Hi Michael, point is that having a magnitude too close to 1 can lead to problems in the FPGA side of DSP – not in GNU Radio.
Best regards, Marcus On 07/07/2017 07:20 PM, Michael Carosino via USRP-users wrote: > 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" <mle...@ripnet.com <mailto:mle...@ripnet.com>> > To: usrp-users@lists.ettus.com <mailto:usrp-users@lists.ettus.com> > Subject: Re: [USRP-users] Random errors while transmitting certain > BPSK/QPSK types on x310s > Message-ID: <595ee75a.4070...@ripnet.com > <mailto:595ee75a.4070...@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.caros...@gmail.com <mailto:m.caros...@gmail.com> > > <mailto:m.caros...@gmail.com <mailto:m.caros...@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@lists.ettus.com <mailto:USRP-users@lists.ettus.com> > > > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > <http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com> > > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
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