BAND WIDTU AND TIOINSMISSION PERl'ORMANCE 



513 



Fig. 12— PPM-FM, CW and Similar System Interference 



The curve showing interference from a similar system of lower jiower was 

 based on a calculation of the beat spectrum between two FM waves, both 

 frequency modulated over the same r-f range (j3 mc) by 8 mc. The phase 

 dillerence between the 8-mc modulating frequencies was assumed to vary, 

 giving rise to various beat spectra. The power in those beat components 

 accepted by a band zero to Fb was averaged over all 8-mc i)hase differences 



20 m 



40 60 80 100 200 400 600 1000 2000 4000 



RADIO SIGNAL BANDWIDTH IN MEGACYCLES PER SECOND 



10,000 



Fig. 11— PPM-FM; performance with respect to fluctuation noise. Relations between 

 bandwidth, power, and audio signal-to-noise ratio for 1000 -i-kc channels. 



and this average power was taken as a measure of the interference to which 

 the baseband signal is subjected. As outlined here, this procedure is valid 

 for interference between idle PAM-FM systems in which the FM waves are 

 frequency modulated as assumed above. For the PPM-FM case in which we 

 are here interested, we take the position suggested by the transient viewpoint 

 that the effect of interference from spaced pulses will not be much different 

 because of their spacing and so we apply the sheer advantage possessed by 

 the wanted system to the total interference calculated above and obtam the 

 curve shown. At the left hand end where the pulse spacing is only slight 



