Page 436 - Complete Wireless Design

Wireless Issues

Wireless Issues 435

design techniques. For instance, the above DC offset dilemma can be miti-
gated by transitioning from a direct conversion, zero-IF architecture to a very
low—but not zero—baseband frequency. This allows the operation of AC cou-
pling into the mixer stage, which eliminates the DC offset, while applying an
LO that is not at the same frequency as the RF. However, costs rise because
of the need for a higher-frequency ADC. (The LO radiation from the antenna
can also be attenuated by a high-reverse-isolation LNA.) Many other
improvements in DCR design will be forthcoming in the next few years.
Despite the above implementation problems, zero-IF receivers are becom-
ing popular in wireless design simply because they require fewer components
than the standard superheterodyne technology, and are therefore much
cheaper to build.

10.7 Prototyping

10.7.1 Introduction
The construction of any electronic prototype encompasses many disciplines:
engineer, technician, assembler, and mechanic. A properly constructed proto-
type can sometimes make or break a wireless project, as a prototype that does
not function as expected can, frequently, damage the confidence level in an
entire wireless design.

10.7.2 Prototyping considerations
When specifying and building a prototype for a wireless project, confirm that
all parts and components selected will not be thermally or mechanically
stressed; if so, the utilization of a heat sink or a more robust component is
indicated. Do not permit components to operate above their maximum volt-
age, current, or power rating, and allow enough derating to permit long and
safe operation. Place the completed circuit or product prototype under ther-
mal, humidity, and vibration stress testing to check for any design or imple-
mentation flaws. Do not purchase unknown or suspect parts simply because
they are low in cost, and test and specify all parts from unknown manufac-
turers. In a multiprototype project build only one first, since the redesign or
reshuffling of circuits or components may be required after the first unit’s
test and measurements are complete. Allot enough time not only to construct
the prototype, but also to debug the myriad problems that will surface in any
complex project.
Presented are some of the more common issues encountered during proto-
type design and construction, along with their solutions:

Confirm that all digital and analog power and grounds remain separated in
order to reduce hash within the RF sections.
Check that all power supplies are fully decoupled, and that all of the
appropriate parts have their proper V and ground.
CC

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.