On Oct 21, 9:55=A0am, "gobruins" <chunmei.k... at (no spam) gmail.com> wrote:
Hi, I am learning multi-channel DDC techniques using FFT. My design is
referenced
tohttp://www.pentek.com/tutorials/15_4/digdown.cfm
Something in this design I do not understand.
The FFT will split the input stream into N(FFT size) bins equally
spaced
in frequency. Totally 4 FFTs are used so that the output data rate is
fs/N*4. Followed by the FFT a compensation filter is used to flatten
the
frequency droop at the passband edge.
What I do not understand is that after FFT each bin output is a
bandpass
signal whose center frequency is fs/N*k (k=3D0:N-1). Do we need a
phase
shifter to translate these bandpass signals down to baseband before
applying the compensation filter?
I am confused on this part, I appreciate for any help.
What you're trying to build is called a "transmultiplexer". The Pentek
article has oversimplified the structure somewhat, leaving out the
polyphase filter that's usually placed on the input of the FFT. With
the polyphase filter you won't need a compensation filter after the
FFT. Do a bit of googling on the term transmultiplexer to get a better
idea of how to build one. You might also throw in the name 'harris',
since fred harris has written a lot about this structure - even has a
pretty good book out on multi-rate systems which goes into a fair
amount of detail on it.
When properly implemented, a transmultiplexer converts each of the
bandpass channels down to be centered at DC.
Eric
