ДОСЛІДЖЕННЯ ДІЕЛЕКТРИЧНИХ ВЛАСТИВОСТЕЙ РЕЧОВИН У ДІАПАЗОНІ РАДІОЧАСТОТ
Journal Title: Вимірювальна техніка та метрологія - Year 2016, Vol 1, Issue 77
Abstract
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 %.
Authors and Affiliations
Андрій Горват, Олександр Молнар, Віктор Мінькович
Keywords
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