Impedance Correction of Wave Filters

HE rapid increase in the demand for long distance or toll telephone service in recent years led to the introduction, about 1920, of carrier systems as a means of securing more intensive use from long telephone lines. The growth of these circuits has resulted, still more recently, in the multiplication of the number of carrier systems in use and in the close association of several similar or different carrier systems on a single pole-line. This development raised a number of totally new engineering problems and demanded careful reconsideration of m a n y other questions of comparatively small importance in earlier systems. Among the factors thus brought into prominence by carrier system development, the chief, for the purpose of this paper, is the impedance mismatch between telephone lines and repeaters or terminal apparatus. The components of a complete transmission system, such as the line 770 IMPEDANCE CORRECTION OF WAVE FILTERS 771 itself, various transmission networks, amplifiers, modulators, electroacoustic apparatus, etc. are quite dissimilar physically and as we might naturally expect, these physical differences manifest themselves in many instances as pronounced dissimilarities in the forms of the impedance-frequency characteristics. For example, the characteristic impedance of a uniform line varies smoothly with frequency, but that of a wave filter changes abruptly as we go from the transmitting to the attenuating range. In spite of the possibility of changing the general impedance level by the insertion of a transformer such inherent "incompatibilities of temperament" between the characteristic impedances of the various components of the telephone circuit must lead normally to impedances which resemble each other only in narrow frequency bands and which may differ widely over large and important portions of the frequency spectrum.