APPLICATION NOTE
3
The sub-circuit has two parameters: the filter cutoff frequency (FC) and its order (ORD). These subcircuit
parameters are given default values, which are arbitrarily set to 1. They will be changed to the required cutoff
frequency and order when the subcircuit is called.
Lowpass Filter Example
To illustrate the use of the models, consider the following example. Here, the Butterworth lowpass filter model is
used to obtain a family of curves for second- through ninth-order responses. Assume the filter has a cutoff
frequency of 10 kHz. Using Capture, the following circuit is drawn.
Figure 3: Butterworth lowpass filter model
The filter Butt_LP has an AC source on its input and a 1K load resistor on its output. On the Butt_LP symbol, the
attribute FC has a value of 10K, and ORD is set to the global variable defined in the global parameter block
(PARAM). Finally, a VDB voltage marker is placed on the output node to view the results.
The PSpice simulation consists of a 1000-point linear AC sweep and a parametric analysis. The parametric
analysis steps the global parameter, ORD, from 2 to 9. The simulated frequency response of the filter for various
orders is shown in Figure 4.
Figure 4: Frequency Response of the Butterworth lowpass filter model
