Cross-Modulation Requirements on Multichannel Amplifiers Below Overload

W H E N 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 effect 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.