Biquad IIR filter design with Q factor sorting?

I have designed biquad IIR filter:

h = fdesign.lowpass('N,Fc', 8, 10e3, 100e3);  % 4-biquad with 100kHz sampling rate and 10kHz cutoff freq
Hd = design(h, 'butter'); 

Warning: The 'butter' design method is obsolete. Using 'butter' with Fc specification sets still works but will be removed in the future. Use a specification with F3dB instead.

Hd_sos = convert(Hd, 'df2sos'); 

Hd_sos = 
         FilterStructure: 'Direct-Form II, Second-Order Sections'                                       
              Arithmetic: 'double'                                                                      
               sosMatrix: [4x6 double]                                                                  
             ScaleValues: [0.085667864748513;0.0719845250125051;0.0641431197006179;0.0605721791831837;1]
     OptimizeScaleValues: true                                                                          
        PersistentMemory: false                                                                         
                                                                                                        

I checked Q-factor and realized the first sos section has the largest Q and the last sos has the smallest. I want the smallest Q to be first and the lagest to be the last sos. Any suggestion to do so?

Answer 1

[Paul]

Hi Jay,

h = fdesign.lowpass('N,Fc', 8, 10e3, 100e3);  % 4-biquad with 100kHz sampling rate and 10kHz cutoff freq
Hd = design(h, 'butter'); 

Warning: The 'butter' design method is obsolete. Using 'butter' with Fc specification sets still works but will be removed in the future. Use a specification with F3dB instead.

Hd is already in DF2SOS form

Hd.FilterStructure

ans = 'Direct-Form II, Second-Order Sections'

so the call to convert seems to be redundant (unless it does something else that I'm missing.

Hd_sos = convert(Hd, 'df2sos');
Hd_sos.FilterStructure

ans = 'Direct-Form II, Second-Order Sections'

We see that

[isequal(Hd.ScaleValues,Hd_sos.ScaleValues), isequal(Hd.sosMatrix,Hd_sos.sosMatrix)]

ans = 1×2 logical array
   1   1

To reorder the sections, the function reorder offers several options.

[Jay]

Thanks Paul for the quick reply!

You are right Hd is already in df2sos. The function reorder was exactly what I've been looking for.