Chapter 14 - Analog Low-Pass Filters

Abstract

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. In this chapter, the basics of analog low-pass filtering are discussed. It reviews the 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. These techniques are useful for designing analog filters as well as prototypes used for digital filters. The Butterworth is a class of filters that provides maximally flat response in the passband. The pole locations for an Nth-order Butterworth filter are found equally spaced around a circle with radius equal to the filter cutoff frequency. On the other hand, the Chebyshev filter has some tipple in the passband, and a sharper cutoff in the transition band than the Butterworth filter. The Bessel filter is optimized to provide constant group delay in the filter passband, while sacrificing sharpness in the frequency response. For high-frequency filters, one option is to build a passive ladder using resistors, inductors and capacitors.