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.
Mike -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Luis Simoes Sent: Friday, March 30, 2007 7:47 AM To: [email protected] 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 signals 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 connector 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 2500 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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
