[USRP-users] Retune stream tag arriving before retune Is complete

Marcus D. Leech mleech at ripnet.com
Tue Sep 9 20:05:55 EDT 2014


On 09/09/2014 08:01 PM, Jacob Gilbert via USRP-users wrote:
>
> Hello,
>
> I am having some trouble with a USRP B210 and quickly retuning it for 
> a wideband survey application. I have an OOT GR block which retunes 
> (DSP only, since the LO retune takes quite a while) the USRP via a 
> call to the usrp_source block's retune method, waits for the stream 
> tag, passes some data forward, then repeats.
>
> The issue I am currently having is on occasion the stream tag 
> indicating the retune will arrive well before the data is actually 
> valid for the frequency the tag is indicating it just tuned to. I had 
> originally thought the uhd source block would insert a stream tag when 
> the USRP indicated the retune had been completed, however in looking 
> into usrp_source_impl.cc:145, it looks like the tag is appended (or 
> indicated it should be appended) immediately after the tune call is 
> made regardless of whether or not the tune in the FPGA has actually 
> completed.
>
> Is this correct? And if so, is there a better way to know when a 
> DSP-only tune has completed and the data stream is valid for the new 
> frequency?
>
>
> Thanks,
>
> Jacob
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
The FPGA-only tune happens so quickly that the way it is currently 
written is "correct", modulo latencies in Gnu Radios internal buffering 
mechanisms,
   which are definitely outside the scope of what the hardware can control.

There will be ambiguous data after a retune, since you don't really know 
how many samples are "in flight" after doing that.



-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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