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[Discuss-gnuradio] Interpretting results from usrp_rx_cfile.py

From: Bahn William L Civ USAFA/DFCS
Subject: [Discuss-gnuradio] Interpretting results from usrp_rx_cfile.py
Date: Wed, 17 Jan 2007 16:37:58 -0700

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. I
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 two
and a 10kHz sine wave for the third.

Each time I captured a few seconds worth of data. I then took the data
files and extracted two streams of single precision floats for each of
the first ten thousand 8-byte groups in the file and plotted them. The
results were very similar regardless of the frequency specified on the
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 for
in the vicinity of sample #530, which looks like an inverted sinc pulse
with a peak amplitude of about -10000 (the first stream, which is made
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 the
sinc pulse. 

This makes absolutely no sense to be whatsoever. Can anyone shed any
light on this?


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