Audio-frequency transformers: Voltage-ratio characteristics determined by the low-voltage cathode ray oscillograph

Abstract

The usual method of determining the voltage ratio of audio-frequency transformers employs a three-element vacuum tube in the circuit. Due to the fact that the results are a combination of both transformer and vacuum tube characteristics this method is not entirely satisfactory. This paper describes a new method, using a low-voltage cathode ray oscillograph, by means of which the voltage ratio of a transformer is determined independent of any other apparatus This newly developed method is accurate and simple in operation, and gives the actual characteristics of the transformer for all frequencies and d-c. components. The ratios of several makes of transformers were determined over a frequency range of 20 to 10,000 cycles and at different degrees of magnetic saturation. The results indicate that the ratios are very good between frequencies of 40 and 3000 cycles. Saturation within the usual operating limits was found to have little effect. The method developed was found to be exceptionally good, as the results are accurate and are readily reproduced. However, precautions must be taken to eliminate stray fields which may influence the tube unless properly shielded.