IMPROVEMENTS IN COMMUNICATION TRANSFORMERS 149 



measurement in order to avoid obtaining a misleading characteristic. 

 For example, the permeability of magnetic core materials tends to rise 

 rapidly from its initial value with increasing voltages. If in the meas- 

 urement, the low-frequency voltages used are materially higher than 

 they are under service conditions, the low-frequency response will 

 appear to be much better than the true response. 



As another example, the transmission of input transformers at the 

 high-frequency end may be critical with the termination of the high- 



w700 

 O 600 



lu 500 

 o 



z 



j< 400- 

 U 



^ 300 

 a. 



O200 



UJ 



^100 



i 



-100 



30 



50 



100 



500 1000 5000 



FREQUENCY IN CYCLES PER SECOND 



10,000 20,000 



Fig. 13 — Impedance-frequency characteristic of an input transformer used to 

 operate from a telephone transmission line into a high quality program repeater. 

 Impedance ratio is 600 to 15,000 ohms. 



voltage winding. If the grid capacitance and conductance conditions 

 are not reproduced faithfully in the measuring circuit, the high- 

 frequency voltage amplification may appear to be much better than 

 the true value. 



In addition to more precise transmission measuring circuits, various 

 other special circuits have been developed for measuring transformers, 

 such as modulation, impedance, and crosstalk measuring circuits. 

 The design of these circuits is of necessity a specialized art. 



In the foregoing, various types of improvements in communication 

 transformers have been discussed. Wherever applicable, several such 

 improvements have been incorporated in an individual design. The 

 improved performance of transformers as described has been an essen- 

 tial step in the development of the communication art. 



References 



H. D. Arnold and G. W. Elmen. Permalloy — an Alloy of Remarkable Mag- 

 netic Properties. Franklin Inst. Journal, May 1923, p. 621-32. 



G. W. Elmen. Magnetic Alloys of Iron, Nickel, and Cobalt. Franklin Inst. 

 Journal, May 1929, Vol. 207, p. 595. 



