DFDUtility The Digital Filter Design Utility is software that allows you to interactively specify and design floating point digital filters.

The software provides a filter design tool for classical design which allows you to specify a FIR/IIR designs by setting design parameters such as filter type, sampling frequency, passband/stopband edge frequencies, passband/stopband ripple levels, and stopband attenuation.
You can select from design methods that include Butterworth, Chebyshev, Inverse Chebyshev, Elliptic, Kaiser Window, Dolph-Chebyshev, and Equi-Ripple.

Do this to reproduce Figures 8-27 and 8-28:

1. Select Elliptic as the Design Method.
2. Select Lowpass as the Filter Type.
3. Set the Sampling Frequency at 1000.
4. Set the Passband Edge Frequency at 1/4 the highest frequency, which is 125.
5. Set the Passband Ripple to 0.3 dB. If the units aren't dB, check Magnitude in dB which is above the magnitude plot.
6. Set the Stopband Edge Frequency to 300. The exact value isn't important as long as the resulting filter order is 3.
7. Set the Stopband Attenuation to 30 dB.
8. Uncheck Magnitude in dB. The Z plane plot should look like Figure 8-27 and the Magnitude plot should look like Figure 8-28.