Вимірювальна техніка та метрологія
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Item Аналіз граничних умов працездатності вимірювальних перетворювачів імітансу на базі операційних підсилювачів(Видавництво Львівської політехніки, 2016) Івах, Роман; Хома, Володимир; Хома, Юрій; Питель, Іван; Національний університет “Львівська політехніка”Показано місце та роль активних вимірювальних перетворювачів імітанс-напруга у структурі частотного аналізатора. Проаналізовано фактори, які обмежують працездатність активних вимірювальних перетворювачів. Наведено формалізовані моделі цих перетворювачів та на їх основі встановлено граничні умови, пов’язані з їх стійкістю. Показано место и роль активных измерительных преобразователей иммитанс-напряжение в структуре частотного анализатора. Проанализированы факторы, которые ограничивают работоспособность активных измерительных преобразователей. Приведены формализованные модели этих преобразователей и на их основе установлены предельные условия, связанные с их устойчивостью. Development of specialized portable measurement tools for impedance spectroscopy is quite challenging and up-to-date technical task. Portable impedance analyzers are required to satisfy certain criteria among which the most important are the following: stability of metrological characteristics of measurement channel over a wide frequency range, high dynamics, fast frequency sweep and the probe signal with appropriate step as well as high resolution of impedance/admittance measurement results. Autobalancing circuits are widely used in impedance analyzers design. However, their applications is followed by some problems related to operation stability caused by global feedback loop. The aim of the paper is to study the stability of the active measurements converter based on autobalancing circuits as well as determining their operation limits. The article shows the place and role of active measuring converters immitance-voltage in the structure of the frequency analyzer. Formal models of active converters were used to analyses operational limits related to system stability. The main results of research can be summarized as following: – To determine operating frequency range of the autobalancing circuit can successfully convert impedance to voltage, following rule have been formulated. The upper limit of the frequency range should be taken at least ten times smaller than operational amplifier bandwidth. The lower limit is formally unlimited. – Circuit operational limits related to system stability depends on relation of operational amplifier bandwidth to closedloop knee frequency as well as relation of closed-loop knee frequency to operational circuit time constant (product of reference resistor to input capacitance). – The system is considered to be operating at normal conditions if relation of operational amplifier output resistance to reference resistance is equal or smaller then one. If the ratio excides this threshold, then measurement circuit will lose amplification properties at high frequency..Item Дослідження діелектричних властивостей речовин у діапазоні радіочастот(Видавництво Львівської політехніки, 2016) Горват, Андрій; Молнар, Олександр; Мінькович, Віктор; Ужгородcький національний університетОписано автоматизовану систему, яка дає змогу досліджувати діелектричну проникність у діапазоні частот 50 кГц − 50 МГц генераторно-резонансним методом з одночасним використанням принципів періодичного порівняння та заміщення. Описана автоматизированная система, которая позволяет исследовать диэлектрическую проницаемость на частотах 50 кГц – 50 МГц генераторно-резонансным методом с одновременным использованием принципов периодического сравнения и замещения.At the frequency range up to 100 MHz the real dielectric with permittivity e and conductivity λ simulated by twoterminal element of the electric circuit with complex impedance Z~ or complex conductivity L ~ , in which interaction of electromagnetic fields with the investigated substance is provided. The values of real e ¢ and imaginary e¢ parts of the complex dielectric constant e e e ¢ + ¢ = * is calculated based on the geometric factors and measured values of Z~ and L). In a simplest case two-terminal element represents as a parallel or series connected capacitor Cx and resistor Rx. In proposed measuring method the investigated two-terminal element with capacitance Cx and conductance Gx=1/Rx or the reference capacitor С0 and conductance G0 connected periodically to oscillatory LC circuit of electric high frequency generator by automatic switching unit with a frequency Ω<<ω, were ω – eigen frequency of oscillatory LC circuit. The output of the measuring generator is a high frequency package voltage, which is modulated both on frequency and amplitude at a switching frequency Ω. The difference on amplitudes ΔU and frequencies Δω of packet output voltage can be used from direct determination of Cx and Gx, but in this case the total measurement uncertainty include instability of ΔU and Δω measurement channels. In addition, on dielectrics with significant losses, neglect contribution of conductivity difference G0–Gx to the Dw is unacceptable. From this regard, in the laboratory layout for automatic measurement of Cx and Gx the method of inverse substitution principles was applied. The principle of periodic comparison and inverse substitution in generator-resonance method allowed to exclude the impact of the circuit element parameters volatility and thereby improve the accuracy and automate the process of measurement. The developed automated system can measure the capacity of 0,05 pF or more in the frequency range 50 kHz – 50 MHz with accuracy better than 2%. The sensitivity of the circuit to conductance changes and consequently to dielectric losses depends on the frequency and amplitude of the generator voltage, and the type and value of measuring conductivity. In practically studies relative conducted measurement uncertainty Gx does not exceed 5%.