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

Michael Carosino m.carosino at gmail.com
Thu Jul 6 19:37:57 EDT 2017

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

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

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.

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