Measurement of the temperature coefficient of ratio transformers

We have measured the temperature coefficient of the output of several ratio transformers at ratios near 0.500 000 using an ac bridge and a dual‐phase, lock‐in amplifier. The two orthogonal output components were each resolved to ±1 ppb of the bridge drive signal. The results for three commercial ratio transformers (an ESI DT72A, a Gertsch RT‐1665u, and an Eaton PRT‐10C) between 20 and 50 °C range from 0.5 to 100 ppb/K for the signal component in phase with the bridge drive, and from 4 to 300 ppb/K for the quadrature component.