46 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



The bandwidth needed to pass the desired signal depends on the fre- 

 quency stabiHty that can be maintained as well as on the type of modula- 

 tion. The allowance for frequency drift includes the variations in both 

 transmitters and receivers. The importance of these figures is indicated 

 in Table II which shows the tolerances needed for frequency instability. 

 For example, with an overall frequency stability of d=0.002 per cent the 

 channel width at 450 mc needs to be 18 kc wider than the minimum band- 

 width required to pass the modulated signals. 



The use of frequency modulation has several important advantages 

 that cannot be readily obtained with AM. The instantaneous gain con- 

 trol and the closer co-channel spacings have already been mentioned. 

 In addition, for the same radiated power, FM with a frequency swing 

 greater than about ±3 kc has the well known advantage of providing a 

 higher output signal-to-noise ratio throughout most of the coverage 

 area than is possible with double sideband amplitude modulation; this 

 FM advantage is substantially reduced when the IF bandwidth is large 

 compared with the bandwidth required to pass the desired sidebands. 



The bandwidth required for frequency modulation of a 3 kc voice 

 band must be at least ±3 kc. For reasonable FM signal-to-noise ad- 

 vantage, particularly in the presence of impulse noise, the frequency 

 swing should be at least ±5 kc which requires a bandwidth of d=8 kc 

 for good quality. The corresponding bandwidth for amplitude modula- 

 tion is ±3 kc; the use of single sideband AM transmission does not seem 

 feasible, at least not for single channel operation. 



LIMITATIONS IMPOSED BY INSUFFICIENT (tOTAL) FILTERING 



The frequency separation between carrier frequencies must be greater 

 than the bandwidth required to pass the desired signal because addi- 

 tional frequency space or guard bands are needed to build up receiver 

 selectivity against undesired signals and to avoid the extra band radia- 

 tion from transmitters. The power of a 100 watt transmitter is about 



Table II — Tolerance Needed for Overall Frequency 



Drift 



