Re: [Discuss-gnuradio] Basic question about filter

[Ian Buckley]

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" <address@hidden> 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

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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
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