How to take the FFT of an undersampled signal I am sampling a 5mhz blocks at an IF of 40 MHZ. I am sampling at 11MHz per bandpass sampling theory, but when I run the FFT I get all the aliases. 

So I am trying to figure out how to constrain the fft interpretation to the appropriate bands. 

IE i may be sampling the band from 80-85MHz by mixing it down to 37.5-42.5 MHZ and sampling at 11 MHZ. How do I take the FFT and plot it showing the peaks appropriately?
If there is other frequency content in other Nyquist regions, you will see those alias into your digitized signal also; there's no free lunch here. If you wish to use bandpass sampling, then you need to first apply a bandpass anti-aliasing filter to suppress the unwanted content.