguide antenna is a useful measure that preserves undistorted radiation pattern in the operating frequency band. Usage of Visual Basic Scripting Edition macros allows to minimizing the time spent creating a model in CAD Ansys HFSS.

Introduction. A wireless communication system based on a relay network where a link between a source and a destination is carried out through a network of relay nodes have been considered. Relay networks operate according with amplifier-and-forward protocol where each relay node performs reception, amplifying, phase shifting and retranslation of a signal to the destination node. As a result a task of powers and phases optimization in the relay nodes (i.e. the complex weighted coefficients optimization) becomes actual. Complex weighted coefficients of the relay nodes are optimized in such a way as to ensure the maximum signal to noise ratio at the receiver while limiting a power emitted by the relay nodes. In the paper, optimization of spatial processing with different a priori channel state information (i.e. instantaneous channel state information and the second order statistics) have been considered.

Aim. Spectral efficiency analysis of a relay network in a multipath channel where the relay network was optimized by using of two types a priori information: an instantaneous channel state information and second order statistics.

Materials and methods. Optimization of spatial signal processing in the relay network was based on methods of statistical theory and optimization using analytics of linear algebra and methods of mathematical programming. Performances of the relay network were analyzed using Monte Carlo simulation. The simulation was performed in MATLAB program environment using CVX toolbox for solving convex optimization task.

Results. In the paper optimal solutions for spatial signal processing in the relay network were presented. The solutions were based on maximum of signal to noise ratio while limiting total relay power and individual power of relay nodes. Monte Carlo simulation was performed to provide performances of the relay network for different types of channel state information and channel parameters. Mean capacities versus mean source power, a budget of relay nodes power and a ratio between random and deterministic power of the channel were gained for the Rayleigh model of multipath channel.

Conclusions. The results have a practical application. Thus, the use of the second order statistics is possible in relay networks when direct visibility with a low level of background from local objects is provided. In urban areas, where shading and multipath propagation of signals occur, it is possible to use only an approach based on the knowledge of channel instantaneous state.

Introduction. PECVD enables fabrication of wide range of advanced materials with various structure such as amorphous, polymorphous, nano-crystalline, nanostructured, microcrystalline etc. and with various electronic properties. The latter can be also changed by different dopingl. PECVD silicon materials are commercially employed in multi-layered PV structures (including ones on flexible substrates). Combining these materials with crystalline silicon active substrate resulted in significant improvement of PCE in hetero junction technology PV structures. Existence of new organic semiconductors (OS) together with understanding of physical properties resulted in fast development of OC PV devices

Aim. To consider both PECVD and OS materials and to present description of fabrication, structure and electronic properties for device application.

Materials and methods. Devices based on non-crystalline materials, devices based on OS, hybrid devices. PECVD and Spin coating technique was used to deposit materials with tunable properties enabling device engineering possibilities.

Results. PECVD and OS materials were analyzed. These materials have different levels of characterization (data volume, interpretation of the results etc.) and of understanding of physics determining device performance. Some examples of these materials in PV including structures with crystalline silicon were considered.

Conclusion. Important advantage of both PECVD and OS materials is that fabrication methods are compatible and allow fabrication of great variety of hybrid device structures on crystalline semiconductors. Advantages of such devices are difficult to predict because of lack of data in scientific literature. However a new area in material science and related devices for further exploring and exploiting has appeared.

Introduction. At present, an optical transmission of a microwave signal is of great scientific and practical interest. Moreover, this transmission line can also be used to create microwave photonic devices. Microwave signal losses decrease with an increase of laser power. Commercial photodetectors withstand radiation with a power of several tens of milliwatts. Using a photodetector with a high photocurrent can improve characteristics of photonic transmission lines; in particular, it can reduce microwave signal losses.

Aim. Investigation of characteristics of a photodetector with a high photocurrent when transmitting microwave radio signals through optical fiber. Research of microwave signal losses as a function of optical power.

Materials and Methods. Experimental studies were carried out on created experimental schemes for studying the characteristics of the photodetector with modulated and unmodulated optical radiation. Theoretical studies were carried out by mathematical modeling of optical path transfer characteristics from the laser power at various powers of an input microwave signal.

Results. The dependencies of photocurrent and photovoltage of the photodetector versus laser power were obtained. The experimental amplitude-frequency characteristics of the photonic transmission line were measured at different optical powers. A frequency dependence of the photodetector sensitivity in the range of 0...12 GHz was obtained. Modeling of amplitude-frequency characteristics of the optical path in the range of 0...12 GHz was performed. An approximate frequency dependence of the photodetector sensitivity was obtained.

Conclusion. Due to the use of a photodetector with a high photocurrent value and with increasing laser power, microwave losses were reduced to about 10 dB. It was shown that for improving the transmission characteristics of an optical transmission line, it is necessary to use a broadband electro-optical modulator.

Introduction. Modern wave buoys due to their design make it possible to determine a directional wave spectrum using five coefficients of the Fourier series. However, up to nine members of the series can be determined from wave surface measurements. To determine the missing four members, information about wave curvature is necessary. It cannot be obtained by direct measurements with wave buoys of a conventional shape - a sphere, a cylinder or a saucer. The lack of information about the curvature of waves when determining the directional spectrum leads to its low resolution and to the presence of negative regions.

Aim. To develop a method for determining the curvature of waves from measurements with conventional shape wave buoys.

Materials and methods. Theoretical substantiations of the proposed method were presented, as well as mathematical modeling of irregular wave processes in a wide range of wave intensities and an experimental study of three wave spectra with a threefold repetition of each of them.

Results. Numerical simulations demonstrated a good agreement between the calculated curvature and the set values. The variance deviation was less than 1%. The experimental study demonstrated a greater deviation in variance - up to 9%, which can be explained by the influence of an instrumental error and by an unaccounted influence of reflected waves.

Conclusion. On the basis of the study, the method for determining the curvature of waves by numerical differentiation of wave slope measurements using information on a wave propagation speed has been developed. The method was supplemented by correcting amplitude values according to the criterion of matching the spectral characteristics of wave processes. Additional studies of the developed method are required to determine the influence of wave factors such as wave steepness, spectrum width, random multidirectional waves, etc. on the calculated curvature and on the directional spectrum.

Introduction. Optical coherence tomography (ОКТ) is a non-invasive instrument for studying optically heterogeneous media with micron precision, including skin cancer. However, ОКТ tissue images are very noisy. It complicates both expert and automated image evaluations. There are almost no systematic comparisons of noise reduction algorithms in the literature.

Objective. To obtain comparative test results on a set of ОКТ images of skin melanoma using various noise reduction algorithms.

Materials and methods. A number of noise reduction algorithms were described, which include two relatively simple classical algorithms: Wiener and median, and more complex ones: a Complex Diffusion Filter (CDF), an Interval type-II Fuzzy Anisotropic Diffusion Filter (ITTFADF) and an Empirical Mode Decomposition (EMD) filter, previously proposed by the author for visualizing of mesh implants. Quantitative metrics were determined: a Signal-to-Noise Ratio (SNR) metrics, an Effective Number of Looks (ENL) metrics, Structural Similarity Index Metrics (SSIM) and a correlation coefficient χ, reflecting two main principles of improving image quality: to reduce noise and to save the borders of tissue layers and heterogeneities.

Results. The results of a comparative testing on a set of images, consisting of 10 melanomas (to which various noise reduction algorithms were applied) were obtained.

Conclusion. The study did not reveal the best algorithm for all four metrics. According to the SNR metric, the EMD and the CDF filters perform the best depending on the type of area. At the same time, the EMD filter is either the best in all respects, or is inferior in SNR in heterogeneous areas and takes the second place in ENL. Taking as the correct hypothesis that the border preservation is more important before an integral noise estimate, it is possible to make an unambiguous conclusion about the need to use the EMD filter. As an alternative to the EMD filter, Wiener filter (which wins on the border preservation metrics) should be used or the ITTFADF, which ranked third in all used metrics.

Introduction. Violations of the microcirculatory-tissue systems of the human body play a key role in the pathogenesis of various diseases complications. However, there are a few unresolved methodological and instrumental problems with non-invasive monitoring of microcirculatory function and metabolic disorders associated with insufficient accuracy, reproducibility and informativeness of diagnostic results. The proposed approach of multiparametric optical diagnostics, when optical non-invasive technologies are used in combination, can be a promising tool that improves the sensitivity and accuracy of detecting microcirculatory and metabolic disorders at an early stage, which is important for the diagnosis and treatment of diseases of various profiles.

Aim. Development of a method and device for assessing the functional state of microcirculatory-tissue systems of the human body based on multiparametric optical diagnostics and assessment of its diagnostic potential in clinical practice.

Materials and methods. In the paper, theories of radiation transfer, applied mathematical statistics and clinical research methods are used. The statistical indicators of the developed diagnostic methods are given, the principle of the device construction is described.

Results. An original method and principle for constructing a device for assessing the functional state of microcirculatory-tissue systems of the human body based on multiparametric optical diagnostics has been developed.

Conclusion. The present work presents a method and device for assessing the functional state of microcirculatory-tissue systems of the human body based on this approach. The basic principles of each of the diagnostic channels (laser Doppler flowmetry, fluorescence spectroscopy and diffuse reflectance spectroscopy) are described. Examples of the clinical application of the described device in various fields of medicine (endocrinology, rheumatology, minimally invasive surgery) are presented. The method proposed in this article and principle for constructing a device with the possibility of its technical adaptation by developing additional probes for specific tasks of biomedical research makes optical non-invasive diagnostics affordable and increases its information content.