USRP2 frequency offset problems

HI all,

My application needs to have accurate frequency outputs. The function
usrp2::set_rx_center_freq only set the daughter board to a frequency
close to what I need. The center frequency of the output has a random
offset to the tune_result. My question is how to control the output
carrier center frequency? How can I set the center frequency to be
processed close to what I need and how to control the output carrier
frequency?
Thanks a lot!