Re: [Discuss-gnuradio] self-interference with wbx

[Matt Ettus]

One possible reason is that your transmitted signal may have a wider spectrum than you think due to the intermittent nature and or intermod.  You should look at the spectrum of the output signal and make sure you are not transmitting wider than you think.  Rapid on-off keying of the transmitted signal without ramping up and down at the beginning and end will create spectral noise.  This is why most standards like bluetooth and 802.11 specify amplitude ramp functions.

Matt

On Thu, Dec 20, 2012 at 5:00 PM, Francois Quitin <address@hidden> wrote:

Hi Marcus,

 

Thanks for your answer. I do expect some of the Tx power to leak into my Rx chain, and my application can live with that. What I don’t understand, is why this power is much higher when transmitting packets than when transmitting a continuous tone.

 

Francois

 

De : discuss-gnuradio-bounces+fquitin=address@hidden [mailto:discuss-gnuradio-bounces+fquitin=address@hidden] De la part de Marcus D. Leech
Envoyé : jeudi 20 décembre 2012 16:52
À : address@hidden
Objet : Re: [Discuss-gnuradio] self-interference with wbx

 

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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