Classic IIR Filter Design

This example shows how to design classic IIR filters. The initial focus is on the situation for which the critical design parameter is the cutoff frequency at which the filter's power decays to half (-3 dB) the nominal passband value.

The example illustrates how easy it is to replace a Butterworth design with either a Chebyshev or an elliptic filter of the same order and obtain a steeper rolloff at the expense of some ripple in the passband and/or stopband of the filter. After this, minimum-order designs are explored.

Lowpass Filters

Let's design an 8th order filter with a normalized cutoff frequency of 0.4pi. First, we design a Butterworth filter which is maximally flat (no ripple in the passband or in the stopband):

N = 8; 
F3dB = .4;
d = fdesign.lowpass('N,F3dB',N,F3dB);
Hbutter = design(d,'butter','SystemObject',true)

Hbutter = 
  dsp.SOSFilter with properties:

            Structure: 'Direct form II'
    CoefficientSource: 'Property'
            Numerator: [4x3 double]
          Denominator: [4x3 double]
       HasScaleValues: true
          ScaleValues: [0.2914 0.2261 0.1929 0.1788 1]

  Show all properties

A Chebyshev Type I design allows for the control of ripples in the passband. There are still no ripples in the stopband. Larger ripples enable a steeper rolloff. Here, we specify peak-to-peak ripples of 0.5dB:

Ap = .5;
setspecs(d,'N,F3dB,Ap',N,F3dB,Ap);
Hcheby1 = design(d,'cheby1','SystemObject',true);
hfvt = fvtool(Hbutter,Hcheby1);
legend(hfvt,'Butterworth','Chebyshev Type I');

A Chebyshev Type II design allows for the control of the stopband attenuation. There are no ripples in the passband. A smaller stopband attenuation enables a steeper rolloff. In this example, we specify a stopband attenuation of 80 dB:

Ast = 80;
setspecs(d,'N,F3dB,Ast',N,F3dB,Ast);
Hcheby2 = design(d,'cheby2','SystemObject',true);
hfvt = fvtool(Hbutter,Hcheby2);
legend(hfvt,'Butterworth','Chebyshev Type II');

Finally, an elliptic filter can provide the steeper rolloff compared to previous designs by allowing ripples both in the stopband and the passband. To illustrate that, we reuse the same passband and stopband characteristic as above:

setspecs(d,'N,F3dB,Ap,Ast',N,F3dB,Ap,Ast);
Hellip = design(d,'ellip','SystemObject',true);
hfvt = fvtool(Hbutter,Hcheby1,Hcheby2,Hellip);
legend(hfvt, ...
    'Butterworth','Chebyshev Type I','Chebyshev Type II','Elliptic');

By zooming in the passband, we verify that all filters have the same -3dB frequency point and that only Butterworth and Chebyshev Type II designs have a perfectly flat passband.

Phase Consideration

If phase is an issue, it is useful to notice that Butterworth and Chebyshev Type II designs are also the ones introducing a lesser distortion (their group delay is flatter):

hfvt.Analysis = 'grpdelay';

Minimum Order Designs

In cases where the 3dB cutoff frequency is not of primary interest but instead both the passband and stopband are fully specified in terms of frequencies and the amount of tolerable ripples, we can use a minimum order design technique:

Fp = .1; 
Fst = .3; 
Ap = 1;
Ast = 60;
setspecs(d,'Fp,Fst,Ap,Ast',Fp,Fst,Ap,Ast);
Hbutter = design(d,'butter','SystemObject',true);
Hcheby1 = design(d,'cheby1','SystemObject',true);
Hcheby2 = design(d,'cheby2','SystemObject',true);
Hellip = design(d,'ellip','SystemObject',true);
hfvt = fvtool(Hbutter,Hcheby1,Hcheby2,Hellip, 'DesignMask', 'on');
legend(hfvt, ...
    'Butterworth','Chebyshev Type I','Chebyshev Type II','Elliptic');

A 7th order filter is necessary to meet the specification with a Butterworth design whereas a 5th order is sufficient with either Chebyshev techniques. The order of the filter can even be reduced to 4 with an elliptic design:

order(Hbutter)

ans = 7

order(Hcheby1)

ans = 5

order(Hcheby2)

ans = 5

order(Hellip)

ans = 4

Matching Exactly the Passband or Stopband Specifications

With minimum-order designs, the ideal order needs to be rounded to the next integer. This additional fractional order allows the algorithm to actually exceed the specifications. We can use the MatchExactly flag to constraint the design algorithm to match exactly one band. The other band will exceed its specifications. By default, Chebyshev Type I designs match the passband, Butterworth and Chebyshev Type II match the stopband and the attenuations of both bands are matched by the elliptic design (while the stopband edge frequency is exceeded):

Hellipmin1    = design(d, 'ellip', 'MatchExactly', 'passband',...
    'SystemObject',true);
Hellipmin2 = design(d, 'ellip', 'MatchExactly', 'stopband',...
    'SystemObject',true);
hfvt = fvtool(Hellip, Hellipmin1, Hellipmin2, 'DesignMask', 'on');
legend(hfvt, 'Matched passband and stopband', ...
    'Matched passband', 'Matched stopband', ...
    'Location', 'Northeast')

Zoom in the passband to compare passband edges. The matched passband and matched both designs have an attenuation of exactly 1 dB at Fpass = 0.1 .

We verify that the resulting order of the filters did not change:

order(Hellip)

ans = 4

order(Hellipmin1)

ans = 4

order(Hellipmin2)

ans = 4

Highpass, Bandpass and Bandstop Filters

The results presented above can be extended to highpass, bandpass and bandstop response types. For example, here are minimum order bandpass filters:

d = fdesign.bandpass('Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2', ...
    .35,.45,.55,.65,60,1,60);

Hbutter = design(d,'butter','SystemObject',true);
Hcheby1 = design(d,'cheby1','SystemObject',true);
Hcheby2 = design(d,'cheby2','SystemObject',true);
Hellip = design(d,'ellip','SystemObject',true);
hfvt = fvtool(Hbutter,Hcheby1,Hcheby2,Hellip, 'DesignMask', 'on');
legend(hfvt, ...
    'Butterworth','Chebyshev Type I','Chebyshev Type II','Elliptic',...
    'Location', 'Northwest')

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