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BELL SYSTEM TECHNICAL JOURNAL 



2125 cycles and were demodulated by a linear discriminator centered about 

 2125 cycles as shown in Fig. 8. The characteristics of the low-pass filtering 

 are shown in Fig. 9. These low-pass filters were adjusted by oscillographic 

 observation of the signal wave form and had cut-off characteristics giving 

 very little characteristic distortion^. The d-c. amplifier was a high-gain 

 nonlinear type designed so as to have a square-wave output having transi- 

 tions established by the passage of the demodulated voltage wave through a 

 narrow amplitude range. The amplitude and wave front slope of the de- 

 modulated wave thus had no effect on the output wave form and could not 

 affect the distortion measuring equipment. 



100 200 300 400 500 600 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 9. — Attenuation versus frequency characteristics of low-pass filters. 



Experimental Results 

 Band Width Before Demodulation 



The band width before demodulation determines the amount of noise and 

 interference which is to be accepted along with the desired signal and thus 

 largely determines the signal-to-noise condition at the antenna at which 

 the system fails to receive intelUgence. The band width at this point 

 (point '*a" in Fig. 5) also limits the signaUng speed capabilities of the system. 

 In the following experimental data the values of band width were measured 

 between the points of 6 db loss above that at midband. 



Efect on Signaling Speed 



For both methods of signaling considered here the band width must be 

 at least twice the maximum signaling speed in dot-cycles per second but it 

 is found that signal distortion rises rapidly for a band width less than thiee 

 times the maximum signaling speed, and that a factor of at least four times 

 is indicated for a system which is to have reasonably low distortion with any 

 margin of safety. The signaling speed capability of a given band width is 

 nearly the same for FS signals as for AM (on-off) signals. In Fig. 10 is 



