MATLAB: FDAtool: Cutoff for “Chebyshev” and “Elliptic”

-1db-3dbchebyshevcutoffellipticfdatoolMATLABSignal Processing Toolbox

Hi all.

I'm about to design a couple of IIR filters in FDA tool. The cutoff-frequency normally is -3 dB, but for Chebyshev and Elliptic it seems like it's only -1 dB.

Am I doing something wrong, or is it simple because I don't know the correct definitions? 🙂

I would appreciate some sort of verification, if possible.

Thanks!

Alex.

Best Answer

Hi Alex,

I think there is some misunderstanding regarding the design specification. In IIR filter, if you specify the filter order, the parameters for Butterworth method and the parameters for Chebyshev/Elliptic are different. For Butterworth, like you said, the parameter is the cutoff frequency, i.e. 3dB point. However, for Chebyshev/Elliptic method, you specify the passband and passband ripples. Based on your picture, I assume you are using the default passband ripple (Apass), which is 1dB. This means that between 20 and 150 Hz, the fluctuation cannot exceed 1dB. If you think you can live with a 3dB drop, you can then change your passband ripple (Apass) to 3dB, although such setting is rare in a Chebyshev/Elliptic design.

HTH

Honglei

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MATLAB: How to design FIR lowpass filter with cutoff frequency, fc1 = 20 Hz and FIR bandpass filter with cutoff frequency fc2 = 10 Hz and fc3 = 35 Hz.

Déjà vu

how to design IIR highpass filter with cutoff frequency of 20Hz and FIR bandpass filter with cutoff frequency of 10Hz and 15Hz?

Make the appropriate changes to get the cascaded filter you want.

MATLAB: Fir and iir filtering

You have to know your filter specifications. If you do, you can do both with fdesign.lowpass and fdesign.highpass

fdesign.lowpass and fdesign.highpass have different specification strings which support different filter designs.

The default string 'Fp,Fst,Ap,Ast' supports both FIR and IIR designs.

Here is an example:

Design a lowpass filter with a passband frequency of 1 kHz, a stopband frequency of 1.5 kHz, passband ripple of 0.5 dB, and stopband attenuation of 40 dB. The sampling frequency is 20 kHz

   d = fdesign.lowpass('Fp,Fst,Ap,Ast',1e3,1.5e3,0.5,40,20e3);
   designmethods(d)  %shows which designs are valid for this spec
   Hdeq = design(d,'equiripple'); % FIR filter
   Hbutt = design(d,'butter'); %IIR design

Best Answer

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MATLAB: How to design FIR lowpass filter with cutoff frequency, fc1 = 20 Hz and FIR bandpass filter with cutoff frequency fc2 = 10 Hz and fc3 = 35 Hz.

MATLAB: Fir and iir filtering

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