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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
noise_ave_psd.jpg
Description: JPEG image
noise_ave_pwr.jpg
Description: JPEG image
noise_wfall.jpg
Description: JPEG image
wifi_psd_ave.jpg
Description: JPEG image
wifi_psd.jpg
Description: JPEG image
capture.py
Description: application/python
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