Network Theory, Filters, and Equalizers

Part III is concerned with ladder and lattice filters, attenuation and phase equalizers, and dividing networks. The fundamental equations and design procedures involved in both ladder and lattice filters are reviewed. Tables and charts are given for aiding in the design of M-derived filters having terminal half sections. It is possible in this way, without making calculations other than placing numbers in simple formulas, to design filters having characteristics that will meet most requirements. Consideration is given to filters terminated with other than half sections, particularly MM′ and fraction terminations. A chart is given for the design of MM′ terminating sections, which give unusually good impedance characteristics at the terminals. Fractional terminations, which are used when filters are to be placed in series or in parallel, are considered with particular reference to complementary filters. The factors controlling the cutoff frequencies, attenuation and phase characteristics, and impedance characteristics of lattice filters are reviewed. The attenuation and phase behavior of such filters can be given almost any desired characteristic by suitably locating an adequate number of critical frequencies (i.e., zeros and poles of impedance) in the pass bands, while the behavior of the image impedances can be similarly controlled by the location and number of the critical frequencies in the attenuating bands. Particular consideration is given to the requirements that the critical frequencies in the pass band must satisfy in order to obtain a substantially linear phase-shift characteristic in the pass band. The relationship between ladder and lattice filters is reviewed.