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[Discuss-gnuradio] Spurious Signals and DC Offset


From: Lee Patton
Subject: [Discuss-gnuradio] Spurious Signals and DC Offset
Date: Fri, 09 Dec 2005 00:39:18 -0500

I am seeing very pronounced DC offset and spurious signals when
collecting data with the DBSRX.  I have updated to the latest CVS for
both gnuradio and USRP.

I found this post, which discusses DC offset being due to the A/D and
FPGA rounding.
http://lists.gnu.org/archive/html/discuss-gnuradio/2005-04/msg00058.html

Also, this post discusses a means of correcting for A/D offset
http://lists.gnu.org/archive/html/discuss-gnuradio/2005-04/msg00257.html

However, I am under the impression that the FPGA is now supposed to be
doing this automatically, but I don't think I'm seeing that.  Do I need
to enable something?

I would appreciate any help making sense out of some plots of my data.
The plots (attached) are as follows.

Data Collection 1:
Just noise, DBSRX connected to RxA, no antenna connected.

  1) noise_ave_pwr.jpg
    * normalized average power over time
    * is there any reason why the power would start high then go low?

  2) noise_ave_psd.jpg
    * average PSD of a chunk of the data
    * why the very large energy at DC?
    * why the spikes at +/- 1 MHz and -2 MHz?

  3) noise_wfall.jpg
    * a waterfall plot (PSD over time)
    * can see the DC +/- 1MHz and -2MHz spikes are present throughout

Data Collection 2:
Antenna pointing at an 802.11b access point, DBSRX connected to RxA

  1) wifi_psd.jpg
    * fft of 0.5 sec. of data
    * very pronounced DC component
   
  2) wifi_psd_ave.jpg
    * average PSD of entire data collection
    * DC component not as pronounced
    * I guess the half-band filter in the FPGA is causing attenuation at
the edges of the band?

I have attached the script I am using to collect the data. It is just a
customized version of usrp_rx_file.py.  I seem to see these affects
regardless of frequency, pga gain, or whether I write complex or short
samples to file.  On a related note, besides space considerations, are
complex or short samples preferable one to another?

Please find attached the plots.

Thanks, 
  - Lee

Attachment: noise_ave_psd.jpg
Description: JPEG image

Attachment: noise_ave_pwr.jpg
Description: JPEG image

Attachment: noise_wfall.jpg
Description: JPEG image

Attachment: wifi_psd_ave.jpg
Description: JPEG image

Attachment: wifi_psd.jpg
Description: JPEG image

Attachment: capture.py
Description: application/python


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