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

mleech at ripnet.com mleech at ripnet.com
Fri Jul 7 13:39:55 EDT 2017

If the modulator blocks always produce in {-1,+1}, then a downstream
multipier would be required to reduce the baseband magnitude. 

It will happen with any radio where there is a translation from floating
point {-1.0,+1.0} into some integer representation on the wire, followed
by interpolation/upconversion in the digital domain on the radio, and
then out the DAC, where it may be driving the mixer a little hard, or
the subsequent RF gain stages. 

On 2017-07-07 13:20, 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" <mleech at ripnet.com>
>> To: usrp-users at lists.ettus.com
>> Subject: Re: [USRP-users] Random errors while transmitting certain
>> BPSK/QPSK types on x310s
>> Message-ID: <595EE75A.4070805 at 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.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
>>> _______________________________________________
>>> USRP-users mailing list
>>> USRP-users at lists.ettus.com
>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com [1]
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