518 BELL SYSTEM TECHXICAL JOCRXAL 



carrier of a PAM-FM system using sinusoidal pulses assumes the particularly 

 simple form of an FM wave modulated by a sinusoidal signal having fre- 

 quency Nfr (8 mc for 1000 channels) and total frequency swing 13/2. Hence 

 the rigorous steady state solution for interference between CW and sinu- 

 soidally modulated FM was calculated and the interfering components falling 

 in the baseband range selected. The gating function was then applied to these 

 components in the same way as described above for fluctuation noise, and the 

 resulting products falling in the audio channel range evaluated. The signal- 

 to-interference ratio was expressed as the ratio of rms signal power received 

 from a full-load channel test tone to the rms value of the audio interference. 

 A range of frequency locations for the CW interference was investigated for 

 each radio signal bandwidth and the one giving maximum audio interference 

 used for the point on the curve. The worst position of the CW was usually 

 found to be near the extremities of the idle channel frequency swing. Cur\'es 

 are shown for a rectangular gate of maximum duration and for instantaneous 

 sampling. 



When the source of interference is a similar system, we assume that the 

 midband frequencies differ only slightly. With both systems idle, we have 

 two sinusoidally modulated FM waves which are identical except for (1) a 

 small variable difference between mean carrier frequencies and (2) a variable 

 phase shift between the two modulating frequencies. The interference 

 falling in the baseband consists of steady state components which are 

 approximately harmonics of the channel slot frequency Nfr- As is charac- 

 teristic of FM interference, the amplitude at the mth harmonic contains a 

 factor proportional to m; and the component near zero frequency, the 

 approximate zeroth harmonic, is very small. If we gate this interference 

 with a rectangular gate of duration 1/Nfr, we find that the gated output 

 vanishes for input components at Nfr, 2Nfr, ... , because these frequencies 

 are located at the infinite loss points of the aperture admittance. The gate 

 would transmit the zeroth harmonic, but this component tends toward zero 

 amplitude. Our conclusion is that two idle PAM-FM systems accurately 

 lined up to occupy the same frequency range are balanced against inter- 

 ference from each other when a rectangular channel gate of full channel 

 allotment time is used. The balance tends to disappear as the channels are 

 loaded because the interference then spreads throughout the base band 

 instead of being concentrated at the blind spots of the aperture. Thus, for 

 the first time in our consideration of pulsed systems, we are obliged to take 

 account of channel loading conditions.-' 



23 A wave could be frequency modulated about a central frequency by P.\M pulses of 

 plus and minus sign and an idle system would thus consist of a wave of constant frequency. 

 The weaker of two such idle systems aligned in frequency would produce no (or very little) 

 interference in the other, using either channel gating or instantaneous sampling. The 

 susceptibility to CW interference would be greater than in the biased modulation assumed 

 above, however. 



