Page 22 - Analog and Digital Filter Design

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CHAPTER

INTRODUCTION

This chapter gives an introduction to filters and signals, and the terminology
used in relation to filters. Experienced engineers may wish to skip this chapter.

Fundamentals

Why Use Filters?
Why are you so interested in filters? This was a question put to me when I was
planning this book. It is ;I very good question. I have been involved with elec-
tronic system design for a number of years and have found that the perform-
ance of an electronic filter can determine whether the system is successful.
Detection of a wanted signal may be impossible if unwanted signals and noise
are not removed sufficiently by filtering. Electronic filters allow some signals to
pass, but stop others. To be more precise, filters allow some signal frequencies
applied at their input terminals to pass through to their output terminals with
little or no reduction in signal level.

Analog electronic filters are present in just about every piece of electronic equip-
ment. There are the obvious types of equipment, such as radios, televisions. and
stereo systems. Test equipment such as spectrum analyzers and signal genera-
tors also need filters. Even where signals are converted into a digital form. using
analog-to-digital converters, analog filters are usually needed to prevent alias-
ing. Computers use filters: to reduce EM1 (electro-magnetic interference) emis-
sions from their power lead; to smooth the output of the switched-mode power
supply: to limit the video bandwidth of signals going to the display.

What Are Signals?
Before describing filters in detail. it is important to understand the characteris-
tics of signals. A signal can be described in the time domain or in the frequency
domain. What does this mean’?