TABLE OF CONTENTS Why study filter theory? What are filters? Where are they used? Can you just get them on integrated circuit chips like many other circuit functions? There are many questions to answer to get started on a journey to understand the electronic filters of today.
Filters are everywhere. Many times they are key elements required to optimize the performance of a complex system. Band-limiting filters are used in front of analog-to-digital converters to prevent signal energy from overlapping in the aliased sampled signal spectrum. They are used to smooth the digital-to-analog reconstructed signals in synthesized speech or other information. Weather-mapping facsimile receiver/transmitters are an example of this digital sampling, transmission, and signal reconstruction process. Weather maps are transmitted continuously and are liberally used by the news media. Filters are used to compensate for frequency response so that closed-loop systems will be stable and provide optimum response to suddenly applied perturbations. Gated ranging radar circuits, such as those used in the Lunar Excursion Module (LEM), use active electronic filters to improve resolution as the target landing spot is approached.
Large aperture seismic arrays (LASAs) are installed underground all around the world. Each installation employs many receiver paths that have 2-Hz lowpass active filters on their input. These filters are designed and produced to exacting theoretical specifications that cannot be met (in size, space, and cost) with other technologies. The filters are guaranteed to outlast other components since it is very expensive to dig up and repair any receiver element. Whoever manufactures...
