We know that the under-sampling results in aliasing and frequencies higher than half of the Nyquist rate is not distinguishable. I've a base band signal that I want to use the higher frequencies which are higher than the half of the Nyquist rate (Nyquist frequency) as well as low frequencies(all parts). I've a special process with this path:
$$\textrm{Input}{\longrightarrow}\boxed{\textrm{anti-aliasing pre-filter}}{\longrightarrow}\boxed{\textrm{decimate}}{\longrightarrow}\boxed{\textrm{FFT}}{\longrightarrow}\boxed{\textrm{tune on special part}\\{\textrm{of the signal}}}$$
The low-pass post-filter that people usually use as anti-aliasing filter removes the high frequencies that are of interest for me. What is the digital or analog anti-aliasing pre-filter that I don't lose high frequencies.
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