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From: | Lee Patton |
Subject: | Re: [Discuss-gnuradio] Spurious Signals and DC Offset |
Date: | Fri, 09 Dec 2005 16:59:14 -0500 |
Thanks, Matt. I appreciate the help. On Thu, 2005-12-08 at 23:25 -0800, Matt Ettus wrote: > > 1) noise_ave_pwr.jpg > > * normalized average power over time > > * is there any reason why the power would start high then go low? > > The first samples could be corrupted because the pipeline may not be > cleared. Drop the first samples Is there a programmatic way of clearing the pipeline before collecting data, or at least know how much data I have to throw away for each collection? It looks like the first 0.02 seconds worth. > > 2) noise_ave_psd.jpg > > * average PSD of a chunk of the data > > * why the very large energy at DC? > > It's less than 10 dB above the noise floor. Turn up the gain and you > won't see it. I see this no matter what gain I use. That goes for the DBSRX and BasicRx boards. Please see attached plots. > > > * why the spikes at +/- 1 MHz and -2 MHz? > > You are seeing synthesizer spurs or harmonics of the reference. You'll > notice that they are only about 3 dB above the noise floor. I figured it was something like that. The weird thing is that they're often much larger than 3dB above the noise floor. I even got rid of them for a very short period of time when I was experimenting with enabling/disabling the halfband filter (although I'm not sure that had anything to do with it.) > > > 1) wifi_psd.jpg > > * fft of 0.5 sec. of data > > * very pronounced DC component > > How many bins in the data? This was a full half second at fs = 4MHz. I think matlab zero pads to reach the next largest power of two for fft. So, that would be Nfft = 2^( ceil(log2( 0.5*4e6))) = 2097152, right? No windowing was done either. That should be about 2 Hz resolution. > > > > > 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? > > What command line options are you using for the script? ./capture -f 2.412G -g50 I've attached two more plots. One is the BasicRx with no antenna, and the other is DBSRX with no antenna. Two log files are attached as well so that you can see the settings. These were taken at 1800 MHz. Note: the PSDs shown here are calculated as follows: 1) 0.5 second data stored in x 2) x is reshaped into length(x)/1024 rows by 1024 columns 3) fft taken across columns of x 4) average magnitude squared of rows is taken and plotted. I guess I just don't know if what I am seeing is expected or not. If it is, that's fine. I just want to make sure there is nothing wrong with how I'm collecting the data. Thanks, - Lee
basicrx_ave_psd.jpg
Description: JPEG image
basicrx_log.txt
Description: Text document
dbsrx_ave_psd.jpg
Description: JPEG image
dbsrx_log.txt
Description: Text document
capture.py
Description: application/python
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