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RE: [Discuss-gnuradio] Artefacts in usrp_fft.py

From: Mike Garcia
Subject: RE: [Discuss-gnuradio] Artefacts in usrp_fft.py
Date: Fri, 30 Mar 2007 21:50:37 -0700

Hi Luis,

Google "receiver spurs" or "rf spurs" or "a/d spurs". Those kinds of things
may be the culprit. One test I often do if I suspect a spur is note the
frequency of the spur, tune the receiver by say 1khz, and then look at the
frequency of the spur again. If it doesn't move or it moves in the opposite
direction or it moves an amount other than 1khz, than it's an internal
artifact and not external. Can you connect a spectrum analyzer in front of
the A/D and see if the spurs are there? You might also consider histograming
your noise-only input. The histogram should be Gaussian shaped and
zero-mean. Any sort of A/D problem could show up in the histogram. For
example, spikes in the histogram may indicate bits that are "stuck". Also,
A/D's tend to have quantization effects that are periodic and so show up as
spurs. Adding dithering noise helps to break up the periodicity and reduce
the spurs. I'd be interested in what you find.


-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Luis Simoes
Sent: Friday, March 30, 2007 7:47 AM
To: address@hidden
Subject: [Discuss-gnuradio] Artefacts in usrp_fft.py

Hi all,

We implemented a spectrum senser based on GnuRadio for our cognitive radio 
research. In order to calibrate the senser we check the performance of the 
RFX2400 and DBS RX d'board with the usrp_fft.py script. We use the norma FFT

and the waterfall plot of the script. Feeding the usrp with generated
by a high performance signal generator we could observe the appearance of 
some strong artefacts in the spectrum. To find out the origin of this 
artefacts we put a calibrated 50 ohm termination on the sma antenna
in order to assure we only measuring the noise floor of the usrp. Now we 
could observe several artefact in almost all frequencies between 800 and
MHz. Sometimes the artefacts are sharp peaks some times wider peaks and the 
total number of peaks varies also from center frequency to center frequency.

Normally we use a decimation rate of 8 to sense a chunk of 8 MHz and we try 
several gains of the d'board. If someone know how the origin of these 
artefacts and/or how to avoid getting them in the sensed spectrum please 
answer. Every advice will be appreciated. Thank you all in advance

Luis Simoes

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