64 



THE BELL SYSTEM TECHNICAL JOUllNAL, JANUARY 1957 



load. With the proposed specific telegraph level (STL) of —30 db (i.e. 

 telegraph signal of —30 dbm per telegraph channel at db telephone 

 transmission level), the telegraph distortion was too low to measure 

 reliably ; it was possible to send clear messages under test conditions with 

 a signal 36 db weaker than this. 



Crystal Tests 



All of the peaks of noise at the crystal resonance frequencies were 

 easily discernible. 



The crystal gain values all lay in the range from 23.6 to 27.2 db, with 

 60 per cent of them lying in the range from 25 to 26 db. Crystal gain, as 

 used here, is the difference between the system gain at a repeater crystal 

 frequency and the average of the gains at 50 cycles above and below this 

 frequency; the latter value is approximately the gain if the crystal were 

 absent. No significant changes in crystal gain have occurred in eleven 

 months of system life, and none were expected. These measurements are 

 to be continued over the years, as an indication of electron tube aging, 

 as explained in the companion paper on the repeater.' 



A series of measurements of the frequency of each of the 51 repeater 

 crystals versus time has been made. (Any of these frequency determina- 

 tions can be checked on the same day within ±0.1 cycle with the special 

 test apparatus and techniques used.) The crystals were designed to be 

 extremely stable in frequency, so that measurements on one repeater, 

 made at the land terminal, would not be affected by the combined effect 

 of 50 other crystals at 100-cycle spacings. The crystal frequency varies 

 only about — | cycle per degree Fahrenheit increase in sea-bottom 



uj 173.403 

 tr 



UJ 

 Q. 



UJ 



173.401 



> 173.399 



173.397 



O 



z 



UJ 



O 



UJ 



tr 

 u. 



170.702 



170.700 



20 



40 



60 80 100 120 140 



DAYS AFTER COMPLETION 



160 



180 



200 



Fig. 17 — Resonant frequency versus time — shore and deep sea crj^stals. 



