[USRP-users] Switching and high spike in spectrum

bob wole bnwole at gmail.com
Fri Oct 31 02:31:23 EDT 2014

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
> > USRP-users at lists.ettus.com
> > 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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