Pomiary komparatorowe stosunku impedancji wzorcowych różnego typu

W artykule przedstawiony jest układ komparatora do wyznaczania stosunku impedancji dwuportowych, współosiowych w zakresie (10 – 100) kΩ, typu R-R, C-C, a także R-C. Główne elementy układu to generator, transformator pomiarowy i cyfrowy moduł pomiarowy. Przeprowadzono badania w celu pewnego porównania wyników otrzymanych komparatorem z tymi otrzymanymi klasycznym mostkiem transformatorowym. Dokonano analizy błędów występujących w systemie pomiarowym.

In the paper a comparator circuit which measures two terminal-pair impedance ratio is presented. The system is able to compare impedances of R-C type elements but with indirect comparison it is possible to achieve impedance ratio of R-R and C-C type elements. The first section describes the principle of operation of the impedance bridge and comparator. The system setup and the measurement method are presented in the second section. In Fig. 1 there are shown: a digital generator G, a measurement transformer Tr, the compared impedances Z_X, Z_N, a digital sampling voltmeter with channels V₁, V₂ and a buffer amplifier B. Stray impedances are depicted as Z_I and they affect mostly the unbalance voltage [2]. The CVR (complex voltage ratio) is gathered when the secondary voltages are in phase and when they are in reversed phase (180˚). Equation (8) describes the relation between the measured CVR_S and the impedance ratio. Table 1 contains the measurement results of the CVR at the transformer output in the circuit with different impedances. It is assumed that the voltage ratios at the transformer output are equal to 1 and -1, respectively. The experimental results are listed in Table 2. There were compared two standard resistors (R₁, R₂) of nominal values equal to 100 kΩ, two resistors (R₃, R₄) of 10 kΩ and two capacitors (C₁, C₂) of 3 nF. The resulting indirect ratio of C₁, C₂ and R₁, R₂ is compared with the direct result from a transformer bridge. The results are contained in the range of ± 5 ppm. The fifth section describes shortly sources of the uncertainty in the system.