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

Ron Economos w6rz at comcast.net
Fri Jul 7 15:20:05 EDT 2017


I've created an OOT module with a little utility I use to print peak IQ 
levels. This allows for precise setting of the gain to insure IQ levels 
are within +1/-1.

https://github.com/drmpeg/gr-iqlevels

Ron

On 07/07/2017 11:28 AM, Ron Economos via USRP-users wrote:
>
> 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" <mleech at ripnet.com
>>     <mailto:mleech at ripnet.com>>
>>     To: usrp-users at lists.ettus.com <mailto: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
>>     <mailto: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>
>>     > <mailto:m.carosino at gmail.com <mailto:m.carosino at 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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>>
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