MATLAB: Filter

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in butter(1,wn) i know wn is normal frequency =(cut off/nyq.freq) in amplitude modulation what will be wn=? if carrier freq=fc and i calulat 0:.001:1 sample and message freq fm

Best Answer

Code:

   dt = 0.001;         % sampling time (seconds per sample)
   Fs = 1/dt;          % sampling rate (samples per second)
   Fnyq = Fs/2;        % Nyquist frequency (hertz)
   Fco = fm + bw;      % Cut-off frequency (hertz)
   wn = Fco/Fnyq;      % Normalized cut-off (dimensionless)

Explanation:

dt is the sampling time (in seconds per sample)
Fs is the sampling rate (in samples per second)
Fnyq is the Nyquist frequency (in hertz)
Fco is the cutoff frequency (in hertz)
bw = bandwidth (in hertz)
Fc is not needed to compute wn

You should make sure that the following conditions are true:

   bw << fm << fc

and

   bw << fm << Fs

HTH.

Rick

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MATLAB: How do i normalise frequency axis after computing fft

If you have a 50 Hz sampling frequency, the Nyquist frequency — in this instance the normalising frequency if you are designing a digital filter — is 25 Hz. So if you want a 15 Hz cutoff for a lowpass filter for instance, the normalised frequency, ‘Wn’ would be:

Fs = 50;                    % Sampling Frequency (Hz)
Fn = Fs/2;                  % Nyquist Frequency (Hz)
Fc = 15;                    % Cutoff Frequency (Hz)
Wn = Fc/Fn;                 % Normalised Cutoff Frequency (Dimensionless)

So Wn = 15/25 = 0.6

EDIT — For fft scaling, the best documentation is in the R2015a fft version. See specifically the code between the first two plot images.

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fc = f;
df = fs/Ns;
f = -fs/2:df:fs/2-df;
plot(f,abs(xdft);

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