Figure 9.23: Top view of
Figure 9.21
. This view is useful in defining the solution space.
The example shown in
Figure 9.23
will produce two solutions spaces:
1. 38
< daughtercard Z
o
< 70
and 40
< motherboard Z
o
< 52
2. 58
< daughtercard Z
o
< 70
and 40
< motherboard Z
o
< 61
Furthermore, it can be used to estimate the minimum spacing. Since the impedance is
varied over a wide area, the maximum tolerable impedance variations can be translated into
minimum spacing requirements by reverse engineering a SLEM model (the SLEM model is
described in
Section 3.6.2
). A SLEM model translates crosstalk, which is a function of line--
line spacing into an impedance variation. It is not difficult to translate an impedance variation
into a spacing requirement for a given cross section. Note: Be certain that the manufacture
impedance variations are accounted for when translating an impedance variation into a
spacing requirement.
It is not necessary to use three-dimensional plots; however, they are generally more efficient
than a two-dimensional approach. The parameter sweeps are then used to limit each
variable that the designer has control of in order to choose the optimal electrical solution
space. It is necessary to perform parametric sweeps for all relevant timing and signal quality
metrics at each of the eight corners defined in
Section 9.4.1
. Often, custom scripts must be
written to automate these sweeps efficiently. However, many simulation tool vendors are
beginning to offer these capabilities.
9.4.3. Phase 1 Solution Space
The purpose of the phase 1 solution space is to limit the electrical solution space from
infinite to finite. Subsequently, the simulations are simplified so that they can run fast.
Ordered parameter sweeps are performed on all the variables as described in
Section 9.4.2
.
To facilitate efficient and fast simulation times, several assumptions are made during phase
1.
1. SLEM transmission line models are used to account for crosstalk. The impedance is
swept and the allowable impedance range is translated into an initial target
impedance and spacing requirement. It is important to allow for manufacturer
variations.
2. Power delivery and return paths are assumed to be perfect.
3. Simple patterns are used (no long pseudorandom patterns).