function Hd = filter_notch

Fs=500; Fnotch=50; BW=1; Apass=1;

[b, a] = iirnotch(Fnotch/(Fs/2), BW/(Fs/2), Apass);
Hd     = dfilt.df2(b, a);

>> freqz(filter_notch);

>> fs=500;
>> f_range=linspace(0,fs/2,2048);
>> h=freqz(filter_notch,f_range,fs);
>> hdb=20*log10(abs(h));
>> plot(f_range,hdb)

h = freqz(___,f,fs) returns the frequency response vector, h, at the physical frequencies supplied in f.