User Guide

1
-
10 Thin
g
s You Need to Know
Probe
g
oal function
A Probe goal function defines how
to evaluate a design characteristic when running any kind of
analysis other than a single-point sweep analysis. A goal
function computes a single number from a Probe waveform.
This can be done by finding a characteristic point (e.g., time of
a zero-crossing) or by some other operation (e.g., RMS value of
the waveform).
For example, you can use Probe goal functions to:
• Find maxima and minima in a trace.
• Find distance between two characteristic points (such as
peaks).
• Measure slope of a line segment.
• Derive aspects of the circuit’s performance which are
mathematically described (such as 3 dB bandwidth, power
consumption, and gain and phase margin).
To write effective goal functions, determine what you are
attempting to measure, then define what is mathematically
special about that point (or set of points).
Note
Be sure that the
g
oal functions accurately measure
what they are intended to measure. Optimization
results hi
g
hly depend on how well the
g
oal
functions behave. Discontinuities in
g
oal functions
(i.e., sudden jumps for small parameter chan
g
es)
can cause the optimization process to fail.
PSpice Optimizer expression
A PSpice Optimizer
expression defines a design characteristic. The expression is
composed of optimizer parameter values, constants, and the
operators and functions shown in Table 1-2.
Example: To measure the sum of resistor values for two resistors
with parameterized values named R1val and R2val,
respectively, use the PSpice Optimizer expression R1val +
R2val.
Refer to the Goal Function
wizard in Probe
and your PSpice
user’s
g
uide for information on
how to develop and specify
g
oal
functions.
Here are some quick tips. In
Probe:
• To test the value returned by
a specified
g
oal function,
select Eval Goal Function
from the Trace menu.
• To see the waveforms and
marked points used to
evaluate a
g
oal function,
select Display Evaluation in
the Probe Options dialo
g
box
(from the Tools menu, select
Options to display this dialo
g
box).
See Gain on page 7-5 for an
example of the YatX
g
oal
function definition.