Page 27 - Complete Wireless Design

Wireless Essentials

26 Chapter One

This will significantly decrease the number of components, and thus the cost, of
high-volume items such as mobile phones, direct-conversion (zero IF) data radio
receivers, cheap Global Positioning System (GPS) receivers, wireless local-area
networks (LANs), pagers, and other (high-volume) systems-on-a-chip.
Many major companies, such as Harris Semiconductor, Hughes Networking,
National Semiconductor, Northern Telecom, and Tektronix, find this technology
important enough that they have obtained expensive foundry licenses for the
IBM SiGe BiCMOS technology.
The first products are just becoming available, and are small-density com-
ponents meant to supersede GaAs products. Inevitably, higher-integration
devices will be introduced that will lower cost and increase performance in
many high-volume wireless systems, with cellular phones being the primary
market.

1.3 Microstrip
1.3.1 Introduction
At microwave frequencies, microstrip (Fig. 1.32) is employed as transmission
lines, as equivalent passive components, and as tuned circuits and high-Q
microwave filters on printed circuit boards. Microstrip is used for these func-
tions for its low loss and ease of implementation, since high-frequency compo-
nents, such as surface mount capacitors, resistors, and transistors, can be
mounted directly onto the PCB’s microstrip metallization layer. The metal-
lization layer can be formed of copper or gold.
Microstrip itself is unbalanced transmission line and, because of its
unshielded nature, can radiate RF. However, radiation from properly termi-
nated microstrip is quite small. Stripline (Fig. 1.33) is similar to microstrip,
but is placed between the metallization layers of a PCB and, because of the
balanced twin ground planes, does not radiate. Both microstrip and stripline
normally have a printed circuit board substrate constructed of fiberglass, poly-
styrene, or Teflon. Since microstrip can utilize standard PCB manufacturing
techniques, it is easier and cheaper to fabricate than stripline.
The characteristic impedance of microstrip is governed by the width of the
conductor, the thickness of the dielectric, and the dielectric constant; low-

Figure 1.32 Microstrip, showing the dielectric and conductive layers.

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.