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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



for unrestricted operation is equal to the required receiver selectivity- 

 given in Table III and can be translated into frequency space in the 

 following manner. 



Both AM and FM transmitters radiate some noise and distortion 

 products outside of the ideal modulation bandwidth. In addition, some 

 of the sideband energy in FM falls outside the desired modulation band- 

 width. The magnitude of the undesired FM sideband radiation is higher 

 than the noise immediately outside of the desired band, but it decreases 

 more rapidly with the result that the noise is usually controlling in the 

 region where the extra band radiation is more than 60 to 70 db down. 



The guard bands that are required between the edge of the desired 

 transmitted band and the frequency at which the necessary suppression 

 of extra band radiation can be obtained are estimated in Table V. 

 These values depend on the width of the voice band and are relatively 

 independent of the radio frequency since ri. selectivity is not possible. 



Measurements on present day transmitters correspond to the above 

 estimates for values of suppression less than 80 db, but a frequency 

 separation of nearly one megacycle or more is needed for suppressions of 

 100 and 120 db. This limitation is not expected to be inherent so more 

 optimistic estimates are indicated in Table V. If the present charac- 

 teristics cannot be improved, that is, if suppressions greater than about 

 80 db cannot be obtained. Table III indicates that some interference 

 may be expected within about one-half mile of an unwanted transmitter. 



A comparison of the information given in Tables III, IV and V in- 

 dicates that the guard bands required for unrestricted operation are 

 approximately 100, 50 and 25 kc for minimum separations between 

 transmitter and receiver of 50 feet, 500 feet and one-half mile, respec- 

 tively. These values together with the bandwidth needed for modula- 

 tion and for frequency instability determine the frequency separation 

 required between channels operating in the same area and are sum- 

 marized in Table VI. 



Table V — Guard Bands Required to Avoid Extra 

 Band Radiation 



