Re: [Discuss-gnuradio] Syncrinization of two USRP2s

[Matt Ettus]

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