Improved Square Wave Inductance Bridge

Abstract

This paper describes an ac bridge useful for comparing an inductance coil with its dual equivalent circuit so as to determine, with one square wave null measurement, the values of the inductance, the equivalent series and shunt resistances, and the distributed capacitance. This circuit, which features a wide band transformer of novel but simple construction, has the important advantage that one terminal of both the inductor and its dual circuit are at ground potential. Shielding problems are thereby minimized, and laboratory-type decade boxes and precision variable condensers may be used to synthesize the dual equivalent circuit or to adjust the distributed capacitance of the inductor. A prototype bridge circuit, operating in the frequency range 0.1 to 350 kc, and incorporating compensation for errors due to residual elements is given, together with a simple alignment procedure and the results of a sample measurement of a standard inductor. It is shown that the frequency limitations of the present bridge circuit are not fundamental; an improved high frequency compensation scheme using the concept of the distributed transformer should permit operation in the vhf range. The resulting transmission line bridge circuit may also have application as a precision wide band pulse reflectometer.