FILTER DESIGN BY ITERATED ANALYSIS.

An alternative to the classical synthesis of an LC ladder filter is to adjust the element values until some property of the filter, computed directly from the ladder, matches what is desired, and thereby confirms that the element values are correct. The author describes a simple way of doing this by matching the poles, zeros, and scale factor of the characteristic function. The motivation behind this approach is to obtain high accuracy in the element values using a simple, noncritical computer program, at the expense of increased computing time. The amount of program to be written is both small and noncritical, particularly so if a packaged subroutine for solving linear equations is available. In almost all applications the element values are found correct to within one decimal digit of the mantissa size in the arithmetic that is used. The accuracy achieved is higher than can be provided by synthesis techniques, and is illustrated by means of an example of a 15th degree elliptic-function filter.