Specifying both passband and stopband frequencies (and without other arguments to the ellip function) means that you are designing a bandpass filter. I appreciate your quoting my code, however I also posted several examples of elliptical filters using bandpass filters.
For your filter, replace ‘Wp’ with:
Wp = [250 512]/Fn; % Passband Frequencies For Bandpass Filter (Hz)
and ‘Ws’ with:
Ws = [0.99 1.01]*Wp; % Stopband Frequencies For Bandpass Filter (Hz)
or define it yourself, similarly to the way I defined ‘Wp’ here. Note that for a bandpass filter, the stopbands must be outside the passbands.
