Dear List,
I am using a USRP-2 with a WBX daughterboard that is operating in full
duplex mode. Both the Tx and Rx gain are cranked up to their maximum
values (tx gain 25 dB, rx gain 30 dB). The Tx and Rx frequencies are
about 70 MHz apart (Tx->964 MHz and Rx->892MHz). I'm having a little
trouble with the self-interference:
-when the Tx chain is sending periodic packets (with 0s in between),
this creates very high noise in my Rx chain (up to 0.01 on gnuradio
companion scope)
-when the Tx chain is sending a continuous pilot tone, there is no
noise in my Rx chain (noise is lower than 0.0005 on gnuradio companion
scope)
I would like that, when transmitting periodic packets, the noise would
be as low as when transmitting a continuous pilot tone. I understand
that periodic packets would create noise in a wider band than the
periodic tone, but still, my Tx and Rx frequencies are very far apart.
Is there some automatic gain in the USRP that is playing tricks here?
If so, is there a way to solve this?
Any input is appreciated,
Thanks a lot,
Francois
PS: for our application, we had to turn the DUC cordic off. But even
with the DUC cordic enabled, I still observe similar trends, so I don'
suppose that would matter. We also had to put the Clock source to
"external" to avoid the automatic clock correction (but again, putting
it to "Default" didn't change a thing).
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I a fully-engineered full-duplex system, there'll be a duplexor to
provide better isolation (70-80dB) for the RX gain an mixer. Your RX
front-end is
"seeing" all of the TX energy you're transmitting, and very likely
being drive into non-linear operating territory -- just because your
mixer is tuned
70Mhz away from the TX, *does not mean* that your RX LNA is "tuned"
only to your tuned frequency. You might able to get away with just using
a notch for your TX frequency in front of your RX, but without some
actual RF plumbing/systems-engineering, you'll run into "desense" issues
like
this.
Any such duplexing arrangements are clearly, *necessarily* application
specific. Which is why there's no duplexor or other filters on the
daughtercards.
Since they're used for a *huge* variety of applications, there's no
way to engineer them to always "do the right thing" regardless of
application.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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