Selective Circuits and Static Interference* 



By JOHN R CARSON 



SvNovsi-.: riu- protiit i>.i|H'r has its iiui-ption in the iummI of ,i corrcri 

 imilcrst.iiKliiij; of the Itchavior of selective circuits when siil>jeileil to ir- 

 roRiilar and random interference, ami of devisiiiK a practically useful 

 figure of merit for conipiiring circuits desiKne<l to reduce the effects of this 

 type of interference. The problem is essentially a statistical one and the 

 results must Ix; expressed in terms of mean values The mathematical 

 theory is developed from the idea of the spectrum of the interference and 

 the res|Hinse of the selective circuit is expressed in terms of the mean 

 square current and mean power al>sorlK-(l. The application of the formu- 

 las deduce<l to the case of static interference is discussed and it is shown 

 that deductions of practical value are possible in spite of meagre informa- 

 tion regarding the precise nature and origin of static interference. 



The outstanding deductions of practical value may be summarized as 

 follows: 



1. Even with absolutely ideal selective circuits, an irreducible minimum 

 of interference will Ix; absorbed, and this minimum increases linearly 

 with the frequency range necessary for signaling. 



2. The wave-filter, when properly designed, approximates quite closely 

 to the ideal selective circuit, and little, if any, improvement over its present 

 form may be expected as regards static interference. 



3. .As regards static or random interference, it is quite useless to employ 

 extremely high selectivity. The gain, as compared with circuits of only 

 mo<lerate selectivity, is very small, and is inevitably accompanied by 

 disadvantages such as sluggishness of response with consequent slowing 

 down of the [wssible speed of signaling. 



4. .A formula is developed, which, together with relatively simple ex- 

 jwrimcntal data, provi<lcs for the accurate determination of the spectrum 

 of static interference. 



5. An application of the theory and formulas of the paper to repre- 

 sentative circuit arrangements and schemes designe<i to reduce static 

 interference, shows that they are incapable of reducing, in any substantial 

 degree, the mean interference, as compared with what can be done with 

 simple filters and tuned circuits. The underlying reason lies in the nature 

 of the interference itself. 



I 



' I ^ H f-1 selecti\e circuit is an e.xtremely important flenieiii of e\er\' 

 -*• radio receiving set, and on its efficient design and operation 

 depends the economical use of the available frequency range. The 

 theory and design of selective circuits, particularly of their most 

 conspicuous and important type, the electric \va\'e filter, ha\'e been 

 highly de%'eloped, and it is now possible to communicate simultane- 

 ously without undue interference on neighboring channels with a 

 quite small frequency separation. On the other hand too much has 

 been expected of the selective circuit in the way of eliminating types 

 of interference which inherently do not admit of elimination by any 

 form of selecti\e circuit. I refer to the large amount of inventive 

 thought devoted to devising ingenious and complicated circuit ar- 



• Presented at the Annual Convention of the .A. I. E. E., Edgewater Beach, Chicago, 

 III., June 23-27, 1924. 



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