THE L3 SYSTEM — DESIGN 805 



earlier applied after 4,000 miles of transmission imply that, with no 

 equalization, stability of the transmission characteristics of the indi- 

 vidual repeaters would have to be of the order of a few ten thousandths 

 of a db. Obviously, stabilities of this magnitude with changes due to 

 temperature, electron tube aging and manufacturing processes cannot 

 be achieved. Therefore, the equaUzation system design must he based 

 on an economical balance between the cost of achieving repeater ac- 

 curacy and stability and the cost of providing and maintaining an elabo- 

 rate system of fixed, manual, and automatic equalizers. 



The equalization problem involves so many variables that no attempt 

 has been made to evolve a unified theoretical basis for evaluating the 

 factors entering into this economic balance. However, in planning and 

 designing the L3 system a number of principles and points of view have 

 been developed which have guided the equalization planning. 



2.231 Misalignment 



The transmission objectives described above are determined on the 

 basis of delivering satisfactorily equalized signals at terminals. In ad- 

 dition to this function the equalizers must limit the signal excursions 

 along the line so that excessive noise or modulation is not accumulated 

 in the repeater system. The amount of signal misalignment that can be 

 allowed to accumulate before the first mop-up equalizer depends of 

 course on the signal-to-noise allowance that has been made for this 

 purpose. The amount of signal-to-noise performance allotted to mis- 

 alignment must represent a balance between the reduced repeater spac- 

 ing and increased complexity of equalizers that it costs and the increased 

 spacing between mop-up equalizers and increased repeater deviations 

 that it allows. 



The engineering method for arriving at this balance represents an 

 interesting example of system design by successive approximations. For 

 example, the total gain area available (over an infinite frequency range) 

 in a coupling network is inversely proportional to the capacity across 

 the network and one of the important design choices is the extent to 

 which one tries to utilize this area in the transmitted frequency band. 

 The degree to which the available gain is concentrated in-band is called 

 the resistance integral efficiency. In the very early stages of the ampHfier 

 design it was necessary to choose resistance integral efficiencies and 

 frequency characteristics for the coupling networks. In a definite but 

 complicated way these parameters are related to the sensitivity of the 

 networks to element variations. Efficient networks give improved sig- 

 nal-to-noise performance but also increase the sensitivity to element 



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