Page 2 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
1

Wireless Essentials

A firm understanding of how passive and active components function at high
frequencies, as well as a strong grasp of the fundamental concepts of lumped
and distributed transmission lines, S-parameters, and radio-frequency (RF)
propagation, is essential to successful circuit design.

1.1 Passive Components at RF
1.1.1 Introduction
At radio frequencies, lumped (physical) resistors, capacitors, and inductors are
not the “pure” components they are assumed to be at lower frequencies. As
shown in Fig. 1.1, their true nature at higher frequencies has undesirable
resistances, capacitances, and inductances—which must be taken into account
during design, simulation, and layout of any wireless circuit.
At microwave frequencies the lengths of all component leads have to be min-
imized in order to decrease losses due to lead inductance, while even the board
traces that connect these passive components must be converted to transmis-
sion line structures. Surface mount devices (SMDs) are perfect for decreasing
this lead length, and thus the series inductance, of any component (Fig. 1.2),
while the most common transmission line structure is microstrip, which main-
tains a 50-ohm constant impedance throughout its length—and without
adding inductance or capacitance.
As the frequency of operation of any wireless circuit begins to increase,
so does the requirement that the actual physical structure of all of the
lumped components themselves be as small as possible, since the part’s
effective frequency of operation increases as it shrinks in size: the smaller
package lowers the harmful distributed reactances and series or parallel
resonances.

1
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.