Page 449 - Complete Wireless Design

Wireless Issues

448 Chapter Ten

A Spice program of some sort will always be required for certain circuit sim-
ulation tasks. Electronics Workbench (EWB), by Interactive Image
Technologies, is quite a popular Spice simulator and is very simple to use.
EWB allows the viewing of a lower-frequency circuit’s output in real time—as
with any real-life oscilloscope or VOM—instead of in batch format as in most
other Spice simulators. And EWB has just released a beta version of its soft-
ware that is capable of simulating at RF frequencies. The RF simulation is
accomplished with special active and passive high-frequency models, along
with a built-in RF spectrum and network analyzer. However, any Spice simu-
lator will have not only convergence problems (the inability to reach a solu-
tion), but will also be significantly slower than linear simulators. There are
many other GUI Spice programs on the market, with some as low as $50.
There are a few other effective S-parameter-based tools available, such as
Stanton’s low-cost text-based NOVA-686, or the much more expensive high-
end programs that are able to model circuit theory and electromagnetic effects
of all kinds, such as the ubiquitous Genesys by Eagleware, the new Microwave
Office by AWR, and EEsof by Agilent. AWR’s and Agilent’s software packages
permit the designer to view the nonlinear effects of any circuit you design, as
well as the harmonic power of its output signal. Eagleware will be releasing
its nonlinear simulation suite shortly. However, Microwave Office, Genesys,
and EEsof, because of their high expense, are aimed strictly for the profes-
sional, as opposed to the amateur, market.

10.10.3 RF software issues
Just because linear S-parameter-based software does not allow a transistor to
be physically biased with a power supply—since the S-parameter model is act-
ing as a prebiased black box—does not mean that the actual amplifier’s bias-
ing network should not be added to the S-parameter model’s input and output,
just as in the final physical design. By adding the actual calculated biasing
resistors and capacitors, the software simulation can take into account the
true effects these biasing components will have on the amplifier circuit.
Many are confused when attempting to add a bias supply to a circuit in a
linear S-parameter simulator, as no battery or power supply model is included
(and if such a device is included it is for looks only, and does not function as a
true bias supply). The engineer need only substitute a ground in the circuit for
this missing bias supply, since a real bias supply—especially with its decou-
pling capacitors—is an RF short to ground anyway.
To reiterate a very important concept: Unless we utilize a simulation program
that is at least as powerful as Caltech’s Puff, our distributed and lumped designs,
whether for filters, matching networks, or transmission lines, will be out of spec
because of calculation inaccuracies, lack of optimization, layout errors, radiation
and ohmic losses, etc. Even with the best simulation program, however, first-
build perfection—especially at the higher microwave frequencies—can be diffi-
cult; some physical tuning will almost always be required.

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