Why dxc_freq never goes to zero after tuning?

Dear fellows,

I am using USRP with d’board Flex 2400 to transmit and receive a
wideband signal. My target frequency (carrier frequency) is flexible
and could be anything within the d’board range 2.3 to 2.9 GHz. I am
trying to set DDC at zero frequency. Non-zero DDC may have some
unwanted effects (not sure here!). I am using usrp_siggen.py script
with my own signal generation program. When I looked at the output of
‘tune’ function the DDC never goes to zero (by reading dxc_freq). I
tried using different target frequencies at the command line of
usrp_siggen but dxc_freq never went to zero.

BTW, I understand the two step tuning process but I was expecting that
for some target RF frequency dxc_freq should be zero. If this is not
possible, I will explain my concerns about the non-zero DDC for
wideband signals.

Any help/pointer is highly appreciated.

Thanks,

Faisal