TABLE OF CONTENTS The basics of analog low-pass filtering are discussed. These techniques are useful for designing analog filters as well as prototypes used for digital filters. This chapter is by no means an all-inclusive tutorial on analog circuit design; rather it is introductory in nature and the reader is referred to other texts for more details, if necessary.
The low-pass filter is a ubiquitous component in many different kinds of signal-processing systems. Channel-separation, A/D antialiasing, and general signal processing are applications for low-pass filters, just to name a few. Even if you are a digital filter designer, it behooves you to know something about analog filter design, since many digital filters begin as analog prototypes, and are then transformed to the digital domain. Also, DSP systems generally have an analog front end that includes an analog low-pass filter for antialiasing purposes (Figure 14-1).
As with any kind of design, the devil is in the details. Your specification will lead you to choices in filter topology and filter order, depending on the attenuation you need, the ripple that you can live with, and also the group delay variation that you can live with. In the following sections, we discuss design issues associated with low-pass filter design. The results can be extended without much trouble to band-pass and high-pass filters as well.
The magnitude response of the ideal low-pass filter is shown in Figure 14-2a. The gain of this...
