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.
