In general that is correct, analog filtering has been applied to the signal chain prior to the ADC to prevent significant spectral folding. Specifically, the individual radio daughter cards provide filters of appropriate bandwidth for each design, there is no analog filtering circuit in the actual USRP1 motherboard. The BasicRX and TX boards are the sole exception to this, they provide no on board filtering.
On Jun 18, 2012, at 9:18 PM, signalswdm wrote: > Thank you > To make it clear, I use USRP1. Does that mean in common the signals > that come out of ADC chips (in USRP case it is the signals come out of AD9862 > before processed in FPGA) is already filtered and aliasing prior is partly > eliminated ? In the datasheet I do not see anything about that. If I am wrong > please correct me. > > Best regards > > > At 2012-06-19 09:54:11,"Marcus D. Leech" <[email protected]> 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 >> [email protected] >> 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. > > > > -- > Principal Investigator > Shirleys Bay Radio Astronomy Consortium > http://www.sbrac.org > >
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