MATLAB: Low pass filter design

convolutionfilterlow pass

Hi, i designed a low pass filter but it is not working to 0.05 sec. After this point filter work normally. How to i can fix it thank you.

Fs=10000;

Ts=1/Fs;

signalLength=1000;

t=(0:signalLength-1)*Ts;

filtLP= designfilt('lowpassfir','FilterOrder',50,'CutoffFrequency',250,…

'SampleRate',Fs);

outputAfterFilterLPFourier = fft(outputAfterFilterLP,NFFT)/signalLength;

Best Answer

Hi Toygar Ozel,

have a look at the step response of your filter, e.g. using filter designer. You can determine easily that the group delay of your filter is around 0.05 s. You would have to use another filter design in order to reduce the group delay.

Kind regards,

Robert

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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

MATLAB: Butterworth lowpass filter design code

I would just substiture 'butter' here:

d=design(h, 'butter');

I would also add 'Ast' to the fdesign.lowpass parameters for a Butterworth design.

Also, use filtfilt for phase-neutral filtering, not filter, so your*|‘xfilter’|* assignment becomes:

xfilter = filtfilt(d,x2);

This applies to all filter designs, so try it with your Chebyshev Type I filter as well. The results will be different, perhaps noticeably so.

Best Answer

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MATLAB: Fir and iir filtering

MATLAB: Butterworth lowpass filter design code

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