Ideal Wave Filters

TN the phenomenal advance of telephone practice during the past twenty years, almost every step has further restricted the distortion which individual parts of a transmission system can be allowed to introduce into the signal. T h e extension of circuits to great distances made it necessary that each link pass on to the next a more faithful copy of the signal so that the accumulated effects of many links might not endanger the intelligibility. The extension of telephone circuits to new uses, such as the transmission of pictures and the distribution of broadcast programs, imposed new demands for accuracy. Each of these has required rising standards of performance for wave filters. More than anything else, however, it has been the introduction of carrier methods, with their comparatively large utilization of selective structures, which has given prominence to the problem of reducing the distortion from wave filters. With the increase in length and complexity of carrier systems, the problem of providing wave filters which will have no harmful effect upon transmission has become one of increasing importance. W h a t this requires of the filters quickly appears if we recall that a structure which transmits all signals without distortion must (1) possess a characteristic impedance which is a pure resistance independent of frequency; (2) attenuate steady sinusoidal signals equally at all values of frequency; and (3) introduce a rotation in phase proportional to the frequency. In filter theory we need consider these requirements over only a limited band, since the signals which filters * T h e reader is referred to the preceding paper entitled " A General T h e o r y of Electric W a v e Filters." 215