USRP1 DBSRX phase noise



I would like to capture signals using the USRP1 (rev 4.5) with DBSRX.
the first test I am using CW signal from generator at 1.501GHz, 0 dBm
(verified to be precise). To capture the signal I use the
( -R a -d 8 -f 1.500G -N 40064 -s -g 45 usrp.dat)

I plot the FFT using Matlab (rectangular window, FFT length 65536).
At first it appears fine (there is a DC component, but OK):

Zooming in the CW peak:
There is a lot of noise, the peak is not sharp (width 2 kHz). Looks like
phase noise to me. Or is there very fast frequency drift?

Thinking the signal might be saturated, I’ve reduced the gain. The
range is smaller, but also the SNR

I’ve tried to use hi quality external reference clock (64MHz), following
instructions form USRP Clocking Notes. Unfortunately there was no

Is there an explanation for this behavior?

Thank you.

PS: Sorry for multiple posts, if any…

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