Theoretical And Practical Aspects of Measuring the A-Parameters of the Equivalent Two-Port Network of the Track Filter

when developing and subsequently updating digital twins of audio- frequency track circuits, a need arises to measure the A parameters (coefficients A, B, C, D) of equivalent two port networks of the equipment being part of a specific track circuit. Measuring the A parameters of the two port network of the track filter TF used in audio-frequency track circuits is complicated by the nonlinearity of the transformer included in the TF, which makes these parameters dependent on the voltage at the TF input. Currently applied methods for measuring the A parameters of linear two port networks are reviewed. Aspects of the practical implementation of the methodology earlier proposed by the authors for measuring the A parameters of the TF two port network are presented. The methodology takes into account the nonlinearity of the TF transformer by performing measurements at a specific operating point of the TF, understood as a given voltage value at its input. The methodology was implemented for a TF 8,9,11 filter tuned to a carrier frequency of 480 Hz with a TF input voltage of 3.3 V. The coefficient matrix of the TF two port network was calculated. Measurement verification was carried out. The relative error of the magnitude and argument of the transformer input impedance, of the magnitudes and phases of the transformer input current and voltage, and of the magnitudes and phases of the TF input current and voltage did not exceed 1 % in absolute value. The use of methods for measuring the A parameters of linear two port networks under open circuit and short circuit conditions, due to the nonlinearity of the TF transformer, may lead to incorrect values of the parameters of the TF two port network. The methodology for measuring the A parameters of the TF two port network, considered in the paper through a specific practical example, makes it possible to determine the values of the parameters of the TF two port network with an accuracy acceptable for use in calculations of adjustment characteristics of track circuits.