After having established an initial time reference, data points can be referenced using the time delta, or dt (pronounced Dee-Tee) of the sample clock. However if this clock is not aligned with a GPS time reference the accuracy of the timestamp data will degrade over time as the clock skews.
Suppose the master clock timebase of a PXI data acquisition device has an stability of 25 ppm. After one hour the clock drift will be the following:
0.000025 drift error * 3600 seconds per hour = .090 drift error seconds per hour
So in the worst scenario our timestamps will be inaccurate by a factor of 90 ms per hour. By continuously phase-aligning the sample clock of the data acquisition device with a GPS-derived clock we ensure the accuracy of the timestamps over long acquisition periods.
There are a two methods to phase-align the sample clock to the GPS-derived clock. In a PXI or PXI Express system, you have the option of using the NI PXI Multi-Device Clock Disciplining Bundle or the NI PXI Express Multi-Device Clock Disciplining Bundle. Additionally, a PXI system has the option of using the Trimble® Thunderbolt®.
The first option for a PXI system is to use the NI PXI Multi-Device Clock Disciplining Bundle. The Clock Disciplining Bundle utilizes the PXI-6682H and PXI-6653 timing and synchronization modules to discipline the 10 MHz backplane reference clock to the 10 MHz clock on the PXI-6653. The NI PXI-6653 offers a controllable 10 MHz high-stability oscillator, and the NI PXI-6682H monitors the timebase. The PXI-6653 10 MHz clock is kept phase-aligned to the GPS-derived 10 MHz reference clock on the PXI-6682H using the Clock Disciplining Bundle software.
For a PXI Express system, National Instruments offers the NI PXI Express Multi-Device Clock Disciplining Bundle. This bundle works in the same manner as the PXI Multidevice Clock Disciplining except that the PXI-6653 is replaced with the PXIe-6674t. The backplane reference clock of the PXI Express system is replaced with the high-stability oscillator of the PXIe-6674t. The oscillator of the PXIe-6674t is phase-aligned to the GPS-derived clock from the PXI-6682H.
By disciplining the 10 MHz clock in multiple systems using the NI Multi-Device Clock Disciplining Bundle, you can align each system to remove drift and keep phase aligned to less than 6 ns standard deviation between systems.
The other option for a PXI system utilizes the Trimble® Thunderbolt® connected to the backplane of the PXI system, replacing the 10 MHz reference clock. This clock allows the sample clocks of the devices in the system to phase align themselves to a GPS time reference assuming they have the capability. Since each rising edge of the 10 MHz clock from the Thunderbolt is within 15 ns of the GPS atomic clock, we eliminate the errors due to drifting and significantly improve the accuracy of our data relative to GPS time.
Figure 4. Using a 10 MHz GPS-derived reference clock and GPS-based initial time ensures accurate data to time correlation.
The accuracy of the timestamps is now a function of the accuracy the hardware and the GPS satellites. Though the atomic clock on the GPS satellites is accurate to 10-13 seconds, the accuracy of the time signals they send for civilian use is only guaranteed to be within ±170 ns of UTC. As the propagation delays in the PXI chassis are within this limit the time accuracy of the acquired data should be within ±170 ns of UTC.
Real-world testing of multi-chassis systems using the synchronization techniques described above achieved results of synchronization between chassis to approximately ; where 0 ns would be perfect synchronization.