On 10/29/2014 01:54 PM, bob wole via USRP-users wrote: > > > > USRPN210r4 with SBX > > > > I am observing a strong spike at the center of the receive spectrum > > when I start burst transmission. > > > > My top flowgraph contains following two hierarchical blocks > > 1) A transmitter flow graph with (tx_time, tx_sob, tx_eob) > > 2) A receiver flow graph > > > > When I run top flowgrpah (without transmitting anything) and observe > > the FFT of the received signal the spectrum does not contain high > > spike in the center. > > > > But as soon as I start transmitting in burst mode I see a very high > > spike in center of the received signal FFT spectrum. It looks like LO > > (transmitter or receiver ) is being received? Which one is it ? And > > why is it happening? How can I avoid it because it is affecting my > > packets. > > > > When I apply the offset in digital using DDC/DUC, the spike moves out > > of the band. > > > > > > -- > > Bob > > > > > > _______________________________________________ > > USRP-users mailing list > > [email protected] > > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > That spike in the middle is a consequence of using direct conversion in > both the RX and TX paths--it'll be there in both to some degree. > > You can use offset-tuning to move the DC offset outside your passband: > > http://files.ettus.com/manual/page_general.html > > > In built-for-a-particular-purpose radios, there will also be undesired > LO leakage and mixing products--those are generally dealt with using an > application/band-specific filter to eliminate them. For > general-purpose SDRs, that isn't possible to do "as manufactured", you > have to deal > with RF hygiene and plumbing issues yourself. > > So, moving the LO leakage outside your passband is part of the > picture--use offset tuning for that. Then, if you have "this won't meet > our hygiene requirements", you have to look at filtering. > > Another thing you really should do is to run the calibration utilities, > which will attempt to balance I/Q amplitude and phase, which can improve > some of these issues, but not, usually, eliminate them entirely. > > > Yes, I know that LO leakage/DC offset is an issue present in direct conversion receiver. But as I mentioned earlier, the received spectrum looks fine (a very little spike at DC around -70dB) while the burst transmission is not running. The spike becomes much more significant ( high spike at DC -20dB) when burst transmission (tx_time,tx_eob, tx_sob ) starts and all the spectrum just shifts up and down with it. I am using TX/RX antenna in both usrp source and usrp sink. I want to know why the burst transmission is affecting the received spectrum on the same node.
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