You can change the gain setting on the interpolating FIR filter to
accomplish this.
The QT GUI Histogram Sink can be used to set a gain value that limits
the IQ values to +1/-1.
Ron
On 07/07/2017 10:20 AM, 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" <mle...@ripnet.com <mailto:mle...@ripnet.com>>
To: usrp-users@lists.ettus.com <mailto:usrp-users@lists.ettus.com>
Subject: Re: [USRP-users] Random errors while transmitting certain
BPSK/QPSK types on x310s
Message-ID: <595ee75a.4070...@ripnet.com
<mailto:595ee75a.4070...@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.caros...@gmail.com <mailto:m.caros...@gmail.com>
> <mailto:m.caros...@gmail.com <mailto: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
>
>
>
>
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