Measurement of Parameters Controlling Pulse Front Response of Transformers

The two-element equivalent circuit commonly used in predicting the high-frequency or pulse-front response of a stepdown or step-up transformer is not applicable to transformers with turns ratios near unity. Furthermore, the transformer leakage inductance (the series element in the circuit) may vary with frequency or time because of skin effects in the windings, and the distributed capacitance (the shunt element in the circuit) varies with frequency or time, source impedance, and load impedance because of variations in voltage distribution. These variations are difficult to take into account in measuring the element values and also in using the values in the prediction of transformer response. A theoretical and experimental analysis has shown that a T circuit containing three inductances and one capacitance (or the mathematically equivalent pi circuit, which contains seven elements) can be used to represent a transformer of any turns ratio, and element values obtained from appropriate measurements give reasonably accurate results over necessary frequency range. Measurements are easily made with an rf generator and a vacuum-tube voltmeter. Greater accuracy with which high-frequency response characteristics can be predicted by this T circuit indicates a marked improvement in accuracy will result when T circuit is used in predicting response of a transformer to fast-rising pulses.