[USRP-users] Random errors while transmitting certain BPSK/QPSK types on x310s

Michael Carosino m.carosino at gmail.com
Fri Jul 7 13:20:58 EDT 2017


Hi Marcus,

you are correct, reducing the magnitude did seem to fix the issue. I am
wondering though, the gnuradio constellation modulator hier block also
seems to suffer from the same issue and doesn't have any options for
reduced magnitude output - do most people just throw in a downstream block
to reduce the magnitude? Also, is there any reference that speaks a bit
about the scaling issues that can happen with the USRP's or is that the
just type of thing you learn on the fly?

Thanks,
Mike


> Date: Thu, 06 Jul 2017 21:43:54 -0400
> From: "Marcus D. Leech" <mleech at ripnet.com>
> To: usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] Random errors while transmitting certain
>         BPSK/QPSK types on x310s
> Message-ID: <595EE75A.4070805 at ripnet.com>
> Content-Type: text/plain; charset="windows-1252"; Format="flowed"
>
> On 07/06/2017 09:36 PM, Michael Carosino via USRP-users wrote:
> > A quick update to this question with more info. I did some further
> > analysis by capturing the received I/Q data from the USRP Source block
> > when transmitting the BPSK that works without errors (symbols are
> > 0.707+0.707j, -0.707-0.707j) and also when using the BPSK that gives
> > errors (symbols are +1/-1). You can see in the attached image there is
> > quite a bit of small magnitude anomalies for the second image (also
> > errors are probably occurring a bit more frequently than I had
> > previously estimated).
> >
> > To me this points to the issue being either something to do with the
> > USRP or possibly with the TX chain blocks.
> With a baseband magnitude near 1, scaling issues/filtering/interpolation
> issues can combine to produce distortions.
> I usually use a baseband magnitude no greater than 0.9 or so.
> >
> > On Thu, Jul 6, 2017 at 4:37 PM, Michael Carosino <m.carosino at gmail.com
> > <mailto:m.carosino at gmail.com>> wrote:
> >
> >     Hi all,
> >
> >     running Gnuradio 3.7.10.2 and UHD 4.0.0 rfnoc-devel latest commit
> >     (tried earlier versions too). I've got a simple tx/rx flowgraph
> >     going on. The simple description is:
> >
> >     Random input data -> Pack 1 Bit->Chunks to Symbols->Interpolating
> >     FIR Filter->USRP Sink
> >
> >     USRP Source-> Polyphase Clock Sync -> Costas Loop-> Constellation
> >     Receiver->Unpack 1 Bit
> >
> >
> >     I'm transmitting on RF-B TX/RX and receiving on RF-B RX. The
> >     system works almost perfectly, except that there are single bit
> >     errors occurring (not many, maybe every couple of seconds at 500k
> >     samp rate).
> >
> >     Now here's the real strange thing, these errors are ONLY present
> >     if running real BPSK (-1,+1), imaginary BPSK (+j,-j), or rotated
> >     QPSK/QAM (+1j,-1j,+1,-1)
> >
> >     If I use a BPSK having symbols with real and complex parts like
> >     (-.707-.707j, .707+.707j) or QPSK (+/- .707+/- .707j) the errors
> >     are NOT present.
> >
> >     A couple more notes:
> >     Happens if using different x310s or the same for tx/rx.
> >     Happens even if I try to add a small complex value before sending
> >     data to the USRP.
> >     Error always happens as a single bit error (not bursty).
> >
> >     Attached are the constellation plots out of the polyphase clock
> >     sync (PFB) and the costas loop. My guess is the issue is either at
> >     the USRP or the Polyphase Clock Sync block.
> >
> >     Anyone seen something like this before? I'll probably start diving
> >     in the polyphase clock sync code to figure what's going on.
> >
> >     Thanks,
> >     Mike
> >
> >
> >
> >
> > _______________________________________________
> > USRP-users mailing list
> > USRP-users at lists.ettus.com
> > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>
>
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