[Discuss-gnuradio] spurious spikes in FFT

[Kutik]

My HW configuration: USRP1 with DBSRX, no input signal.

Using the script to plot FFT usrp_fft.py -f 1003.25M -d 8 -g 72 I see two
spurious spikes at 1000 MHz and 1004 MHz (the next one is at 1008 MHz).

http://www.nabble.com/file/p23854396/fft_no%2Bsignal.png 

Where does these spurious signals come from? In the thread "Spurious Spikes
Seen with usrp_fft.py" 
http://www.nabble.com/Spurious-Spikes-Seen-with-usrp_fft.py-td14747991.html#a15025079
one of the explanations is the 64MHz clock, but then the spikes should be in
64MHz intervals.

On the picture below with 0dBm CW input signal at 1015 MHz you can see the
spurious spike is significantly strong. Reducing the gain removes the both
signals - the CW and the spurious.

http://www.nabble.com/file/p23854396/fft_with_signal.png 

Does anyone have experience how to remove/suppress the spurious signals?
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