Page 19 - Analog and Digital Filter Design

1 6 Analog and Digital Filter Design

are also described (amplifier parameters can have a significant effect), as are
measurement techniques.

Chapter 1 1 describes current software availability, including integrated
circuit-specific software. The actual filter design process can be considerably
automated. Indeed, I have written a program with Number One Systems Ltd.
called FILTECH, which designs and simulates filter circuits. I outline how
FILTECH operates at a systems level. There are also other programs on the
market. Some of these only design active filters; they are offered free because
they enable users to design filters using certain manufacturers’ integrated
circuits.

Executable PC programs, capable of designing useful filters, are supplied at
www.bh.com/companions/0750675470. This chapter basically serves as a user
guide. describing their operation. These programs are far simpler than
FILTECH and give a netlist compatible with SPICE-like analysis programs.

Chapter 12 describes how transmission lines can be used to filter signals.
Quarter-wave lines of either short or open circuit termination can be used to
pass or stop certain frequencies. One application of this is to allow a radio
carrier signal into a receiver from an antenna while preventing internal radio
signals from radiating back to the antenna.

Printed circuit board (PCB) filters are also described. Tracks on a PCB can be
transmission lines when the signal frequency is high. The width of a track on a
printed circuit board defines its impedance; sections of wider or narrower track
become inductive or capacitive. Concatenation of narrow and wide track sec-
tions can therefore form an LC (inductor capacitor) filter.
Phase-locked loop filters are usually quite simple, but poor design can cause
instability of the loop. Many people avoid designing phase-locked loops for this
reason. Chapter 13 provides some examples that may help remove some of this
fear.

Chapter 14 provides an introduction to switched capacitor filters. Commercial
filter ICs (integrated circuits) are described and plots of some practical exam-
ples are given. Problems with this type of filter are described, as are some of the
benefits such as being able to make the filter cutoff programmable or adjustable.

Chapter 15 outlines the process of digital filtering. In this chapter I cover the
data sampling operation (under-sampling, over-sampling, interpolation, and
decimation) and the advantages or problems of each. A brief outline of digital
filtering techniques provides some understanding of digital signal processing.
Digital signal processors (DSPs) are described, along with the mathematical
methods by which they handle data during signal processing.