So it looks like MATLAB is applying the filter to the "negative" and "positive" frequencies separately and I end up with two band pass regions with a null around zero. I want the whole spectrum filtered once.
[b,a] = butter(2,[.18 .3739],'bandpass');
hm = filter(b,a,Y);
figure;
plot(20*log10(abs((fftshift(fft(hm))))))
Any ideas?