64 THE BELL SYSTEM TECHNICAL JOUKNAL, JANUARY 1953 



electrical characteristics. This paper discusses the number of potential 

 interferences, and in effect, envisages a situation where potential inter- 

 ferences are strong enough to be actual interferences.* 



GENERAL 



Intermodulation products are commonly referred to as 2nd, 3rd, 4th 

 ••• nth order products depending on the order of nonlinearity which 

 gives rise to the products. Interference within a system is not generally 

 experienced from the even order products because the frequency sepa- 

 ration between the channels involved and the product formed by them 

 is so great that the selectivity of the transmitter and receiver radio fre- 

 quency circuits is sufficient to reduce it to a negligible amount. Some of 

 the odd order products can be discounted also for the same reason. 

 There are odd-order products, however, involving both sums and dif- 

 ferences of operating frequencies in such fashion that the frequencies 

 of the products formed are very close to those which generated them. 

 These products are those referred to throughout the remainder of this 

 paper since they are the most likely to cause interference. The most 

 general form of 3rd order interference occurs when three frequencies, 

 A, B and C, intermodulate in such fashion as to produce interference on 

 a channel operating at frequency D. In this case 



A+ B - C = D 



Another form of 3rd order interference occurs when the second har- 

 monic of A intermodulates with B to produce interference on a channel 

 operating at frequency C. In this case 



2A - B = C 

 In like fashion the following forms of 5th order interference may occur. 

 A + B + C-D-E^F 

 2A+B-C-D = E 

 A+B+C-2D = E 

 2A + B -2C = D 

 3A - B - C = D 

 3A -2B = C 



* Bullington, K., Frequency Economy in Mobile Radio Bands. Page 42 of 

 this issue. 



