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


From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Question about benchmark_tx and benchmark_rx
Date: Wed, 14 Dec 2011 21:41:32 -0500

On Wed, Dec 14, 2011 at 7:24 PM, Marcus D. Leech <address@hidden> wrote:
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

and

./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

Regards,
Muhammad

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
http://www.sbrac.org


Can someone put this in the FAQ page on gnuradio.org? It probably deserves a more detailed explanation than a FAQ-level answer, but it'd be a start.

Tom
 

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