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

From: John Andrews
Subject: Re: [Discuss-gnuradio] usrp_fft.py question and benchmark related question
Date: Thu, 2 Dec 2010 12:12:30 -0600

On Wed, Dec 1, 2010 at 5:14 PM, Eric Blossom <address@hidden> wrote:
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...

Thanks for your reply Eric but can you please explain what it means or direct me to a place  where I can find out? That will be very helpful of you. I looked at the messages but I didn't not find which one you were referring to.


> 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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