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Re: [Discuss-gnuradio] Can threading disturb sensing continuety

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Can threading disturb sensing continuety
Date: Mon, 23 Mar 2009 10:36:11 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Mon, Mar 23, 2009 at 08:00:52AM -0700, kaleem ahmad wrote:
> Hi,
> I have one more confusion about this sampling rate of USRP. 
> At FPGA level it is 64MSps and when we decimate (D = 4...256), the sampling
> rate is reduced to 64MSps/D. Now I am confused that if the actual sampling
> rate of USRP is changed from 64MSps to the new decimated sampling rate or
> the USRP is still sampling at 64MSps and only internal sampling rate (i.e
> just for channel selection) is changed.

The ADCs on the USRP always run at 64MS/s.  The decimation occurs
inside of the FPGA.  This is explained in the USRP FAQ on the wiki.

If this concept is still confusing, you might want to spend some time
looking at a text book that talks about decimating filters and/or
multirate signal processing.  http://gnuradio.org/trac/wiki/SuggestedReading

> Addition to this as maximum value of decimation rate is 256 so sampling rate
> can be reduced to 250ks/s, if I want to set it to 3ksps then how it is
> possible.

You will need to do additional decimation on the host.  You can do
that after the usrp.source_c block with a gr.fir_filter_ccf.  
grep the examples for examples.


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