Page 445 - Complete Wireless Design

Wireless Issues

444 Chapter Ten

Computer wireless circuit simulation programs can involve one or more
methodologies. The most common is Spice, developed at Berkley University
some 30 years ago. Spice works quite well at frequencies below 20 MHz
(depending on the Spice model, some of which are optimized for much higher
frequencies), and permits simulation of linear and nonlinear circuit behavior
in the time, frequency, and transient domains. It also allows the modeling of
most real-life effects that occur when a transistor or diode is biased to almost
any desired value. However, Spice is very slow (a relatively simple circuit may
take up to an hour to simulate), high-frequency component models are quite
rare, and convergence can be a problem (convergence is the ability for Spice to
come up with a correct answer during a simulation run). Spice is still invalu-
able in the simulation of many circuit designs even when another simulation
methodology is used, especially for confirming proper active device bias and
viewing the output waveform of a circuit in the time domain.
Linear simulators, such as Eagleware’s Genesys and the included Caltech
Puff, are the dominant program types employed in the RF and microwave
world. These operate with S-parameter models, which are the most accurate
way of representing a device in the RF and microwave regions (see Sec. 1.5, “S
Parameters”). However, S parameters of active devices are already biased with
a certain I and V when these models are taken, so a linear simulator’s main
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limitation is that a circuit cannot be biased to any chosen current and voltage
level. In fact, the model’s input and output parameters will be quite different
in a real circuit if the designer later chooses to bias the transistor to any other
value. Time domain views are normally not possible (unless a reverse FFT is
taken), and the linear simulator’s display, using the program’s own Bode ana-
lyzer tool, will always be in the frequency domain. Nonetheless, linear model-
ing is very fast, allows rapid circuit tuning and optimization, and is quite
accurate, and the models are prevalent.
Harmonic Balance (HB) simulation methods are practiced in very expen-
sive, high-end simulators to model both linear and nonlinear circuit effects.
Unfortunately, HB component models are not only difficult to obtain, but it
may take a very long time to simulate even a simple circuit. Harmonic Balance
will also not measure transient effects and is very poor at measuring the IMD
products and higher-order harmonics of mixers and saturated amplifiers. If
this type of simulator can be afforded ($30,000 and up), it is still an incredible
and enlightening look into the functionality of a particular RF circuit design.
A cousin to Harmonic Balance is the Volterra Series, which accurately models
the slightly nonlinear effects present in all linear circuits. This method is not
meant to model grossly nonlinear circuits such as mixers, Class C amplifiers,
oscillators, or frequency multipliers.
Three-dimensional planar electromagnetic analysis software employs the
method of moments or the method of lines technique to simulate planar
(microstrip, stripline, etc.) microwave structures. This type of simulator is able
to display the gain and return loss of distributed microwave filters, transmis-
sion lines, waveguides, spiral inductors, planar antennas, and more, as well as

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