Evolution of passive and active microwave filters

“All the World is a Filter” is true today, but was it always so? The study of passive networks stemmed from the need to understand the performance issues associated with the early power grid at the beginning of the 20^th century. Development of radio communication necessitated careful attention to parasitic elements and the limitations of simple representations of inductors, capacitors, etc. The evolution of distributed elements as extensions of lumped descriptions led to significant developments in the area of synthesis, meaning that performance could be tailored to meet needs. Approximation of passive elements by active equivalents is still a work in progress. The danger is ignorance of basic principles and dependence on simulation rather than analytic thinking. This paper will explore history, discuss the present, and try to look into the crystal ball to see what might be just around the corner!