THE L3 SYSTEM — EQUALIZATION AND REGULATION 847 



deviation and some constant part of the equalizer characteristics. If 

 this is not the case the residues can produce new variable deviations 

 shapes and thereby lead to increased complexity in following stages of 

 equalization. 



As previously described, equalization of both gain and loss is required 

 and this implies active equalizers. Although a number of methods were 

 studied, noise and modulation requirements led the L3 system to the 

 so-called ''block of gain-block of loss" design. The equalizers are passive 

 networks and in order to realize both gain and loss adjustability, the 

 normal setting loss must at least equal the total amount of gain ad- 

 justment. The various equalizers are combined into two to four groups 

 whose loss is compensated by corresponding numbers of flat gain 

 amplifiers.^ 



The required number of such blocks of loss and gain depends upon 

 the amount of system gain variation to be equalized. The determination 

 of the shapes and magnitudes of system variations is an important 

 system problem. Large amounts of study are necessary in order to evalu- 

 ate the system sensitivity to various changes; the determination of the 

 magnitude of these causes, such as temperature variation, aging rates, 

 etc.; and the determination of maintenance intervals that provide an 

 economic balance between maintenance expense and system cost. These 

 must be studied in detail to provide the equalizer designer with shape 

 and range data for his dynamic equalizer designs. The equalization 

 characteristics and maximum ranges for the two most important dynamic 

 equalizers aside from cable temperature, namely, repeater temperature 

 (T) and vacuum tube aging )u/3 are shown on Fig. 3. In order to main- 

 tain satisfactory transmission during a maintenance interval, the dy- 

 namic equalizers must be able to match any characteristic within the 

 maximum ranges and throughout the transmission band to within 1 to 

 2 per cent. As noted previously the necessity for simultaneous delay 

 equalization requires the dynamic networks to also make at least an 

 approximate correction in the out-band region. This is shown on Fig. 4. 



The control element in the equalizers is a thermistor whose available 

 resistance range is 30 to 1050 ohms. (V7 ^^ ^^^^ amplifier^' ^ 125 to 2000 

 ohms). Thus the regulation ranges shown in Fig. 3 are realized using 

 this one variable resistance element in each equalizer. 



The following is a summary of the dynamic equalizer requirements. 



1. Provide symmetry in regulation characteristic. 



2. Minimize flat loss. 



3. Match prescribed gain variation to a high degree of accuracy 

 within transmission band. 



