Page 446 - Complete Wireless Design

Wireless Issues

Wireless Issues 445

exhibit the actual RF current flow and density running through these struc-
tures. The common circuit theory–based algorithms of other simulation pro-
grams are typically inadequate to accurately model these types of microwave
structures and their stray coupling interactions, box modes, and discontinu-
ities. In fact, the EM simulator supplied free with this book, Sonnet Lite, is the
light version of one of the premier EM simulator packages on the market
today, Sonnet em® Suite, and is perfect for this type of wireless simulation.
If some type of valid computer simulation is not performed—even after care-
ful design and layout—a lot of rework will normally have to be performed in
order to get a microwave or RF circuit to function properly. This is because of
the abundance of undesired real-world internal component reactances and
resistances, along with the various component tolerances and temperature
effects, all conspiring to lower the expected performance of the circuit. And
unless an EM simulation is run, complex interactions of electromagnetic cou-
pling and certain stray parasitic reactances will decrease the resonant fre-
quency of a design below what was expected. Eagleware’s Genesys package is
one popular program that will simulate the unpredictable effect of lumped
components at high frequencies by permitting the designer to build a virtual
RF circuit within the computer itself, and then tuning it to function as desired.
With the inclusion of Eagleware’s EM module Empower, both circuit theory
and electromagnetic effects can be combined to allow the design of a more pre-
dictable circuit. However, even with less expensive linear computer simulation
packages—such as Caltech’s Puff—the development cycle can be tremendously
reduced by viewing how various circuit parameters are affected by component
tolerances, temperature variations, internal component reactances, and other
annoying contributors to disappointing circuit operation.

10.10.2 RF programs
There are many free and low-cost software programs that will assist the RF
designer in producing an electronic circuit—or even a complete wireless sys-
tem—in one-hundredth the time it would take to design with a hand-held cal-
culator, and will permit the engineer to tweak or change the design until it is
optimized. This is just not possible with hand calculation techniques in any
reasonable time. All of these programs as presented below run quickly and
reliably on everyday PCs, some with as old a CPU as a 486.
The first is the Windows™ version of Agilent’s AppCad. (The older DOS ver-
sion was indispensable in RF design, but is difficult to get to run on all
Pentium™-based Windows systems.) AppCad is a completely free program
from Agilent, and is included for your convenience with this book’s CD ROM.
AppCad helps the engineer to instantly design bias networks for BJTs, FETs,
and MMICs, as well as detector circuits and microstrip and stripline trans-
mission lines. It also has a reflection calculator to compute VSWR, return loss,
and mismatch loss for any desired input and output impedance of a circuit, a
noise calculator to compute a receiver’s NF, a standard-value calculator for

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