IMPROVEMENTS IN COMMUNICATION TRANSFORMERS 143 



in the shape of the transmitted wave. However, departures from 

 Hnearity change the wave shape, and this form of distortion is referred 

 to as phase or delay distortion. The delay at any frequency is a 

 measure of this departure from Hnearity, and is dependent upon the 

 frequency derivative of the phase shift at that frequency. Differences 

 in delay of the various frequency components of the signal wave which 

 transformers tend to produce result in distortion that may be especially 

 serious in circuits intended for program transmission. 



For wide-band transformers the delay caused by the shunting effect 

 of the mutual impedance usually predominates. In fact for audio 

 transformers the delay at higher frequencies is relatively so small that 

 the delay distortion is practically equal to the mutual impedance delay 

 at the lowest transmitted frequency. Delay distortion is also of im- 

 portance in transformers to be used in television and telephotography. 

 In these circuits phase distortion causes a space shift in the image of 

 certain frequency components with respect to others with consequent 

 blurring of the image. 



The delay characteristic of a transformer used in program circuits 

 to connect a telephone transmission line to the grid circuit of a re- 

 peater amplifier is shown in Fig. 9. This characteristic is compared 

 in the same illustration with the delay characteristic of a repeater 

 transformer developed some years ago for use in what then was re- 



NEW 



\OLD 

 _\ 



eiZei 



■AV/ 



: 300W 



PHASE DELAY = CI/^^^ WHERE /3 = ere2 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



10,000 



Fig. 9 — Phase delay-frequency characteristic of an input transformer used in 

 recent program repeaters, compared with that of an input transformer used in an 

 older repeater. 



