DESIGN OF SYSTEM 



NORTH ATLANTIC LINK 



41 



where A^o = system random noise, in dba,* referred to zero transmis- 

 sion level 

 Nin — random noise per repeater in dba, referred to repeater 



input level 

 G — repeater gain in db 

 TL — transmission level of repeater output 

 n = number of undersea repeaters 



dr = db increase in noise due to differing output levels of the 



various repeaters as compared to the highest level repeater. 



At the top frequency of 164 kc, TL = — 6 db as seen above, G = 60.7 



db, and Nin is about —55.5 dba, which corresponds to a noise figure of 



about 2 db. Hence for 52 repeaters, 



ATfl 



■55.5 + 60.7 - (-6) + 10 log 52 + ck = 28.4 + dr 



At lower frequencies, the noise power referred to repeater input is 

 greater because part of the equalization loss is in the input circuit; the 

 repeater gain is less, to match the lesser loss of a repeater section of 

 cable; and the repeater output level is lower on account of the equaliza- 

 tion loss from output grid to repeater output. This is shown in Table I. 



Table 1 



In order to estimate dr (which is a function of frequency) before the 

 cable was laid, the factors were studied which might contribute to differ- 

 ences between the levels of the various repeaters. Estimates were made 

 of probable total misalignment — by which is meant the level difference 

 between highest-level and lowest-level repeaters. These values together 

 with estimates of the resulting noise increases, are shown in Table II. 

 This is based on the assumption that the repeater levels would be dis- 

 tributed approximately uniformly between highest and lowest. The posi- 

 tion of a repeater along the cable route is not significant. 



* "dba" is a term used for describing the interfering effect of noise on a speech 

 channel. Readings of the 2B noise meter with FIA weighting may be converted 

 to dba b,y adding 7 db. dba may be translated to dbm (unweighted) by noting that 

 flat noise having a power of 1 milliwatt over a 3,000-cycle band equals approxi- 

 mately -(-82 dba, and that 1 milliwatt of 1,000-cycle single-frequenc}' power equals 

 -t-85 dba. 



