FYI I've noticed a very similar issue, isolated it to non usrp gnuradio blocks in tx chain. Specifically the constellation or *psk modulator blocks. Certain combos produced garbage IQ data(visible as several spikes in PSD display) at regular intervals (seemingly related to sample rate), this is visible even with a very simple flowgraph, dumping data to a file, no USRP involved. I actually changed the underlying data I was modulating and the bad data appeared at the same position in time no matter how many times I ran the flowgraph. I did try moving the symbols closer away from unity, so used something like (-.707, .707) but that didn't change anything. Still haven't completely eliminated user error, so I was going to try to make an example flowgraph later today and submit to discuss-gnuradio.
On Jul 6, 2017 9:38 PM, "Michael Carosino via USRP-users" < usrp-users@lists.ettus.com> 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. > > On Thu, Jul 6, 2017 at 4:37 PM, Michael Carosino <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 > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > >
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