MULTICHANNEL AMPLIFIERS BELOW OVERLOAD 601 



difficult one, and the other made to contribute a negligible amount. 

 Usually one type of modulation product will predominate for a given 

 order and the allowable noise for the order may be assigned to this 

 type. If such is not the case division of the noise between the various 

 types may be estimated. 



8. Addition of Products in a Multirepeater Line 



When a number of amplifiers are used in a carrier system, contribu- 

 tions to interchannel interference occur at each repeater. The con- 

 siderations previously discussed have a bearing on the modulation 

 requirements on the system as a whole, but it is evident that the 

 individual amplifiers may have to meet much more severe require- 

 ments. The relation between total system modulation and single 

 amplifier modulation depends to a considerable extent on the phase 

 angles between products originating in the various repeater sections. 



A discussion of the general problem of addition of modulation prod- 

 ucts from multiple sources is to be given in a forthcoming paper by 

 J. G. Kreer. A point of particular interest in connection with broad 

 band systems is the effect of a linear phase characteristic on the phase 

 shift between modulation products originating in different repeater 

 sections. The curve of phase shift vs. frequency throughout the 

 frequency range occupied by a considerable number of adjacent 

 channels will in general depart but little from a straight line, but the 

 intercept of this straight line if produced to zero frequency is in general 

 not zero or a multiple of 2t. The intercept of such a linear phase curve 

 is effective in producing phase difference between contributions to 

 modulation from successive repeater sections of all the second order 

 products and of some of the third order products, namely the types 

 2>A, 2A -{- B, B - 2A, A -{- B -j- C, and A - B - C. The phases of 

 third order products of types 2A — B and A -\- B — C however are 

 unaffected by the value of the intercept and the contributions from 

 the different repeaters of these types of modulation will add in phase to 

 give the maximum possible sum whenever the phase curve is linear 

 throughout the channels involved. Third order modulation require- 

 ments on individual repeaters of a system may therefore have to be 

 based on the very severe condition of in-phase or voltage addition of 

 separate contributions. 



Experimental verification of in-phase addition of third order modula- 

 tion products from the repeaters of a 12-channel cable carrier system 

 are included in the paper by Kreer previously mentioned. Corroborat- 

 ing data obtained on an experimental system capable of handling 480 

 channels are shown in Fig. 3. The measurements there shown were 



