about decimation

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why do we need decimation in DDC,before the data go into the FIR digital filter,whose rate is fast.assume that the sample rate is 105MHZ,the single bandwidth is 40MHZ,if i need FIR to select 8MHZ's bandwidth from 40MHZ, is it ok,can the fir permit fast clock rate?how fast rate the fir can work.

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Your questions are far from clear. In a DDC you normally receive signal at high sample rate e.g. 105Msps (if I understood you). Your target sampling rate is then decided by what signal you are working with. If your final signal bandwidth is 8MHz(complex) then a sampling rate of 8Msps may be what you aim at. This should cover +/- 4MHz provided your signal is around 0 frequency(baseband).