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Re: [Discuss-gnuradio] usrp_fft.py question and benchmark related questi

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] usrp_fft.py question and benchmark related question
Date: Wed, 1 Dec 2010 15:14:37 -0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Dec 01, 2010 at 04:57:11PM -0600, John Andrews wrote:
> Hi,
> Can we change the DDC value to 0 in usrp_fft.py? Why is this not 0 as one
> would expect to input the center frequency same as the baseband frequency;
> i.e. if I know that my signal is on a carrier at 2.4G then I would use
> usrp_fft.py to display the spectrum by having the center frequency as 2.4G
> instead of typing in 2.404G (as DDC is -4M).

It's non-zero to move the RF DC offset problem away from zero (and in
most cases out of the range that anybody cares about).  See earlier
message today...

> In benchmark_tx/rx programs do I have to account for this DDC=-4.0M? If one
> USRP is transmitting at 2.4G ( ./benchmark_tx.py -f 2400M ) then should I
> have the receiving USRP receiving at 2.404G ( ./benchmark_rx.py -f 2404M )?

No you don't.  If you do -f 2400M then the complex baseband signal is
equivalent to the RF signal that's centered at 2400M.


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