GPS Timestamp

[annayaj]

I’m a LabView and SignalExpress novice so I thought I’d try this forum before trying the LabView forum.

I’m trying to acquire analog data with a GPS time stamp.  My application requires that I acquire approximately 10 hours of data at a rate of 500kS/s.  The system is a PXI1000B containing 2X5922 cards and a 6682 card.  Due to the long duration of the collect I’d like to account for any drift in the sample clock.

I can see two options.

1.                  1.  Use a NI-SCOPE Acquire step with a corresponding step to get a GPS timestamp for each read length of data (I’ve set it to 512000).  This might not work as there is no            step for GPS functions.

2.                  2.  Using a SMB cable route the CLK OUT from the 6682 to the CLK IN on the 5922.  Choose CLK IN on NI-SCOPE acquire – Advanced TAB.  This seems too simple.

Again I’m a novice so any advice or help is appreciated.

Thanks

[DFGray]

Option 2 should get you where you need to go.  You have a lot more options in LabVIEW, where you should be able to fully synchronize clocks and triggers of both 5922s and the 6682 using the backplane triggering lines.  Due to the complexity of supporting all combinations of all hardware SignalExpress can use, SignalExpress does not currently support all these modes.  It is a problem we are well aware of, but not one we can easily solve.

Let us know if you need more help.

[annayaj]

Thank you for the quick response.

Just to confirm,choosing CLK IN on the NI-Scope Acquire – Advanced Tab - with the appropriate wiring - is enough to have the sample clock on the 5922 (I’m using channel 0 on one card) is enough to slave the sample clock to the 10MHz clock on the 6682?  The 10 MHz reference clock on the 6682 will itself be slaved to GPS time?  I have chosen GPS as the time reference under MAX. 

The reason I ask is that the following solution to account for clock drift

http://zone.ni.com/devzone/cda/tut/p/id/9734

requires the use of the Trimble Thunderbolt.  I’m confused by this solution as it requires two GPS receivers which seems redundant.

I did try setting the reference clock under SignalExpress to CLK IN.  I get an error which I assume is because I haven’t connected the CLK OUT from the 6682 to the CLK IN on the 5922, assuming nothing else needs to be changed.

Thanks again for your response.  I require a quick and correct solution but I’ll post on the LabView forums to see if there are better solutions.

[DFGray]

Due to the quick response and my faulty memory, I gave you incorrect information.  The 6682 10HMz clock is not synchronized to the GPS, which is why the example you reference requires the use of the Trimble Thunderbolt.  The 6682 has a temperature stabilized clock (better than the default PXI backplane clock), but it is not used by default and there is no current method in SignalExpress to activate it.

The 6682 will give you GPS accurate timestamps for triggers.  You can use this information by simultaneously triggering the 6682 and the 5922s with the same signal (watch out for line length issues) or triggering one with the other (watch out for delay issues).  Check out the 6682 manual for details.  Unfortunately, there are no native steps for doing this in SignalExpress.  You would need to create one or more user steps to set up the 6682, get the trigger data, and replace the timestamp in the 5922 data.  Much as I hate to admit it, it would probably be easier to do the whole thing in LabVIEW, if you have it available.  SignalExpress does not currently support the 6682.

Once again, my apologies.  If you need further assistance, let us know.