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

Marcus D. Leech mleech at ripnet.com
Thu Jul 6 21:43:54 EDT 2017

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 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
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