Re garding uhd_fft.grc

Yesterday i ran uhd_fft.grc using gnuradio companion.I tried to sense
spectrum from 2.4 to 2.5 GHZ.In the graph that i got, i could see a
vertical line at center frequency,even if i change the frequency also
i am
getting same graph .I am not understanding why i am getting this sharp
vertical line at center.

I attached screen shots of the graphs.
I am using gnuradio with USRP2 - XCVR daughter board.

Thanks & Regards,

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My guess is it’s a DC component resulting from the way the XCVR
downmixes from carrier frequency (direct conversion). I have that
problem too with an SBX daughterboard. There is a way to offset the LO
in the mixing stage to do low-IF instead of direct conversion and that
apparently removes the DC although I haven’t tried that personally.

Also worth mentioning, it gets worse if you set the gain too high.