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

Anon Lister listeranon at gmail.com
Fri Jul 7 13:19:01 EDT 2017

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 at 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.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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