Amplitude variations inn FFT sink

Hi all

I’m trying to use the USRP1 as a beacon receiver i.e. measure the
amplitude and frequency of a signal then store the detected values to
file. I’m using the DBSRX daughter board to measure satellite beacons.

However I have ran into some problems. When viewing the FFT plot from I can see amplitude variations of up to several dB when
using a highly stable frequency generator. The amplitude variations
seem to increase with decreasing input power i.e lower C/N. Even with
the averaging function turned on the fluctuations are noticeable at
about 10 dB C/N.

My question is therefore: What is causing this ? And is there a way to
avoid or reduce this effect ?

I have already tried to reduce bin size an added FIR filter to the

Thanks in advance.