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I need help in Matlab

  1. Dec 18, 2007 #1

    I need some help in matlab.
    I am required to take the FFT (fast fourier transform) of the data obtained from my experiment, however, I have some problem understanding the code written by other people, just wondering if anyone could help me with this.

    I also need to understand how to fit the response into a rational function. I have pasted a code below, but I dont understand what it means. What does those 1:50001 means? and how to find out the length of wt and f_norm (they are suppose to be of the same length, as mention in matlab help). also, what does it means by wt?

    % Fitting the response to a rational function
    ff = f(1:50001);
    f_norm = ff / f(50001)*pi;
    [num,den]=invfreqz(tf(1:50001), f_norm,3,4,wt)
    [h2,w2] = freqz(num,den,50001); % compute approximated magnitude response
    % plot measured and approximated magnitude response
    plot(ff(1:501), abs(tf(1:501)), 'r', ff(1:501), abs(h2(1:501)), 'b');
    xlabel('Frequency (Hz)'), ylabel('Magnitude')
    title('Magnitude Response')

    Also, I am told to pass my data through a 5th order low pass butterworth filter, which is limited to 5kHz, before performing the FFt.
    However, after writing the code, i realised that the waveform is not correct in term of the magnitude and the data point.
    May I know how am I suppose to work out on the butterworth?

    I had written the butterworth code as shown below, please take a look and advise me.

    %5th order lowpass butterworth filter,limited frequency 5kHz
    n = 5; Wn = 5000/250000; %n=order, 0<Wn<1, Wn= cutoff frequency=5kHz/sampling frequency
    [Vsec,Vpri] = butter(n,Wn, 'low');
    % perform FFT where Y=fft(yy,length) / (length/2)
    Vpri_FT = fft(Vpri,10000)/5000;
    Vsec_FT = fft(Vsec,10000)/5000;
    f = 250000*(0:5000) / 10000; % f=sampling frequency*(0:(length/2))/length;
    tf = Vsec_FT ./ Vpri_FT; % transfer function=FFT(secondary) / FFT(primary)

    Thank you very much .:)
  2. jcsd
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