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Re: [Discuss-gnuradio] Spurious Signals and DC Offset


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

Attachment: basicrx_ave_psd.jpg
Description: JPEG image

Attachment: basicrx_log.txt
Description: Text document

Attachment: dbsrx_ave_psd.jpg
Description: JPEG image

Attachment: dbsrx_log.txt
Description: Text document

Attachment: capture.py
Description: application/python


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