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RE: [Discuss-gnuradio] Interpretting results from usrp_rx_cfile.py

From: Bahn William L Civ USAFA/DFCS
Subject: RE: [Discuss-gnuradio] Interpretting results from usrp_rx_cfile.py
Date: Fri, 19 Jan 2007 09:49:14 -0700

// SIGNED //
William L. Bahn
Instructor of Computer Science
United States Air Force Academy
e-mail: address@hidden

> -----Original Message-----
> From: Eric Blossom [mailto:address@hidden
> Sent: Wednesday, January 17, 2007 4:45 PM
> To: Bahn William L Civ USAFA/DFCS
> Cc: gnuradio mailing list
> Subject: Re: [Discuss-gnuradio] Interpretting results from
> usrp_rx_cfile.py
> On Wed, Jan 17, 2007 at 04:37:58PM -0700, Bahn William L Civ
> wrote:
> > I have a function generator outputting a sine wave into the RX-B
> > connector of the BasicRX board connected to the RX-B side of a USRP.
> > am trying to capture the waveform and store it to a file.
> >
> > Here are the commands I tried:
> >
> > # ./usrp_rx_cfile.py -R B -d 256 -f 1000 sine_1k1.dat
> > # ./usrp_rx_cfile.py -R B -d 256 -f 10000 sine_1k10.dat
> > # ./usrp_rx_cfile.py -R B -d 256 -f 15000 sine_10k15.dat
> >
> > The function generator was producing a 1kHz sine wave for the first
> > and a 10kHz sine wave for the third.
> >
> > Each time I captured a few seconds worth of data. I then took the
> > files and extracted two streams of single precision floats for each
> > the first ten thousand 8-byte groups in the file and plotted them.
> > results were very similar regardless of the frequency specified on
> > command line (and I have no idea what that frequency is for) or the
> > actual frequency of the signal that was applied to the inputs.
> >
> > What I got in all cases was basically a flat line (-3 to +3) except
> > in the vicinity of sample #530, which looks like an inverted sinc
> > with a peak amplitude of about -10000 (the first stream, which is
> > up of the first float value in each sample pair), and what looks
like an
> > extremely heavily damped sine wave (the second stream) with a peak
> > amplitude of about 20,000 and roughly centered at the same place as
> > sinc pulse.
> >
> > This makes absolutely no sense to be whatsoever. Can anyone shed any
> > light on this?
> >
> > Thanks
> The Basic RX only passes signals > 100kHz.

That helped quite a bit. When I raised the frequency to 200kHz and
looked at the data, I now saw related data in the captured file after
the large transient event. The data looked strange, but appeared to have
a quadrature-like pattern to it. So that suggested to me that the
frequency specified on the command line is the frequency for the
oscillator of a sin/cos mixer. Changing the frequency to zero then
produced a cosine waveform and a flatlined-waveform, which makes sense.

As for the spike at about sample 530, I am now of the opinion that this
is a switching transient as the board configures the hardware.

I only see data when the signal is applied to the RX-A input, not the
RX-B (here I am referring to the two inputs on the daughterboard, not
the daughterboard slots on the USRP. I am still trying to figure out how
to get data from the other input, but that is fairly low priority.

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