vivkr
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Hi min,
You should remember that the modulator will react to signals both inband and out-of-band. Since you are interested in inband signals, it makes sense to prefilter the input to minimize energy outside your band of interest.
The NTF poles lie outside the signal band of interest and if the input to the modulator contains significant power at frequencies close to the NTF poles, this can excite it and induce oscillations, which are worse if high-Q poles are used in the NTF. Hence, the need for prefiltering.
For more details, please see Chapter 5 by Bob Adams in the book by Norsworthy, Schreier and Temes.
Regards Vivek
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