144 



BELL SYSTEM TECHNICAL JOURNAL 



garded as a high quality circuit. The delay characteristic of a high 

 frequency transformer is shown in Fig. 10. 



7 0.45 

 O 



0.2 5 



80^ 



QeiZei 



%< ^zl3z 



PHASE DELAY=dyyc|(jj WHERE /3= 91-02 



100 200 300 400 500 600 700 800 900 1000 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. lO^Phase delay-frequency characteristic of an input transformer designed to 

 transmit radio frequencies. 



Reduction in Audio Frequency Modulation 



The present exacting requirements for transformer performance 

 have made it necessary to lessen greatly certain second-order distor- 

 tion effects inherent in transformers having magnetic cores. Nearly 

 all core materials tend to generate extraneous frequencies because of 

 magnetic non-linearity — a property referred to as magnetic modula- 

 tion. In audio-frequency circuits intended for high quality service, 

 magnetic modulation may cause serious distortion in that harmonics 

 of the lower frequencies appear higher in the audible range. The 

 energy present at the lower frequencies is usually so much greater than 

 that over the rest of the band that the modulation products may 

 approach the order of magnitude of the signal components at higher 

 frequencies. 



This form of distortion in no way is revealed by the ordinary trans- 

 mission loss characteristic; in fact, a transformer having a very flat 

 loss characteristic over a wide-frequency range may nevertheless be 

 definitely objectionable from a modulation standpoint. In present 

 audio-frequency transformers, the total modulation products are some 

 40 to 80 decibels down from the energy of frequencies around 35 cycles 

 per second producing them. This represents an improvement of about 

 30 decibels over older types. 



Another second-order effect resembling modulation is microphonic 

 noise caused by magnetostriction phenomena, that is, changes in mag- 

 netization accompanying the physical deformation of the magnetic 



