Digital Signal Processing Reference
In-Depth Information
circuit board traces that have a constant width, spacing, height above the ground
plane, and thickness is a good example of the type of structure appropriate for 2D
analysis.
The field solver calculates the resistance, capacitance, inductance, and conduc-
tance of the 2D structure and tabulates them across a user-defined frequency range.
The frequency range must be carefully selected because, as we saw in Chapter 1,
digital pulses contain high-frequency harmonics and (as we will see in Chapters 6
through 8) losses increase with frequency.
Since the shape is uniform, the electrical parameters are specified per unit
length. The value for a specific trace is found by multiplying the field solver results
by the actual length of the structure. For example, if the capacitance of a trace is
reported as 140 pF/m, the capacitance for a 1-cm-long trace is 1.4 pF.
The best solvers allow the user to choose the output formant from a variety
of circuit model types, so a library of circuit models can be created that are usable
by a number of different circuit simulators. At a minimum the field solver that
you choose should produce both an RLGC output file and an S-parameter model
(preferably in Touchstone format). The RLGC data of single traces can be vali-
dated by comparing it with the results from the approximate formulas presented in
Chapter 17. This is a good way to verify that the model has been setup correctly.
As described in Chapter 7's Problems, the RLGC information can be used to create
a W line model in those cases where the field solver does not directly create one.
Alternatively, it can be used with the SPICE LTRA model (noting the limitations
discussed in Section 3.3).
3.5.2 What Are 3D Field Solvers?
Circuit models for physical structures that are not uniform in shape (such as vias or
traces that vary in width or spacing or that bend around objects) are created with a
3D field solver. Some of these solvers require knowledge of radio frequency terms
and concepts to produce valid models, but other simulators are specifically designed
to be operated by signal integrity engineers having little RF knowledge.
Because the structure is simulated in three dimensions, setting up the problem
space is usually more time-consuming and error-prone than with 2D simulators.
Before purchasing a 3D simulator, check that entering the circuit board stack-
up and defining the material properties are intuitive. Furthermore, it should be
straightforward to verify that the values have been assigned properly and assigned
to the correct layer in the stackup. Surface roughness (as shown in Chapter 8, an
important factor when calculating conductor loss) should be easy to apply to traces
and planes. When working with high-speed signals, the ability of the simulator to
include the way in which frequency changes the dielectrics electrical properties is
also very important.
As part of the selection process, the drawing interface should be thoroughly
tested to determine the ease in creating and editing complex 3D structures. Al-
though some 3D simulators can directly import the 3D mechanical drawings cre-
ated by CAD drafting software, or the stackup from circuit board artwork tools,
it is routinely necessary for the SI engineer to “clean up” the drawing and to crop
regions once it has been imported.
