DSP Blockset

Discrete-Time IIR Filters.   For IIR filters, the cutoff frequency is defined as the frequency where the magnitude response drops to -3 dB below the passband level (one half the passband power). Passband ripple, Rp, is defined as the peak-to-peak ripple magnitude in the passband, and stopband attenuation, Rs, is defined as the peak magnitude in the stopband. The following figure shows the response of an elliptic lowpass IIR filter with 1 dB of passband ripple and 60 dB of stopband attenuation. The right plot shows a zoomed view of the passband.

The Digital IIR Filter Design block uses the following Signal Processing Toolbox functions to design its filters:

• The Butterworth design uses the toolbox function butter.
• The Chebyshev type I design uses the toolbox function cheby1.
• The Chebyshev type II design uses the toolbox function cheby2.
• The elliptic design uses the toolbox function ellip.

For more information on the filter design algorithms, see the Signal Processing Toolbox documentation.

Arbitrary Shape Discrete-Time FIR Filters

Example: Chebyshev Type II Lowpass Filter