# Usrp_fft.py question and benchmark related question

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).

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
)?

Thanks

On Wed, Dec 01, 2010 at 04:57:11PM -0600, John A. 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.

Eric

Got it. Saw the earlier message too.

Thanks

On Wed, Dec 1, 2010 at 5:14 PM, Eric B. [email protected] wrote:

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…