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372 H. R. KlMBALL [J. S. M. P. E.
Wave-Filters. — Electric wave-filters, like attenuation equalizers, are four-terminal networks having a pair of input terminals and a pair of output terminals. Between the input and output terminals is an orderly array of electrical elements arranged so as to produce a specified insertion-loss characteristic when connected between the proper terminal impedances. Unlike equalizers, resistive elements are excluded from wave-filters; only inductive and capacitive elements being used to interconnect the input and output terminals. The reason for the exclusion of resistive conductors is contained in the purpose of a filter; i. e., to transmit, without appreciable loss, all frequencies of the transmission band, and to attenuate by a prescribed amount frequencies lying outside this band. If resistive elements were used attenuation would result within the transmission band.
Electric wave-filters consist usually of a number of filter sections of unit four-terminal networks connected in tandem on a matchedimpedance basis to form the complete filter. It is not necessary that a filter consist of more than one section, but usually the transmission characteristic desired is such as to require the use of multiple sections. In this respect filters are different from equalizers, where in a great many cases, the desired equalization curves may be secured without tandem operation of sections.
Conventionally designed wave-filters seldom provide constant impedances at their terminals over the operating frequency range, and for that reason it is usually not possible to achieve a match of impedance between the wave-filter and the system to which it is connected, even though the impedances of the latter are constant resistances. In general, the terminal impedances of filters are largely resistive in their transmission bands and reactive in the attenuation ranges. In addition, in the transmission range the resistive characteristics vary with frequency, especially in the cut-off region. While various methods are available of stabilizing these impedances to almost any desired precision, most filters, as arranged for commercial purposes, provide some mis-match at their terminals. The insertion loss of a filter takes into account these terminal effects and for that reason it is important in operating filters to make sure that the proper connecting impedance conditions are obtained.
Although wave-filters transmit the frequencies of their transmission band without appreciable attenuation loss, they do shift the relative phases of all the frequencies. This is an inherent feature