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)?
Från: Matt E. [mailto:email@example.com]
Skickat: den 14 maj 2009 19:07
Till: Ulrika U.
Ämne: Re: [Discuss-gnuradio] Syncrinization of two USRP2s
Ulrika U. wrote:
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
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