Selective Circuits and Static Interference

Abstract
The present paper has its inception in the need of a correct understanding of the behavior of selective circuits when subjected to irregular and random inteference, and of devising a practically useful figure of merit for comparing circuits designed to reduce the effects of this type of interference. The problem is essentially a statistical one and the results must be expressed in terms of mean values. The mathematical theory is developed from the idea of the spectrum of the interference and the response of the selective circuit is expressed in terms of the mean square current and mean power absorbed. The application of the formulas deduced to the case of static interference is discussed and it is shown that deductions of practical value are possible in spite of meager information 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 be 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.

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