216 BELL SYSTEM TECHNICAL JOURNAL 



are meant to transmit, whether voice, telegraph, or television, are of 

 a specified type having energies concentrated in certain portions of the 

 frequency spectrum. We can therefore say that an ideal filter is one 

 which has the ideal phase, impedance and attenuation properties in 

 the frequency range of the desired signal and which totally suppresses 

 all other frequencies. 



The conventional ladder type filter structures which have been so 

 extensively studied may be made to yield any desired suppression at 

 the unwanted frequencies. In the range of wanted frequencies, 

 however, they show wide departures from all three ideal properties. 

 The impedance characteristic can be greatly improved by suitable 

 elaboration of the filter structure itself, but to approximate uniformity 

 of loss or linearity of phase shift it has been necessary to make use of 

 supplementary networks of empirical design.^ 



The design of such corrective networks is by no means an easy task, 

 primarily because the filter characteristics for which they are supposed 

 to compensate change very rapidly with frequency in certain intervals. 

 Nevertheless, much has been achieved. Thus it has been found 

 possible to limit reflection coefficients to 2 per cent, in contrast with 

 coefficients of 50 per cent not uncommonly tolerated in the systems 

 of ten years ago. Improvements in the other characteristics have 

 been comparable. In modern systems variations in attenuation of a 

 few hundredths of a decibel, or in phase slope of a few per cent, can 

 be attained if need be. These limits, however, demand the most 

 patient and skillful design, and can seldom be met unless control of a 

 single one of the characteristics is especially important. Since 

 amplitude equalizers introduce non-linear phase, phase correctors non- 

 uniform loss, and so on, the problem becomes increasingly difficult 

 when close requirements must be met in several characteristics 

 simultaneously. 



By contrast, the method proposed in this paper gives the various 



characteristics simultaneously in a single network without recourse to 



auxiliary corrective structures. The method is a systematic one, 



requiring comparatively little in the way of cut and try design work. 



At the same time it preserves a measure of the flexibility of the existing 



technique, so that when considerable deviation from the ideal is 



tolerable in one or more characteristics, a corresponding economy of 



materials may be effected. 



^ The distortion problem has been discussed by several writers in this Journal. 

 See, for example, S. P. Mead, "Phase Distortion and Phase Distortion Correction," 

 April, 1928, p. 195; O. J. Zobel, "Distortion Correction in Electrical Circuits . . .," 

 July, 1928, p. 438; C. E. Lane, "Phase Distortion in Telephone Apparatus," July, 

 1930, p. 493; E. B, Payne, "Impedance Correction of Wave Filters," October, 1930, 

 p. 770. 



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