206 BELL SYSTEM TECHNICAL JOURNAL 



possible since as has been shown by Carson/ a frequency modulated 

 wave of this character consists of an infinite series of fixed frequencies 

 spaced at regular intervals either side of a "fundamental" carrier 

 wave. Obviously only a small part of such a series could get out of 

 the transmitter or into the receiver due to circuit selectivity. For 

 the lower modulating frequencies, however, the approximation in- 

 volved in the assumption of a simple sinusoidal variation is not far 

 wrong since the amplitudes of these side frequency components fall 

 off rapidly as their order in the series increases. While 150 wave 

 lengths difference in path length has been assumed for the synthesis 

 of the wave shapes in Fig. 47, this difference may according to the 

 data obtained amount to much more than this. 



It may well be asked why this frequency modulation, since it pro- 

 duces such marked distortion at night in certain places, does not also 

 give rise to distortion by day or in locations where transmission is 

 steady. A full answer to this question would be far from simple. But 

 in brief it is because the carrier and side-bands shift in absolute fre- 

 quency together as a unit so that their relative or difference frequencies 

 which determine the audio signal remain unchanged. Another way to 

 put it is that the detector operates on the envelope of the high-fre- 

 quency signals and is blind to the frequencies contained within the 

 envelope except insofar as they affect the latter. However, since 

 frequency modulation appreciably widens the frequency band occu- 

 pied by the radio signals it is to be expected that the tuned circuits 

 in the receiver would have some reaction on those louder portions of 

 the signal for which the amplitude modulation and therefore the 

 frequency modulation is large. The perfection with which broadcast 

 signals may be received under suitable conditions leads one to believe 

 that this effect must be small. 



Fading in Relation to Form of Transmission 



It has been shown that serious wave form distortion of the repro- 

 duced signal may result if frequency modulation occurs with the am- 

 plitude modulation and the transmission is subjected to night-time 

 conditions. This distortion from frequency modulation can be elimi- 

 nated by stabilizing the carrier frequency. There remain some wave 

 form distortion and the annoying amplitude changes caused by selec- 

 tive fading which is one of the most serious present day problems in 

 radio transmission. Let us now consider the nature and cause ot this 



^ See "Notes on the Theory of Modulation," by John R. Carson, Proc. Institute 

 of Radio Engineers, February, 1922. 



