At first glance, one would think that undersampling is easy-peasy, after-all we are eliminating an RF chain, and we all know how RF is somewhat a art gained through experience. Well after reviewing Nyquist-Shannon theory, we learn that undersampling or harmonic sampling is not without trade-offs, and a lack of theory will leave you with a severely distorted output on your A to D. One of the reasons I have not responded to your latest question is that alas, I have less than the required experience in this area to give proper guidance.
You are not left without resources however, as a quick Google search reveals a good deal of information on this topic. One paper in particular that I found quite good at explaining undersampling can be download from the following link: "
What Nyquist Didn’t Say"
I would suggest you give the above mentioned paper a read.