User Guide

Chapter 11 Parametric and temperature analysis
278
Example: frequency response vs. arbitrary
parameter
You can view a plot of the linear response of a circuit at a
specific frequency as one of the circuit parameters varies
(such as the output of a band pass filter at its center
frequency vs. an inductor value).
In this example, the value of a nonlinear capacitance is
measured using a 10 kHz AC signal and plotted versus its
bias voltage. The capacitance is in parallel with a resistor,
so a trace expression is used to calculate the capacitance
from the complex admittance of the R-C pair.
Setting up the circuit
Enter the circuit in Capture as shown in Figure 71
To create the capacitor model in the schematic editor:
1 Place a CBREAK part.
2 Select it so that it is highlighted.
3 From the Edit menu, choose PSpice Model.
4 In the Model Text frame, enter the following:
.model Cnln CAP(C=1 VC1=-0.01 VC2=0.05)
5 From the File menu, choose Save.
Set up the circuit for a parametric AC analysis (sweep
Vbias), and run PSpice. Include only the frequency of
interest in the AC sweep.
T
h
is tec
h
nique
f
or measuring
b
ranc
h
capacitances works well in both simple and
complex circuits.
Figure 71
RLC filter example circuit.
Pspug.book Page 278 Wednesday, November 11, 1998 1:14 PM