APPLICATION NOTE
4
Highpass Filter Behavioral Models
Three highpass filter models are developed similar to the lowpass filter models. The three models are:
Butt_HP (Butterworth filter)
Ch2p5_HP (0.25dB ripple Chebyshev filter)
Bessel_HP (Bessel filter)
Each model is capable of simulating up to a ninth-order filter and is used in a similar manner to that of the
lowpass as described in Example 1
Highpass Filter Example
To illustrate the use of the models, consider the following example. Here, the Butterworth highpass filter model is
used to obtain a family of curves for second- through ninth-order responses. Assume the filter has a cutoff
frequency of 1 kHz. Using Capture, the following circuit is drawn:
Figure 5: Butterworth highpass filter model
The filter Butt_HP has an AC source on its input and a 1K load resistor on its output. On the Butt_HP symbol, the
attribute FC has a value of 1K, 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 1001-decade logarithmic 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 6.
