Dealing with TX/RX cross-talk on USRP

I’m looking for tips on how to deal with the USRP TX/RX leakage. I’m
transmitting with 30db (max) gain while trying to measure RSSI on the
RX2
channel and experiencing about 40db of energy leakage from the TX/RX. My
TX
and RX chains are separated by 50MHz. Not sure how I can filter it out.

I’m thinking an antenna amp would work, so I could turn down the gain on
the USRP; however, I want to try and avoid this.

Is there a fast way to switch between RX/TX channels? I did some quick
tests with just turning off the TX channel flow graph, but the leakage
takes a long time to dissipate.

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Hi Ken,

first question: Which USRP/Daughterboard?
Maybe you should be asking this question on the
[email protected] mailing list; I’m going to go ahead and CC:
that list in my reply.

Basically, limited crosstalk is very hard to avoid; especially when
trying to deal with a weak RX while TX’ing high power you’ll run into
problems you can’t really circumvent due to the fact that RF signals
on the same PCB inevitable leak.

An RX LNA close to the antenna is usually the way to go here, or even
separating RX and TX.

You could try turning off the TX amp (gain to 0 and transmit
complex(0,0)), maybe that might help more than shutting down the flow
graph. If you manage to get success using that way, I’d greatly
appreciate you notifying the list.

Greetings,
Marcus

On 05.04.2014 22:01, Ken A. wrote:

quick tests with just turning off the TX channel flow graph, but
the leakage takes a long time to dissipate.

_______________________________________________ Discuss-gnuradio
mailing list [email protected]
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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Hi Ken,

I’m looking for tips on how to deal with the USRP TX/RX leakage. I’m
transmitting with 30db (max) gain while trying to measure RSSI on the RX2
channel and experiencing about 40db of energy leakage from the TX/RX. My TX
and RX chains are separated by 50MHz. Not sure how I can filter it out.

I’m going to assume WBX/SBX/RFX DB here (which have pretty much the
same TX/RX + RX2 path switching).

40 dB seems a bit low for the isolation, usually I’d expect 50-60 dB.

But then, the baseband filter should help you gain more. If you’re not
already doing it, you can offset tune so that the carrier you’re
transmitting/receiving is more “on the edge” of the baseband low pass.
You need to plan this depending on the daughter card bandwith.

If you need more, there is some hw mod you can do on the board.
Removing the cap between RX and TX switch and connecting both end to
ground is the easiest fix to increase isolation. One guy even went as
far as sawing a RFX board in two to increase isolation (yeah, I know
that sounds crazy but both half are pretty much independent … it
probably voids your warranty though, you’ll have to check with ettus
:stuck_out_tongue: )

Cheers,

Sylvain

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