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GENERAL NETWORK THEORY 213
tion of the frequencies of a given transmission band. Except for the unavoidable dissipation of the electrical elements, these networks do not attenuate the frequencies of their transmission range. Phase correctors are used with transmission systems to correct for system phase distortion similar to the manner in which attenuation equalizers correct for attenuation distortion. These networks are not within the scope of this material. (See Chapter XIII.)
It has been found that the engineering of networks is considerably facilitated by the use of distinctive unit structures operated in tandem to form configurations which, in many respects, resemble a carpenter's ladder. Many of the symbols and conceptions used in network theory thus have their origin in these "ladder-type" networks. For instance, referring to Figure 15 6A, the circuit shown is a ladder-type structure having series impedances denoted by the symbol ZA and shunt impedances denoted by ZB. If we arrange this circuit as shown in Figure 156-B, and remove a portion of the network by cutting through two adjacent series arms at their mid-points, as shown by "AA" and "BB," we obtain the familiar T type section of Figure 156-C. The reason then for designating the series arms of this section by the symbol ZA/2 and the shunt arm by the symbol ZB is thus made clear. Also, the origin of the term "mid-series terminals" when referring to either of the pairs of terminals of a T section is evident. In a similar way, if we remove another portion of the ladder-type network by cutting through two adjacent shunt arms at their mid-points in the manner shown by "CC" and "DD," we obtain the -k type section of Figure 156-E. Following the same method of designation, the series arm impedance of this section is denoted by the symbol ZA> the shunt arms by the symbol 2 ZB) and the two pairs of terminals are called "mid-shunt" terminals. Another familiar ladder-type section which can be obtained by removing a portion of the general ladder structure is the L type section of Figure 1 56-D. This section has a series arm of ZA/2 and a shunt arm of 2 ZB. The 1-2 terminals of the L section of Figure 156-D are the midseries terminals and the 3-4 terminals are the mid-shunt terminals. L type sections are often referred to as half sections for the reason that two L sections connected together form one T or one ir type section depending upon whether they are joined at their mid-shunt or midseries terminals.
Transmission systems employing networks usually are arranged so that the impedances connected to the input and output terminals of the networks are resistances. That is, the impedance looking back into the system at the network input terminals is chiefly resistive in character as is also the load impedance connected to the output terminals.