Stanowisko do pomiarów wybranych parametrów elektrycznych linii transmisyjnej

W artykule przedstawiono sposób wykonania pomiarów wybranych parametrów wysokoczęstotliwościowych linii transmisyjnych metodą reflektometrii w dziedzinie czasu z wykorzystaniem szybkiego oscyloskopu cyfrowego, generatora skoku jednostkowego oraz przyrządu wirtualnego wykonanego za pomocą programu LabVIEW. Przedstawiono teoretyczne podstawy metody reflektometrycznej. Podano wstępne, przykładowe pomiary wybranych parametrów elektrycznych testowanej linii transmisyjnej. Omówiono sposób oszacowania niepewności pomiaru jednego ze zmierzonych parametrów - prędkości propagacji.

The paper presents a measuring stand for measurement of selected parameters of high frequency transmission lines by reflectometry method in time domain using a fast oscilloscope, voltage step generator and a virtual instrument constructed in LabVIEW. The theoret-ical foundations of the reflectometry method are presented. The paper describes a construction of stand for reflectometry measurement [Fig.2]. The stand uses a sampling oscilloscope, a fast voltage step generator and fast voltage divider. The paper also describes preliminary sample measurements of selected parameters of the tested electrical transmission line. Fig. 4 shows voltage wave, generated by voltage step generator, reflected from H155 coaxial cable with open, short circuit termination and terminated by 50 ? load. The paper also discusses, how to estimate the measurement uncertainty of the speed of propagation. As a result of first mea-surements it was found that the rise time value in the reflectometry system affects not only the resolution of the measurement results, but also the uncertainty propagation velocity and a dielectric constant of substrate of the transmission line. Fig.5 shows measurement of the pulse shape deviation from a rectangular. The proposed method is based on two measurements: tmin and tmax. These values determine the area of velocity results caused by deviation of the measured pulse from the rectangular shape. The table 1 shows uncertainty budget for cable of length 994 mm and tmin i tmax times for tested H155 cable. For measured times tmax i tmin the velocity of propagation Vpmin and Vpmax was computed. From equation (10) the combined uncertainty was computed. The results are presented in the table 2. The range comprising the true value of propagation velocity, according to the assumptions made, there is shown in Figure 6 (part A).