MULTICHANNEL AMPLIFIERS BELOW OVERLOAD 603 



amplitude of fundamental to that of the {2 A — 5) -product falling at 

 1000 kc. A band elimination filter having more than 100 db loss at 

 1000 kc. and suppressing a band approximately from 940 to 1070 kc. 

 was inserted in the line at the repeater station to remove all contribu- 

 tions to the modulation product originating ahead of the station at 

 which measurements were made. In this way, the modulation con- 

 tributed by each amplifier and by various combinations of amplifiers 

 could be measured without disturbing the operating levels throughout 

 the system. 



The data shown on Fig. 3 include measured modulation from indi- 

 vidual amplifiers, and from tandem amplifiers with intervening cable 

 sections. The summation of amplifiers proceeds in the same order 

 as the plotted individual amplifier values. The crosses show the 

 calculated sums of the individual contributions assuming in-phase 

 addition. Agreement between these values and the measured sums 

 is well within the accuracy of the measurements, considering the 

 difficulties involved and the length of time required to complete the 

 run. The dots show the resultant modulation which would be obtained 

 by adding the power in the individual components instead of the 

 voltages, which would be the expected result for a large number of 

 components with random phase angles. The modulation thus calcu- 

 lated is much smaller than the measured values indicating that a 

 hypothesis of random phasing is untenable for this product. 



In actual systems both the magnitude and phase shift of modulation 

 products in the different repeater sections exhibit variations because 

 of non-iiniform output levels, differences in tubes and other amplifier 

 parts, and unequal repeater spacings. The addition factor for con- 

 verting system requirements to single amplifier requirements should 

 therefore contain a marginal allowance for these irregularities in 

 performance. 



Summarizing our conclusions on addition of modulation from 

 multiple sources, we may state that the third order requirement in- 

 vokes the most severe condition — that of in-phase addition. Second 

 order products on the other hand will have enough phase shift, either 

 inherent or from simple reversals of terminals at alternate sections, to 

 make the addition no more rapid than on a power basis. In fact if 

 there is a high degree of similarity with respect to both amplitude and 

 phase increment of products from successive amplifiers throughout the 

 system, the total second order modulation may be much less than 

 calculated from addition of power. In setting the requirements which 

 each amplifier must meet, marginal allowances should be made for 

 differences in lineup throughout the system and aging effects which 

 may take place after the amplifier is put in service. 



