BA.\D W IDI II AM) TRAXSMISS/OX PERFORM ANCE 



511 



Fig. 9— PPM-AM, Fluctuation Noise 



The curves of Fig. 9 were computed from the sheer advantage derived in 

 Appendi.x I\'. The asymptotic slope of the constant power curves of ^ db 

 per octave of bandwidth reflects the 6 db advantage due to the two-fold 

 greater pulse slope (sheer advantage) diminished by the 3 db increase of 

 noise accepted by the two-fold wider band. In the marginal power case 



Fig. 10— PPM-x\M; performance with respect to C\V and similar system interference 

 for 1000 4-kc channels with ratio of pulse to interference marginal. Relations between 

 bandwidth and audio signal-to-interference ratio. 



the power is increased with bandwidth so the sHcer advantage is preserved 

 and the slope is 6 db per octave. 



The sharp reduction of signal-to-noise ratio with reduced bandwidth 

 appearing at the left end of the curves arises from immobilizing the pulse 

 position as the widened pulse uses up more of the total channel time allot- 

 ment. According to the definition of bandwidth used here and the plan 

 of Fig. 8, no modulation is possible when the bandwidth is IjT and T is 

 half of the channel time. For 1000 channels the channel time is 0.125 

 microseconds and T = 0.0625, which makes the audio signal-to-noise ratio 

 zero at 32 mc. 



