# Filter Tables and Computer Programs (The Physical Approach to the Approximation Problem)

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## Abstract

From the filter specifications and the transfer function *T*(*s*) = *N*(*s*)/*D*(*s*), classical network theory provides a passive LCR filter network. This is obtained from a filter table or a computer program. To understand how the filter components and tables are obtained, we must reformulate the approximation problem. It now becomes the “physical problem formulation” as opposed to the “mathematical problem formulation” discussed in Chap. 3. Instead of the characteristic function, as in the mathematical formulation, we now introduce a concept borrowed from transmission line theory, namely, the concept of a *reflection coefficient*.

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