Re: Inject "tx_time" tags in a stream

[jnunez]

Hi Derek,

Thank you very much for your detailed response. I thought that tag's  
insertion could be done directly from GRC. I hoped that there would be  
some library to insert these tags in a stream. The truth is that I  
don't have enough knowledge to write my own block.

On the other hand, I think my problem (which is essentially a burst  
stream synchronized with a trigger signal) is conceptually very simple  
and must have been encountered by many more people before. I've  
searched the whole forum and it's not like that. I am also surprised  
that no one else has answered my question.

Regards,
José

> Hi Jose,
>
> You'll have to write a block to add those tags where you want them  
> in the stream (at the start of each burst) and with the time you  
> want them at (the time is relative to the USRP's internal timekeeper  
> clock). You'll also need to read from or set the time on that clock  
> at the start of the application.
>
> The first part you can do with an embedded Python Block to add those  
> tags. The second part you'll either need to modify the generated  
> Python flowgraph or use the very latest 3.9 development code which  
> added the Python Snippets block.
> https://github.com/gnuradio/gnuradio/pull/3169
>
> Ettus Research's UHD manual includes a helpful page on  
> synchronization and how to read/set the time.
> https://files.ettus.com/manual/page_sync.html
>
> Regards,
> Derek
>
> On 27/02/2020 19:33, address@hidden wrote:
> > Has anyone worked with tx_time tags?
> >
> > address@hidden escribió:
> >
> > > Hi, all!
> > >
> > > I am working on a proof of concept of a radar system for my  
> > > doctoral thesis. I recently installed the great gr-pdu and  
> > > gr-timing utilities from Sandia Labs and I am trying to develop a  
> > > little application with GNURadio Companion (GRC), based on a USRP  
> > > N200 system.
> > >
> > > I essentially want to use the rising edges of a pulsed signal to  
> > > generate another signal. For this purpose, first the trigger  
> > > signal is sampled with a USRP-Source block, then is threshold  
> > > detected by means of a "Threshold trigger" block and finally, a  
> > > message with a "Tag message trigger" block is generated. The  
> > > output of this last block feeds a "PDU to Bursts" block which  
> > > enters a USRP-Sink.
> > >
> > > I have checked with a "Tag Debug" that the threshold detection and  
> > > message generation are both working fine. At the output of the  
> > > "PDU to Bursts" block, each burst is tagged with the "tx_sob" and  
> > > "tx_eob" tags. The problem arises when those burts enter the  
> > > USRP-Sink. As far as I've read, this blocks transmits all burts  
> > > sequentially ignoring the time separation that should be between  
> > > them. Please, take into account that the trigger pulses are not  
> > > equally separated in time, so I must transmit one burst with each  
> > > received pulse.
> > >
> > > I think the issue is that no "tx_time" tags are being generated  
> > > and added to each burst. I wonder if anyone have the same  
> > > explanation for this issue, and hopefully, if anyone can guide me  
> > > on how can I inject those "tx_time" tags properly in my stream  
> > > with GRC.
> > >
> > > Any help will be appreciated!
> > >
> > > Regards,
> > > José