24 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



frequency modulation curves are shown. Figure 10(c) shows the single- 

 frequency load characteristic. Most of the modulation crosstalk results 

 from the many third-order combinations of carriers and speech sidebands 

 in a repeater. Third-order products of this type add in phase in a string of 

 repeaters. Twenty repeaters are 26 db worse in modulation crosstalk than 

 one repeater. 



Repeater and Terminal Levels 



The operating levels of the system are all referred to the strengths of the 

 individual carriers which are each made 15 db above one milliwatt (+15 

 dbm) at reference level of one sideband. Thus, a + 5 dbm signal at level 



+ 10 







-10 



-20 



5 



m -30 

 o 



-40 

 -50 

 -60 

 -70 



TERMINAL A 



HIGH-GROUP 



TRANSMITTING 



UNIT 



OUTPUT +4- 



■53 



INPUT 



168 KG 



256 



+ 10 







-10 



20 



TRANSMISSION A TO B 



HIGH-LOW 

 REPEATER 



-^OUTPUT 

 .136 "~"*"«*^ 

 *^^ 48 KG 



12 



+10 







-10 



-20 



CD -30 

 Q 



-40- 



TERMINAL B 



HIGH-GROUP 



RECEIVING 



UNIT 



OUTPUT 

 -5.5 



-53 INPUT 



12 1 



CHANNEL NUMBER 



Fig. 11(a) — N-1 repeater and group unit level diagrams. 



in the voice circuit is modulated to produce two sidebands each of +5 dbm 

 at single sideband level in the carrier part of the system. 



Six db of loss must be inserted between single sideband level and voice 

 level at the output because of the in-phase addition of the two sidebands 

 upon demodulation. Thus, the two +5 dbm sidebands become +5 dbm 

 at voice level in the output. Zero dbm of 3700 cycle tone is used for signal- 

 ing at level and, since it is inserted after the compressor and removed 

 before the expandor, each sideband is 15 db below the carrier. Limiting of 

 speech peaks in the compressor restricts maximum values to about +9 

 dbm at level. In-phase addition of the maximum speech sideband peaks 

 nearly 100% modulate the carrier (+15 dbm at level). 



In Fig. 11, (a) and (b), carrier level information is given for repeaters 

 and terminals. High-group output repeaters and terminals have carrier 

 outputs sloped from —3 to +4 dbm with a total of +12 dbm of carrier 



