Lowpass Filter

Construct a default lowpass filter specification object using fdesign.lowpass.

designSpecs = fdesign.lowpass; 

Determine the available design methods using the designmethods function. To design a type 1 Chebyshev filter, pick cheby1.

designmethods(designSpecs,'SystemObject',true)

Design Methods that support System objects for class fdesign.lowpass (Fp,Fst,Ap,Ast):


butter
cheby1
cheby2
ellip
equiripple
ifir
kaiserwin
multistage

While designing the filter, you can specify additional design options. View a list of the options using the designoptions function. This function also shows the default design options the filter uses.

designoptions(designSpecs,'cheby1','SystemObject',true)

ans = struct with fields:
           FilterStructure: {1x6 cell}
              SOSScaleNorm: 'ustring'
              SOSScaleOpts: 'fdopts.sosscaling'
              MatchExactly: {'passband'  'stopband'}
    DefaultFilterStructure: 'df2sos'
       DefaultMatchExactly: 'passband'
       DefaultSOSScaleNorm: ''
       DefaultSOSScaleOpts: [1x1 fdopts.sosscaling]

Use the design function to design the filter. Pass 'cheby1' and the specifications given by variable designSpecs, as input arguments. Set the filter structure to 'df1sos' to design a filter with Direct Form I SOS structure.

LowpassCheb1 = design(designSpecs,'cheby1','FilterStructure','df1sos',...
    'SystemObject',true)

LowpassCheb1 = 
  dsp.BiquadFilter with properties:

                       Structure: 'Direct form I'
                 SOSMatrixSource: 'Property'
                       SOSMatrix: [5x6 double]
                     ScaleValues: [6x1 double]
      NumeratorInitialConditions: 0
    DenominatorInitialConditions: 0
        OptimizeUnityScaleValues: true

  Show all properties

Use fvtool to view the frequency response of the designed filter.

fvtool(LowpassCheb1)

Highpass Filter

Construct a highpass filter design specification object using fdesign.highpass. Specify the filter order, passband edge frequency, and the passband ripple to get the filter exactly as required.

designSpecs = fdesign.highpass('n,fp,ap',7,20,.4,50); 

Determine the available design methods. To design a type I Chebyshev IIR filter, pick cheby1.

designmethods(designSpecs,'SystemObject',true)

Design Methods that support System objects for class fdesign.highpass (N,Fp,Ap):


cheby1

While designing the filter, you can specify additional design options. View a list of the options using the designoptions function. This function also shows the default design options the filter uses.

HighpassCheb1 = design(designSpecs,'cheby1','SystemObject',true)

HighpassCheb1 = 
  dsp.BiquadFilter with properties:

                   Structure: 'Direct form II'
             SOSMatrixSource: 'Property'
                   SOSMatrix: [4x6 double]
                 ScaleValues: [5x1 double]
           InitialConditions: 0
    OptimizeUnityScaleValues: true

  Show all properties

Use fvtool to visualize the highpass frequency response.

fvtool(HighpassCheb1)

By design, cheby1 returns filters that use second-order sections (SOS). For many applications, and for most fixed-point applications, SOS filters are particularly well-suited.