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Re: [Discuss-gnuradio] Selecting specific USB ports for USRP

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Selecting specific USB ports for USRP
Date: Thu, 09 Aug 2007 12:05:05 -0700
User-agent: Thunderbird (X11/20070604)

Jeffrey Ung wrote:

> Is there any way I could use one USRP to both transmit and receive a
> signal?  That is, I want to use benchmark_tx.py to send out a dbpsk
> modulated signal and use benchmark_rx.py to receive this same signal. 
> Would this be possible?  I'm using a FLEX400 daughterboard.  I actually
> have two of them.

Yes.  It's easiest to use both of them, one in each side.

For benchmark_tx.py, use '-T A' to transmit using the FLEX in side A.

In another terminal, use benchmark_rx.py with '-R B', to receive with
the FLEX on side B.

Signal isolation isn't the greatest, though I haven't tested it between
different daughterboards in the same USRP case.

Whatever you do for an external antenna, do NOT just cross-connect the
Tx output and Rx input connectors.  There must be at least 20 dB of
attenuation, 40 dB is recommended.  Otherwise you'll damage the input
stage of the receiver.

You can do the same with a single FLEX board, but you have to modify the
examples to enable using separate Tx and Rx antennas.  Leakage is likely
to affect your results.  In fact, I recall someone saying that leakage
on the board was sufficient that they didn't need to hook any antennas
or cables between the Tx and Rx ports.

Johnathan Corgan
Corgan Enterprises LLC

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