SV: [Discuss-gnuradio] Syncrinization of two USRP2s

[Patrik Eliardsson]

We have already implemented start_streaming_at(0)  (see attached file). The problem is that the FPGA doesn't reset the ddc on PPS so the sample marked with 'time stamp' = 0 is delayed by an unknown fractional number of samples realtive to the output rate.

 

regards

 

Patrik and Ulrika

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Från: address@hidden [mailto:address@hidden För Douglas Geiger
Skickat: den 8 juni 2009 15:08
Till: Ulrika Uppman
Kopia: address@hidden
Ämne: Re: [Discuss-gnuradio] Syncrinization of two USRP2s

 How are you starting the streaming samples from the two 
USRP2's? The default start_rx_streaming call (and thus, the standard 
usrp2_source block) just starts streaming when it receives the command from the 
host PC. So any delay between when each USRP2 receives the command over the 
gig-E interface means will be reflected in the timestamps of the frames they 
send back. In other words, the internal clocks are/maybe synchronized, but you 
won't be able to tell that from the timestamps alone.
 I'm sitting on a 
(at least mostly working) patch to add a start_rx_streaming_at(...) call to the 
USRP2 host and firmware code, which lets you specify a time (according to the 
USRP2's internal timer) to start streaming at. I use this to send a 
sync_to_pps() command, and then start streaming at a specific time (since the 
sync_to_pps() should result in the timer getting reset to zero at the same time 
on each USRP2). From what I understand this will be much easier to do with the 
usrp2-vrt rewrite, but in the meantime, I expect to be sending the patch to the 
list soon.
 Doug

On Mon, Jun 8, 2009 at 6:51 AM, Ulrika Uppman <address@hidden> wrote:

And again we are back to this misspelled topic.

Thank you Matt for your reply. It turned out that we used non synchronous 1PPS and 10 MHz signals after all, and by using a 1PPS synchronous to the 10 MHz we got different results.

However, we still don't get the time stamped samples to be perfectly synchronous in time (yes we do use the time field in the received data). We believe that the DDC is the issue here. Is there any present solution to reset the DDC at a certain time (for example at the PPS signal)?

Best regards
Ulrika

-- 
Doug Geiger
address@hidden

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