718 BELL SYSTEM TECHNICAL JOURNAL 



stage of a repeater, a section of the associated transmission line, and the 

 first stage of the succeeding repeater of a simplified system. 



The specification of a flat transmission characteristic over the useful 

 freciuency band l)et\veen A and B in the figure indicates that equaHzation 

 for the Une loss of the section must occur in either or both coupling circuits, 

 in the line equipment, or in all three of these circuits. For feedback amplifiers, 

 the most desirable type, a flat characteristic between A and B can be specified 

 only if the feedback circuits, or /3 circuits, of the amplifiers are designed to 

 have no transmission variation with frequency. In general, it is possible to 

 suppose the feedback factor, /3, of the amplifiers to be the appropriately 

 varying function of frequency to equalize a part of the line loss, thus altering 

 the transmission specification from A to B. However, the /3 circuits must 

 include regulation of other types in most cases. Hence, it is impractical to 

 include much loss equalization in these circuits. 



Since satisfactory performance of the section is dependent also on the 

 maintenance of a large signal-to-noise ratio, it is important that the line 

 contain no sources of additional loss. It is clear, then, that the best trans- 

 mission performance is obtained (1) without the use of equalization in the 

 line^ and (2) when the reactive input and output coupling circuits equalize 

 as large a percentage as possible of the total line loss. 



Physically, the coupling circuits will be transformers, plus any number of 

 tuning and shaping elements. In addition to the primary function of metal- 

 lically separating the line from the repeater amplifiers, it will be seen later 

 that the transformers provide the means of adjusting, independent of the 

 value of the prescribed line impedance, the final impedance level of the net- 

 work to conform with the value of the parasitic capacitance present. 



Besides the contribution of the various networks in the system to the 

 overall transmission performance, there is the problem of matching the 

 coupling circuits to the line. For constant-resistance equalization, this 

 problem is immediately solved by the relationship Z1Z2 = R-. Well-estab- 

 lished techniques make it a relatively simple matter to design for a specified 

 attenuation variation with frequency at the same time that the impedance 

 of the equalizer is matched to the line. This same procedure, with certain 

 modifications, can be carried over to the design of reactive equalizers. In 

 Fig. 2, the transformers of the input and output coupling circuits are un- 

 terminated. That is, the input of the output circuit and the output of the 

 input circuit are terminated in substantially open circuits. In order to pre- 

 vent the reflection of power at the junctions of the coupling circuits and the 

 line, the impedances of the input and output circuits as viewed from the 

 line must be made equal to the impedance of the line. This impedance re- 



' In practice, the /3 circuits and constant resistance networks associated with the line 

 actually equalize a certain percentage of the total line loss characteristic. 



