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 <[email protected]> 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 :* [email protected] [mailto:discus > [email protected]] *De la part de* Marcus D. > Leech > *Envoyé :* jeudi 20 décembre 2012 16:52 > *À :* [email protected] > *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). **** > > ** ** > > ** ** > > _______________________________________________**** > > Discuss-gnuradio mailing list**** > > [email protected]**** > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio**** > > 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**** > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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