My group and I are currently working on a school project and are having
some troubles. We can't currently find any models for the parts of a
project that we are working on and were curious if it was possible to
create the models based off of the datasheets?
You can, but the accuracy you get is probably not going to be that great --
especially for something like a mixer, which is necessarily a non-linear
device, so coming up with an accurate SPICE model is very difficult unless you
settle for linearizing ing it about your particular operating point. What one
usually does if they need a highly accurate model of a part that doesn't come
with such a model to begin with is to obtain a sample of the device in
question and sit down and characterize it themselves -- this is a
time-consuming process, and requires a fair degree of experience to do well
(i.e., unless you can find someone in your department who's done it already
and can help, I wouldn't suggest doing it unless this is strictly a "learning
experience" kind of project).
What source of device are you building, and what parts of it do you want to
simulate? Although you can use SPICE for RF simulation, the results are very
slow to achieve since transient analysis is usually the only kind of SPICE
simulation that'll work OK for mixers and modulators, and the time steps get
to be painfully small if you're dealing with high frequencies (as it would
appear that you are form your parts specs).
I'm by no means an expert on how people typically perform the sort of mixed
digital/RF design you seem to be aiming at here, but my understanding is that
many companies don't have the resources to be able to perform completely
"integrated" simulations with any one tool. Instead various "point" tools are
used to simulate the pieces, and if the interfaces between them have been
defined well enough, there's a decent chance that everything will work once
it's all plugged together. The various types of simulators used include:
-- Linear frequency domain simulators: These programs (stuff like Agilent's
EESOF or Eagleware's Genesys linear simulator... oops, wait, Agilent now owns
Eagleware too...) accept component data in the form of S parameters and let
you quickly perform functions such as filter design, matching networks, power
supply decoupling design, etc. They're also good for small signal amplifier
design, although they require you to pick an operating point first and somehow
come up with the S paramaeters of, e.g., a transistor that corresponds to that
operating point.
-- Non-linear frequency domain simulators: These programs -- dominated by
so-called "harmonic balance" simulators -- figure out the steady-state
response of a non-linear network to a single tone (or small handful of tones)
input. This makes it very easy to get the first draft of a mixer or non-class
A amplifier going, and the results are computed orders of magnitude faster
than what SPICE can do.
-- System simulators: These programs -- such as Systemvue by Elanix, now owned
by Eagleware/Agilent -- model all your components "parametrically." For
something like an amplifier, you just input items such as gain, noise figure,
excess noise vs. frequency, etc. For mixers you provide the input-output
responses (e.g., how much of the signal at the mixing port is passed through),
etc. In general you get to provide a transfer function in the form of a
polynomial, and the simulator takes care of the rest, computing items such as
noise margins, conversion gains, etc. at some specified output port. This
sort of "simulation" can actually be performed pretty well with an Excel
spreadsheet, although a tool designed for the task tends to be a lot easier to
set up and obtain results from.
-- Signal integtiry simulators: For items such as your FPGA, typically all
you'll get out of the vendor is a so-called IBIS model. IBIS models
characterize the input-output characteristics of the I/O cell of your FPGA in
the form of tabulated or curve-fitted data around a handful of "bias" points;
this is sufficient to allow you to check items such as eye patterns and setup
and hold times of digital signals running around your board. Hyperlynx used
to be one of the well-known, low-priced simulators here, although they're now
owned by Mentor graphics, which usually indicates at least another zero was
appened to the price tag.
-- Field solvers: The idea here is to take a relatively arbitrary physical
structure in a defined environment (known dielectrics, known conduction of the
materials of the metals involved, etc.) and extract a model of the device to
use in (typically) a linear simulator. These are used all the time if you're
designing spiral inductors on ICs, antennas, interconnects (bond wires, vias,
BGAs, etc.), etc. HFSS is one of the heavyweights here, although there are
plenty of more restrictive (2D and "2.5D") field solvers that often work fine
(Sonnet, Agilent's Momentum, etc.) and have nowhere near the price tag or
learning curve of something as sophisticated as HFSS.
Unless you're trying to squeeze the last once of performance for some design
that you're going to manufacture and make a million of anyway, I'd suggest you
perform a system level simulation (this will force you to spend time getting
familiar with the parts' datasheets) and then just go and build your board and
see what happens. It appears that you're performing board-level design using
discrete RF "building blocks," and as such effects such as the PCB material
you use, your layout technique, etc. will have a small but noticeable effect
on the operation of the device and -- if this is the first time you're doing
it -- it's unlikely you'll be able to properly account for all of these
effects in your simulation.
If you do want to try out some of the fancier simulations, note that most
companies such as Agilent, Ansoft, Sonnet, etc. will either give you or sell
you a fraction of the usual cost their products if you're associated with an
educational institution. Before persuing that route, you might want to sit
down and play with some of the tools that are completely free, such as Sonnet
Lite, RFSim99, etc.
---Joel Kolstad
These are the parts
