CARJUER TELEGRAPH METHODS 



509 



locations, if ihc filter characteristic is symmetrical about the midband 

 frcquenc}', if the pass band is narrow, and if the carrier frequency is high 

 compared to the dot speed, as in the arrangement tested. Consequently 

 there was no point in duplicating measurements for carrier locations at the 

 lower edge of the band except perhaps to discover the second order effect of 

 slight asymmetry in the channel filters. 1998 cycles is at the right-hand 6 

 (lb point of the tilter characteristic given by curve A of Fig. 1 ; and as there 

 is not much choice in the region from 5 cycles below to 17 cycles above this 



50 



45 



K 25 



-40 -35 -30 -25 -20 -15 -10 -5 5 10 15 



CARRIER FREQUENCY IN CYCLES PER SECOND (DEPARTURE FROM 1998~) 



20 



Fig. 12 — Distortion vs. carrier frequency characteristics of 1998-cycle single- 



sidclmnd arrangement using spacing carrier current 6 dh below marking 



current at 160 w.p.m. (61 d.p.s.). 



A: Fixed adjustments. 13: Constant marking output level from receiving filter, and 



lixed relay l)ias current. C: Same as B, excei)t receiving signal liias adjusted to zero by 



varying receiving relay l)ias current while transmitting reversals. 



frequency, according to curve C of Fig. 12, it was considered satisfactory' to 

 locate the carrier at 1998 cycles for certain other single-sideband tests covered 

 in this pai)er, although a slightly higher or lower value would probably have 

 done about as well. As previously indicated, the reason for assuming a 

 spacing carrier current 6 db below the marking value for single-sideband 

 tests was to reduce the distortion due to the cjuadrature component"'. Still 

 less effect from the latter would have existed if the spacing carrier current 

 had been further increased, but this would have reduced the difference be- 



