User Guide

Derivatives 4
-11
Example: If there are two specifications, each of which requires
a DC analysis of the same circuit file, the optimizer will run a
single simulation for each parameter, then load the data file
(
.dat) into Probe and evaluate the perturbed values of f. If
there are M specifications (all using the same analysis type and
the same circuit file) and N parameters, then forming the
Jacobian takes:
• N simulations, and
• M Probe goal function evaluations per simulation.
Note
The time needed to simulate is usually much
g
reater than the time needed to evaluate the Probe
g
oal functions. This means that the time taken to
optimize a desi
g
n depends heavily on the number
of variable parameters.
Limitations of Derivative Data
A derivative analysis calculates a linear relationship between a
parameter and a specification. It assumes that the function is
linear near the initial value within a region defined by the value
set for the Delta option. If the data is well-behaved in this region,
then this is a valid assumption; the PSpice Optimizer can use the
derivative to approximate specification values based on the
linear relationship.
However, when the function is not well behaved in the region
around the initial value, the approximation may not be valid.
Example: Assume that the function shown in Figure 4-3 is the
plot of a specification’s behavior vs. a parameter value. Note
that the function is approximately linear between the dashed
lines, but not necessarily linear outside of that region.
If you pick an initial value for the parameter which is between
the two dashed lines, then subsequently compute the derivatives,
the derivative data provides a reasonable approximation for any
other parameter value between those lines. However, if you try
to use the same derivative data to estimate new specification
values for parameter values outside of those lines, the estimates
are not reliable.
See Controlling Finite
Differencing when Calculating
Derivatives (Delta Option) on
page 4-13 for more information
on Delta.
Fi
g
ure 4-3
Hypothetical
Function