Page 434 - Complete Wireless Design

Wireless Issues

Wireless Issues 433

Metallization is the conductive layer of the interconnecting traces and pads
placed onto the substrate. It can be made of copper, gold, or silver deposited on
the substrate board material. The entire PCB itself can be called a metallized
substrate in these hybrid applications. The board material is usually a type of
ceramic, such as alumina, aluminum nitride, or beryllia. These ceramics are
extremely rigid and are quite temperature stable, besides having a very high-
strength characteristic, attributes that make ceramics perfect as a substrate
material for hybrid applications. Alumina (aluminum oxide) is by far the low-
est in cost and most popular. It is a high-frequency (up to 25 GHz), very hard
substrate material that does not require a carrier (heavy metal stiffening
plate), and has a very high dielectric constant of around 9.8 (for small circuit
layout sizes). This material is used in applications that require rigidity,
strength, and temperature stability, along with decent thermal conductivity.
The next substrate, aluminum nitride, is found only in specialized hybrid
applications that require better thermal conductivity, but at a substantially
increased cost, over alumina. Beryllia (beryllium oxide), which is even higher
in cost than aluminum nitride, is seen in applications where low dielectric con-
stants are needed (around 6), as well as improved thermal conductivity.
However, Beryllia dust particles are toxic, and must not be inhaled when this
dangerous substrate material is machined.

10.6 Direct-Conversion Receivers
10.6.1 Introduction
Direct-conversion receivers (DCRs, also called zero-IF receivers; Fig. 10.16)
have seen only limited use because of implementation complexities. A DCR is

Figure 10.16 A direct conversion receiver block diagram.

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