SF.LECTIIE CIRCUITS AND STATIC INTERFERENCE 267 



(5) Formulas given below for comparing (he relative efficiencies 

 of selective circuits on the basis of siRnal-to-interference cnerRV ratio 

 are believed to have considerable practical value in estimatinR the 

 relative utilit\- of selective circuits as regards static interference. 



II 



Discriinin.ition between signal anil inlcrfcrcnce liy means of selec- 

 tive circuits depends on taking advantage of differences in their wave 

 forms, and hence on differences in their frequency spectra. It is 

 therefore the fimction of the selective circuit to respond effectively 

 to the range of frequencies essential to the signal while discriminating 

 against all other frequencies. 



Interference in radio and wire cotnmunication may be broadly 

 classified as systematic and random, although no al)solutely hard and 

 fast distinctions are possible. Systematic interference includes those 

 disturbances which are predominantly steady-state or those whose 

 energy is almost all contained in a relatively narrow band of the 

 fre(|uency range. For example, interference from individual radio- 

 telephone and slow-speed radio telegraph stations is to be classified as 

 systematic. Random interference, which is discussed in detail later, 

 may be pro\isionally defined as the aggregate of a large number of 

 elementary disturbances which originate in a large number of un- 

 related sources, vary in an irregular, arbitrary manner, and are char- 

 acterized statistically by no sharply predominate frequency. An 

 intermediate type of interference, which may be termed either quasi- 

 systematic or quasi-random, depending on the point of view, is the 

 aggregate of a large number of individual disturbances, all of the same 

 wave form, but having an irregular or random time distribution. 



In the present paper we shall be largely concerned with random 

 interference, as defined above, because it is believed that it repre- 

 sents more or less closely the general character of static interference. 

 This question may be left for the present, however, with the remark 

 that the subsequent analysis shows that, as regards important prac- 

 tical applications and deductions, a knowledge of the exact nature 

 and frequency distribution of static interference is not necessary. 



Now when dealing with random disturbance, as defined above, no 

 information whatsoe\-er is furnished as regards instantaneous values. 

 In its essence, therefore, the problem is a statistical one and the 

 conclusions must be expressed in terms of mean values. In the 

 present paper formulas will be derived for the mean energy and tnean 

 square current absorbed by selective circuits from random interfer- 



