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Re: [Discuss-gnuradio] Question about benchmark_tx and benchmark_rx

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Question about benchmark_tx and benchmark_rx
Date: Wed, 14 Dec 2011 19:24:33 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20111108 Fedora/3.1.16-1.fc14 Thunderbird/3.1.16

Hi there,

I made some decent progress but refining the parameters for the benchmark_tx.py and benchmark_rx.py without no errors. However, I can only transmit but I could not receive anything at the receiver side. Could somebody suggest what might be the problem? I tried to work it but still could not find answer on why I did not received anything.

Used code:

./benchmark_tx.py -f 400M -r 250k -S 4


./benchmark_rx.py -f 400M -r 250k -S 4

OS: Ubuntu 11.10 (which I read in blog said that it has some problem with gnuradio)
Gnuradio: 3.5.0 rc 0 (which I git from master branch)
Hardware: USRP1(transmitter), USRP N210(receiver)
Machine: Lenovo T510 (transmitter), Dell Desktop


A common reason for this type of problem is frequency-offset between RX and TX. On the RX side, use uhd_fft.py to observe the spectrum and see where the receiver thinks the peak of the spectrum is. Then adjust your '-f' accordingly on a subsequent run of benchmark_rx.py.

Crystal oscillators aren't perfect. Even an error of a few 10s of PPM can add up to several Khz at center frequencies in the hundreds of MHz. Unless the receive flow-graph has a way of correcting gross frequency error, you will have to do that manually.

This is such a common problem that I'm surprised you haven't covered it in your coursework yet. Real radios (and radio channels) have
  impairments that often aren't adequately modelled by simulations.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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