Introduction. Intensive use of broadband signals in RF devices for various purposes is associated with the need to develop broadband elements of RF systems. Iterative methods for designing such elements are frequently uninformative and ineffective, while analytical methods give solutions only for simple models. The problem is the small set of classical approximations, which impedes dealing with complex models of elements.
Aim. Development of a wide-band matching technique based on generalized Darlington synthesis using flexible approximating functions (AF) for load models with zeros of transmission at infinity.
Materials and methods. The paper is based on the generalized Darlington synthesis method. To extend the capabilities of the method, approximating functions with increased variation properties are used. In order to use the results in engineering practice, a synthesis algorithm was developed, which includes three stages: formation of the frequency response, control of analyticity of the used functions and limits of matching. The method is analytical and does not use iterative procedures. The mathematical apparatus of the method is based on the analysis of residues in the zeros of transfer function of load resistance.
Results. Flexible approximating functions proved to be an effective tool for designing matching circuits with multiple transfer zeros in infinity. Variative properties of the function facilitate the realization of both smooth and wave frequency characteristics. A combination of both is also possible, ensuring the best properties of both. The proposed approximating functions allow a smooth change in the frequency response, while preserving the normalization characteristic of classical approximations. Application of such functions allowed us to virtually remove the limitations inherent in the classical AF on the minimum values of the load capacitance and more than 30 % of the limiting values of inductance in the above examples.
Conclusion. The developed methodology makes the process of wideband matching physically transparent and can be applied to other classes of loads.

Introduction. Bandpass filters are important components that determine the basic characteristics of transmitting and receiving radio electronic equipment. Such filters implemented on surface acoustic waves (SAW) not only demonstrate excellent electrical parameters, but also meet compactness requirements. The relevant research task of reducing the design time and optimizing the filter’s cost can be solved by either using modern computational software or improving existing modeling tools.
Aim. To describe the current state and main features of approaches to calculating SAW-based bandpass filters using the model of coupled modes and its formalization based on P-matrices. To describe the main principles and approaches on the example of designing a combined-mode resonator filter on leaky SAW and comparing the calculated and experimental data.
Materials and methods. A theoretical study was carried out using the mathematical theory of differential equations presented in a matrix form, as well as the methods of finite element analysis and circuit theory. The results were processed in MatLab and COMSOL.
Results. The current state of the analytical approach to designing SAW-based filters using the model of coupled modes and its formalization based on P-matrices was described. An original design for a resonator filter based on leaky SAW at 49° YX-cut of lithium niobate was proposed. The filter has a relative bandwidth of 5.8 %, an insertion loss of –3.7 dB, and a stop-band rejection of –50 dB. A technique for calculating SAW-based filters was proposed.
Conclusion. The proposed analytical approach to designing SAW-based bandpass filters allows the filter characteristics (e.g., transmission factor) to be reliably predicted at the modeling stage, thereby reducing the number of experimental iterations and increasing the development efficiency.

Introduction. Measuring harmonic distortions of a baseband pulse signal constitutes a problem due to the continuous nature of their spectrum. In order to obtain nonlinear distortions of a signal, a comparison should be conducted either of the object’s responses to two different test signals or those of the real object and its linearized model. However, such an approach does not distinguish between various physical factors that cause nonlinear distortions. This, as a result, complicates the optimization of devices.
Aim. To develop an approach capable of determining the contribution of various sources to the nonlinear distortion of baseband pulse signals.
Materials and methods. The method under consideration involves a synthesis of a nonlinear behavioral model for an object and a comparison of the model’s output signals when linearizing some (or all) of the characteristic functions in this model. This allows distinguishing the contribution of inertialess, capacitive nonlinearity and nonlinearity associated with signal recirculation in feedbacks. An example of a three-stage baseband pulse amplifier (with step functions as test signals) is provided, for which a behavioral model was synthesized in the form of a second-order nonlinear recursive filter.
Results. The aggregate signal of nonlinear distortions obtained using the presented method was found to be similar to that obtained by subtracting the responses to two different test signals. Further, the distortions caused by static amplitude nonlinearity, capacitive reactivity nonlinearity and energy recirculation between different reactive storages were distinguished. The nonlinearity of the amplitude characteristic exhibits its effect at the end of the transient process, the nonlinearity with the capacitive nature – at the beginning of the transient process, and the nonlinearity of energy recirculation – in the middle part of the transient process. It is shown that even parts of the nonlinear distortions at step impact, caused by individual physical nonlinearity factors, exceed the harmonic distortions of a RF-pulse signal.
Conclusion. The considered method is particularly useful when designing wideband devices with feedbacks, since the nonlinearity of energy recirculation takes effect long after the visual end of the transient process.

Introduction. A two-dimensional configuration of the receiving antenna array (AA) is used to measure the angular coordinates of radar targets – azimuth and elevation. A transformation of one-dimensional uniform AA into a flat two-dimensional AA with a fixed number of antenna elements (AEs) and constant aperture size leads to a nonuniform arrangement of AE in the AA rows. As a result, the AA becomes sparse, which negatively affects the quality of the AA three-dimensional antenna pattern (AP). The section method based on the modified parametric Burg algorithm is a promising and relevant method for constructing directional characteristics. This method can be recommended for spatial processing of reflected signals in a passive coherent radar with a two-dimensional sparse receiving AA.
Aim. To analyze the azimuthal and elevation sections of three-dimensional APs obtained using a modified Burg method for spatial processing of reflected signals in a passive coherent radar, the AEs of which are located horizontally and vertically with a step that is a multiple of the half the wavelength λ of the used illumination signal carrier oscillation.
Materials and methods. The construction of directional characteristics was implemented via computer simulation in the MATLAB environment with the effect of uncorrelated additive complex normal noise on the receiving channels in each AE as an interference.
Results. The possibility and conditions for the application of the modified parametric Burg method in the problems of single signal detecting and angular resolution of equal-power signals in a passive coherent radar, which includes a two-dimensional sparse AA, were determined. The obtained Burg method directional characteristics were compared with the directional characteristics obtained using conventional algorithms based on the discrete Fourier transform. The use of the Burg method allowed the AP side lobe level to be reduced to a practically acceptable level of -12 ... -17 dB at a signal to noise ratio 6 dB. In addition, the Rayleigh resolution of signals in the AA was significantly improved.
Conclusion. The presented modified Burg method is suitable for signal processing in two-dimensional sparse AA, subject to restrictions on the AE placing method and the AA aperture size. This allows the Burg method to be recommended for use in passive coherent radars.

Introduction. The article proposes a new principle for designing a low-range tracking radio altimeter based on a phase-locked loop (PLL) for frequency-modulated continuous-wave radar (FMCW) systems.
Aim. To develop a model of a low-range tracking radio altimeter, which uses a PLL to estimate the height, as well as to verify its performance via computer simulation.
Materials and methods. To solve the problem, we develop a mathematical model of the tracking radio altimeter with an estimator that uses the principles of PLL to generate a reference signal.
Results. Computer simulation of a radio altimeter with the PLL circuit to measure the height above a rough surface proves the altitude estimate to be efficient. When operating over a perfectly flat surface, the altimeter provides an efficient altitude estimate for a signal-to-noise ratio greater than 10 dB. When operating over a rough surface under the selected scenario parameters, and the signal-to-noise ratio of 20 dB, the resulting height estimate provides a bias, with its standard deviation growing with increasing the surface roughness. When the standard deviation of the surface roughness is twice the transmission wavelength, the bias and standard deviation of the estimate equal 1 m and 5 m, respectively, under the altimeter height of 150 m. The conducted simulation revealed that the quality of the altimeter performance is subject to abnormal errors, which are caused by deep fading of the received signal due to the signal reflecting from a rough surface.
Conclusion. The altimeter under study can be used for estimating the altitude of aircraft flights. Further research will investigate the effect of various factors on the performance quality of the radio altimeter, its circuit implementation and full-scale tests.

    
Introduction. Real-time calibration is essential for maintaining the performance of modern digital phased antenna array (DPAA) systems. Previous papers have proposed a method of real-time internal calibration for all receiving channels. This method uses a calibration signal (CalSig) of the same frequency spectrum as the received signal, modulated in phase and amplitude by the binary phase-shift keying (BPSK) and on–off keying (OOK) codes, respectively. With the purpose of improving the method, we propose an algorithm for estimating the phase and amplitude parameters of each receiving channel on the basis of continuous phase correlation accumulation of CalSig samples.
Aim. Synthesis of algorithms and procedures for real-time internal calibration of receiving channels in digital phased antenna arrays.
Materials and methods. Calibration algorithms and calibration procedure were analyzed and synthesized using the methods of systems analysis. In addition, the methods of systems engineering and technology, digital processing of radar signals and synthesis of building test models close to actual requirements were applied.
Results. The advantage of the proposed calibration algorithm and calibration procedure consists in using CalSig modulated by the BPSK and OOK codes. The results obtained on a small DPAA system with four receiving channels gave the error of phase and amplitude lower than 0.3° and 0.05 dB, and the error of main beam direction lower than 0.2°. The results of testing the developed DPAA model confirmed the simplicity and high calibration accuracy of the approach under study.
Conclusion. The proposed calibration algorithm and calibration procedure have the advantage over those proposed in previous research in terms of simplicity and resource efficiency. This fact determines the prospects for using the obtained results.

Introduction. An experimental study of the structural and electrophysical properties of multicomponent films of solid solutions of barium titanate-zirconate and barium titanate-stannate on sapphire substrates has been carried out. These materials are an alternative to the more studied barium-strontium titanate for use in microwave technology, due to the relatively high controllability. In this paper, it is shown that when using post-post high-temperature annealing, films with a component composition close to the composition of the sprayed targets are formed on the substrate. Optimal deposition temperatures of thin films of barium titanate-zirconate and barium titanate-stannate have been determined to obtain the best electrophysical parameters.
Aim. Investigation of structural and microwave properties of BaZr_xTi_1-xO₃ (BZT) and BaSn_xTi_1–xO₃ (BSnT) films on dielectric substrates. These ferroelectric materials are promising in terms of losses and nonlinearity, and the formation of planar structures based on these materials on a dielectric substrate allows for a significantly higher level of operating power of the microwave device.
Materials and methods. The crystal structure and phase composition of the obtained films were studied by X-ray diffraction using a DRON-6 diffractometer on the emission spectral line CuKa1 (λ = 1.5406 Å). Capacitance C and Q-factor (Q = 1/tg δ) of capacitors were measured at frequencies of 1 and 3 GHz using a resonator and an HP 8719C vector analyzer.
Results. It is established that high-temperature annealing after film deposition has a significant effect on the crystal structure, phase composition of films and their electrical characteristics. For the first time, a low level of dielectric losses of planar capacitive elements based on titanate-stannate and barium titanate-zirconate films in the frequency range of 1…60 GHz with acceptable controllability has been demonstrated.
Conclusion. The results obtained indicate the prospects of using thin ferroelectric films of BaSn_0.5Ti_0.5O₃ and BaZr_0.5Ti_0.5O₃ solid solutions in microwave devices.

Introduction. Diabetic retinopathy is a complication of diabetes mellitus caused by high blood sugar levels damaging the retina. Diabetic retinopathy leads to changes in ocular blood vessels and the appearance of solid exudates and microaneurysms. When diagnosed and treated in the late stages, this disease can cause blindness. The most common diagnostic method for diabetic retinopathy is based on ocular fundus imaging. However, the background interference and low contrast of such images significantly hinders the timely detection of vascular lesions. Therefore, the development of a method for detecting signs of diabetic retinopathy, particularly in its early stages, presents a relevant research task.
Aim. Development of a method for diagnosing diabetic retinopathy based on an analysis of ocular fundus images using the decision-tree approach.
Materials and methods. Methods based on image segmentation with identifying characteristic features and their binary classification were used. A verified database was used to access the accuracy of the proposed method for detecting diabetic retinopathy.
Results. A method for detecting signs of diabetic retinopathy was developed, which includes the segmentation of vessels, exudates and microaneurysms based on digital processing of ocular vascular images using binary classification. The developed method showed a high level of diagnostic accuracy. Thus, the sensitivity, specificity and accuracy of diabetic retinopathy detection comprised 87.14, 88.50 and 87.81 %, respectively.
Conclusion. The developed method allows diabetic retinopathy to be diagnosed with sufficiently high accuracy. The method can also be used for supporting decision making when managing patients with diabetic retinopathy.