Intermodulation Interference in 

 Radio Systems 



Frequency of Occurrence and Control by Channel 



Selection 



By WALLACE C. BABCOCK 



(Manuscript received August 25, 1952) 



Intermodulation interference becomes a serious factor in frequency usage 

 when a block of consecutive channels is provided for a given type of radio 

 service in a confined area. Formulas are presented which show the number 

 of potentially interfering 3rd and 5th order intermodulation products that 

 can be formed in a band of n consecutive channels. The probability of en- 

 countering interference when a number of operating channels are picked 

 at random from this band of n channels is developed and the number of in- 

 terference free operating channels that can be obtained by careful selection 

 in this same band is also derived. 



When a block of consecutive radio channels is used in a confined area 

 to provide a given type of service, interference becomes a serious prob- 

 lem. The situation is aggravated by the fact that whenever energy at 

 two or more radio frequencies combines in a nonlinear circuit, as in 

 transmitter output stages or in receiver input stages, products at other 

 than the original frequencies are created. These are called intermodula- 

 tion products, and they are capable of causing serious interference 

 within the block of channels assigned to a given type of service as well 

 as in other bands assigned to other types of service. 



It is important in engineering a service to know something about the 

 nature of these products in order to evaluate their interference poten- 

 tialities and to study means of controlling or minimizing that interference. 

 The numbers and locations of various types of intermodulation products 

 are susceptible to mathematical computation. Whether or not all of 

 these products would produce actual interference depends on the geo- 

 graphical locations of transmitters and receivers, and on their specific 



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