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Homework Help: Digital filter design with matlab

  1. Dec 14, 2009 #1
    I'm to design a 2nd order, N=2, butterworth HPF starting from the normalized transfer function for a LPF:

    HN(s)=1/(s^2+1.414*s+1)

    Given:
    T=0.004
    Ωc=0.8 pi
    wc=192 pi

    To find the transfer function of the HPF, substitute s for wc/s:
    H(s)=HN(wc/s)=s^2/(s^2+853*s+363833)

    Here's my MATLAB code:

    Code (Text):
    T=.8/192;
    Fs=1/T;
    nums=[1 0 0];
    dens=[1 853.03 363833.1];

    figure
    [Hws,w]=freqs(nums,dens);
    plot(w,abs(Hws))

    [numz,denz]=impinvar(nums,dens,Fs);
    tf(numz,denz,T)
    figure
    [Hwz,w]=freqz(numz,denz);
    plot(w,abs(Hwz))
    The transfer function of my analog filter looks good, but the transfer function of the digital filter is not right. Could someone help please?
     
  2. jcsd
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