User Guide

4
-
12 Understandin
g
Optimization Principles and Options
When, for a given parameter, the difference between its initial
value and the value of interest is large (that is, the relative
difference is much bigger than Delta), modify its initial value
and restart the simulation.
Note
When you modify the current value for a parameter
to recalculate specification values (by editin
g
the
value appearin
g
in the PSpice Optimizer window),
the PSpice Optimizer uses the derivative data
rather than a resimulation to determine the new
values. Therefore, you should periodically verify
the results with another simulation (see “
Ensuring
reliable results when tweaking values ” on pa
g
e 3-24 ).
Tar
g
et Value Scalin
g
When there is more than one goal, or a combination of goals and
constraints, the PSpice Optimizer needs to scale the raw
measurements before combining them.
Example: Consider a least-squares optimization with two
specifications:
• collector-base capacitance
• collector resistance
The first of these can have values of several picofarads; the
second of tens of kohms. Clearly, adding the squares of the
errors for these specifications will lose the significance of the
capacitance term. Instead, the optimizer scales the values as
follows:
Suppose the specified target and measured values are as shown
below.
T
Scaled
T
measured
T
tetarg
–
Range
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=