On Jun 18, 2012, at 6:54 PM, Marcus D. Leech wrote: > On 18/06/12 09:46 PM, signalswdm wrote: >> >> Hello everyone: >> You see in theory there should be a low-pass filter after ADC for >> eliminating the image. Also after DDC there should also be a >> filter. But in USRP there are not such things. So do the cic filter and half >> band filter implement that function besides decimation? >> >> Best regards >> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > Actually, in typical architectures using I/Q, there are low-pass filters *in > front* of the ADC to eliminate aliasing prior > to sampling. So the CIC decimator "sees" a signal that has already been > band-limited appropriate to the sample > rate of the ADCs. > > The CIC decimation process *is* filtering, so out-of-band products appearing > at the output are usually suppressed > by at least 80dB, and usually more. > > >
Indeed, to re-enforce Marcus's point with precise English terminology: Downsampling is a process that reduces the sample rate of a digital signal by simply discarding samples. Decimation is the compound operation that incorporates downsampling and an appropriate low-pass filter.
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