INDUCTIVE LOADING FOR TELEPHONE FACILITIES 



751 



Additional information regarding the loading terminations is given in the 

 description of the terminal loading units which are used in these terminations. 



Control of Impedance Irregularities; Loading Layouts: The carrier loading 

 systems are engineered and installed to meet unusually severe limits on 

 impedance irregularity among the individual loading sections and at the 

 terminals. In installations involving more than one carrier system, it is 

 especially desirable to restrict the individual impedance irregularities in order 

 to control intersystem reflection crosstalk. The significance of this is under- 

 standable when one appreciates that usually the dominating reason for using 

 carrier loading on the incidental cable is to avoid the objectionable reflection 

 crosstalk that would result from the impedance irregularities caused by non- 

 loaded cables. An additional important reason for the control of impedance 

 irregularities is to avoid large humps in the insertion loss-frequency charac- 



Table XVII 

 Carrier Loading Attenuation Data 



teristics which might cause objectionable frequency-distortion within the 

 individual channels. 



The procedure for controlling impedance irregularities in the loaded 

 incidental cables involves the adjustment of the total capacitances of the 

 individual loading sections to values close to the theoretical design values by 

 means of adjustable building-out condensers. Ordinarily, a precision limit of 

 about ±1% is involved. To make these limits economically practicable, 

 precision types of capacitance measuring-instruments have been made 

 available, along with low cost building-out devices capable of simple pre- 

 cision adjustments. 



The theoretical total loading capacitances for the different carrier loading 

 systems are given in Table XVI, along with theoretical values of coil spacing 

 in terms of the "nominal capacitance" of the usual type of cable involved. 

 The actual geographical coil-spacing is usually well below this theoretical 

 spacing because of the unavoidable capacitance deviations that occur in 

 commercial paper-insulated cables. The loading layout procedure is such 



