. The Bell System technical journal . he use of trans-formers. One such circuit is shown schematically by Fig. 14. Filters made by using either series or shunt coils in conjunction ELECTRICAL WAVE FILTERS 423 with condensers and crystals make very acceptable band-pass filterscapable of moderate band widths. It is often desirable to obtain lowand high-pass filters having a very sharp selectivity. The filter of can be modified to give a high-pass characteristic by leaving outthe coils in the series or lattice arms of the network. However, it willbe found that the cross-over points in the
. The Bell System technical journal . he use of trans-formers. One such circuit is shown schematically by Fig. 14. Filters made by using either series or shunt coils in conjunction ELECTRICAL WAVE FILTERS 423 with condensers and crystals make very acceptable band-pass filterscapable of moderate band widths. It is often desirable to obtain lowand high-pass filters having a very sharp selectivity. The filter of can be modified to give a high-pass characteristic by leaving outthe coils in the series or lattice arms of the network. However, it willbe found that the cross-over points in the impedance curve of necessitycome very close to the pass band and hence no appreciable loss can bemaintained at frequencies remote from the pass band. A broader andmore useful characteristic is obtained by using a transformer havinga preassigned coefficient of coupling, in conjunction with crystals andcondensers, as the element for broadening the separation of an element is shown by Fig. 15^. As is well known, a trans-. fa A FREQUENCV Fig. 15—Impedance characteristic of a transformer, condenser, and crystal. former with a specified coupling can be replaced by a T network ofthree inductances as shown by Fig. \SB. The impedance character-istic, as shown by Fig. 15 C, has two anti-resonant frequencies/i and/a,and two resonant frequencies f^ and fi. Suppose now that an element of this type is placed in one arm of thelattice and a similar element having a condenser in series with it isplaced in the other arm as shown by Fig. \6A. If the elements are soproportioned that the anti-resonances of one arm coincide with theresonances of the other arm and vice versa, as shown by Fig. \6B, theimpedances of the two arms are of opposite sign till the last , a low pass filter results. It is possible to make the two im-pedance curves cross five times, so that an attenuation correspondingto five simple sections of low-pass filter results. Other arrangements ofthe r
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