IMPROVEMENTS IN COMMUNICATION TRANSFORMERS 145 



material. For instance, slii^ht jarring of input transformers used in 

 very high-gain ampHfiers (100 decibels or more) may induce in this 

 manner disturbing noise voltages. Freedom from magnetostriction is 

 a unique characteristic of permalloys containing approximately 80 

 per cent nickel, and their use accounts for the superiority of telephone 

 transformers from this standpoint. 



Reduction in Carrier Frequency Modulation 

 Magnetic modulation is even more serious at carrier frequencies 

 where a transformer may transmit many channels, frequently at widely 

 different levels. The modulation from the higher level channels may 

 produce very objectionable interference in other channels. Carrier 

 transformers now have been improved to such an extent that highly 

 sensitive testing circuits are required to detect and measure the modu- 

 lation products contributed by them. Representative values of these 

 modulation products expressed as current ratios to the fundamentals 

 are of the order of one-millionth of the fundamental frequencies, com- 

 pared to one-thousandth in older types. 



It is of interest to point out that in such transformers the presence 

 of magnetic material, other than the special permalloys, in the vicinity 

 of the transformer must be avoided with great care. For example, 

 a small steel screw near the field of the transformer will seriously 

 impair its performance from a modulation standpoint. Common prac- 

 tice is to use brass parts for the assembly and to confine the field of 

 the transformer by completely enclosing it in a copper or aluminum 

 case. The transformer then may be mounted by any convenient 

 means without affecting its performance. 



Improvements in Shielding and Balance 



In the very nature of the service that it renders, the telephone plant 



involves many independent communication circuits in fairly close 



proximity. The minimizing of interference between these independent 



communication circuits has constituted a major problem in telephone 



engineering. Where such interference occurs between like circuits, 



that is, between two voice circuits or between two carrier circuits of 



overlapping frequency bands, the interference commonly is referred to 



as crosstalk, as distinguished from the interference from other types 



of circuits such as power and telegraph circuits. 



In order to avoid crosstalk and other interference, balanced * cir- 



* The new coaxial cable circuits under development are an interesting exception. 

 In this tj'pe of cable a grounded outer conductor completely encloses the central 

 conductor, and shielding rather than balance is relied upon to protect the circuit 

 from interference. The shielding depends upon the size, thickness, permeability, 

 and conductivity of the outer conductor and the frequency of the disturbance. If 

 the frequency band used in the transmission over coaxial circuits is chosen properly, 

 the circuit may be made substantially immune to effects from outside fields. 



