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You can design IIR filters with approximately linear phase using the DFD Least Pth Norm Design VI. You must set filter type to Symmetric or Antisymmetric.

Although it is theoretically impossible to design causal IIR digital filters with exactly linear phase, you can design IIR filters with approximately linear phase. For example, suppose you want to design an approximately linear phase IIR lowpass filter with a passband frequency range of [0, 0.2] and a stopband frequency range of [0.3, 0.5]. You can set the specifications as shown in the following figure:

The following figure shows the magnitude response of the designed filter.

The following figure shows the phase response of the designed filter.

Notice that this filter has greater stopband attenuation than the linear phase FIR filter designed, and this filter keeps the passband phase response roughly linear.

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