Cross-Modulation Requirements on Multichannel 

 Amplifiers Below Overload 



By W. R. BENNETT 



Interchannel interference caused by non-linearity of multi- 

 channel amplifier characteristics is analyzed in terms of second and 

 third order sum and difference products of the bands of energy 

 comprising the various channels. Methods of relating the resulting 

 disturbance to discrete frequency measurements are described and 

 means for arriving at modulation requirements on individual 

 amplifiers thus established. 



1. Introduction 

 'ITT' HEN a repeater is used to amplify a number of carrier channels 

 ^ " simultaneously, departure from linearity in the response as a 

 function of input amplitude tends in general to produce interference 

 between the channels. The non-linear component of the amplifier 

 characteristic in efifect acts as a modulator, changing the frequencies 

 in the input wave and producing components which fall in bands 

 assigned to channels other than the original ones. This phenomenon 

 has been called "interchannel modulation" or "non-linear crosstalk." 

 In formulating the requirements which are imposed on a repeater to 

 insure that the resulting interference between channels will not be 

 excessive it is convenient to treat separately two aspects of the 

 problem namely — the condition when the total load on the amplifier 

 is within the range for which the amplifier is designed and the severely 

 overloaded condition. Actually a transition region between these two 

 cases must also exist but when a considerable amount of negative 

 feedback is used the break in the curve of response vs. input is quite 

 sharp so that for practical purposes the input may be said to be either 

 below or above the overload value. 



Below overload, the amplifier characteristic may in most cases be 

 represented with sufficient accuracy by the first few terms of a power 

 series and the interchannel modulation analyzed in terms of the 

 resulting combination tones of the frequencies present in the different 

 channels. The total interference resulting from the combination tones 

 falling in individual channels must be kept below prescribed limits. 

 Above overload on the other hand the resulting disturbance in all 

 channels becomes quite large and requirements are based on making 

 such occurrences sufficiently infrequent. 



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