604 BELL SYSTEM TECHNICAL JOURNAL 



Let Ax represent the ratio expressed in db between the total x-type 

 modulation received from the system and the contribution of x-type 

 from one amplifier, assuming the amplifiers contribute equally. For 

 example, if there are K amplifiers in the system and if the contributions 

 to the product add in phase Ax = 20 logio K. If power addition 

 occurs, Ax — 10 logio K. A favorable set of phase angles may reduce 

 this factor by an amount depending on the uniformity of the repeaters. 

 If the system is divided into K\ links having Ki amplifiers in each link, 

 with phase shifts and changes of frequency allocations of individual 

 channels present at the link junctions such that amplitude addition oc- 

 curs within links and power addition from link to link, ^1, = 10 logio i^i 

 + 20 logio -K^2- We shall also introduce a lineup factor Fx defined as 

 the number of db by which the x-type product requirement must be 

 increased to allow for irregularities in lineup operating levels of the 

 amplifiers. These may be due to initial differences in repeaters or 

 cable sections and to subsequent changes which may occur because of 

 aging effects. If Kx^ represents the requirement on the ratio of 

 fundamental to x-type product in the output of a single amplifier when 

 one mw. of test tone power is delivered at zero level, 



iJ,0 = iJx + ^x + Fx, (8.1) 



where H^ is the system requirement given by (7.8). 



9, Testing Methods 



It is difficult to test a broad band carrier system under conditions 

 simulating normal operation because of the large number of inde- 

 pendent conversations required to load the channels. We have seen 

 that much information applicable to speech load can be deduced from 

 current analyzer measurements of modulation products when discrete 

 frequencies are applied. Since rather extensive calculations are re- 

 quired to evaluate the performance of the system from single frequency 

 data, an overall test under conditions comparable to actual operation 

 has considerable value as a check. A convenient method of simulating 

 the speech load in the high frequency medium by means of a uniformly 

 distributed spectrum of energy such as thermal noise or the output of 

 a gas tube applied through a narrow band elimination filter has been 

 developed ^ for this purpose. The band elimination filter suppresses 

 the energy which would fall in several adjacent channels, hence any- 

 thing received in these channels at the receiving end of the line is 

 introduced by the system. We can thus measure interchannel modula- 

 tion if it exceeds the background of noise from other sources. A 



8 E. Peterson, Bell Laboratories Record, Nov. 1939, Vol. 18, No. 3, pp. 81-83. 



