Hi Khalid,
I had a similar issue with an LFTX that I had been working
with for about two years. I actually had two boards running on
separate USRP2's and both were working fine for quite a while,
but I recently noticed that one had a drastically lower output
range. I ended up replacing the LFTX and all is working well
now, so I'm assuming it was a hardware issue that came up at
some point.
I didn't take the time to look for the problem, but I
figured I'd share this info since what you're seeing might be
a hardware issue rather than something you can resolve with
settings.
-Mike