After using butter function to design a filter (as follow), I wanted to check the frequency response. However I suddenly find that the frequency response seems to be different using different representation. That doesn't make sense to me. Thanks a lot for answering me.
[N, Wn]=buttord(Wp,Ws,0.1,30); %Design band pass filter.
fvtool(sos,'Fs',Fs); % Plot Freq Response using state space representation.
fvt=fvtool(b,a,'Fs',Fs); % Plot Freq Response using Transfer Function.