wow. luckily i did ask. =)
anyway, for notch filter, the cutoff freq. is found with similar way like normal filter?
i.e. just check the frequency value at -3dB ?
But you should know that in most cases the 3-dB-bandwidth is less interesting since the notch filter is mostly applied to supress only one spurious frequency.
That means, the specification could be for example: attenuate fz by at least 60 dB.
Yes, we usually interested in one frequency or narrow area around it. However, there is one important point: around this central frequency of bandstop filter group delay variations are huge and must be accounted.
It's a bandstop filter; don't look real due the 100 dB attenuation, but only a simulation.
Also the large bandwidth is very strange, usually bandstop are quite narrow.
This for my experience.
If the sampling frequency is 2π then you can only use the (0,π) band, in this case this is low pass filter, you can notice because the the (π,2π) band is a mirror of the (0,π).