SV: [Discuss-gnuradio] Syncrinization of two USRP2s

[Ulrika Uppman]

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

-----Ursprungligt meddelande-----
Från: Matt Ettus [mailto:address@hidden 
Skickat: den 14 maj 2009 19:07
Till: Ulrika Uppman
Kopia: address@hidden
Ämne: Re: [Discuss-gnuradio] Syncrinization of two USRP2s

Ulrika Uppman wrote:
> Hi,
> 
> We've set up a test comparing a recieved white noise signal recieved 
> by two usrp2 recievers. The two receivers were clocked (1PPS and
> refclk) by a GPS diciplined OCXO which outpus a 1PPS signal which is 
> phase locked to the 10 MHz reference. The daughterboard was the RFX400 
> configured with a center frequency of 450 MHz and a decimation factor 
> of 4. Several cross spectrums between the two received signals shows 
> that the syncronisation is drifting by +/- 1 sample. We would like the 
> difference to not drift. Is this possible?


I am not sure exactly what you mean by drifting by 1 sample.  Are you making 
use of the time fields in the received data?

The first thing to do would be to make sure that both USRP2s are locked to the 
reference.  You can do this by calling

        clocks_enable_test_clk(true, 1);

in the firmware.  This will turn on the clock on J503 of the USRP2 
motherboards.  Pins 1 and 4 of that connector are ground, and 2 and 3 are the 
differential clock.  Connect an oscilloscope to those pins on each USRP2 and 
make sure that the clocks do not drift relative to each other.  They should be 
perfectly locked and in phase.


The second issue is the 1 pps signal.  It must be synchronous with the 10 MHz 
reference clock.  GPSDOs have 2 types of PPS outputs, those that are 
synchronous to the OCXO (often called ocxo-derived) and those that are directly 
generated by the GPS.  You need to use the former.

One final issue is that depending on the relative delay of your PPS and 10 MHz 
signals, you might want to sample PPS on the positive edge or negative edge.  
If you are using the wrong edge, there may be timing issues.  To set this, in 
the firmware call:

        timesync_regs->tick_control = 0xC;  // Positive edge or
        timesync_regs->tick_control = 0x4;   // Negative edge, default

Matt