Why does my waveform info timestamp return time since last reboot, instead of current time?

[go_sox]

Hi,

I am using the NI recording system to record data that we would like to time stamp. I was using the "niRFSA Fetch IQ (1D I16)" because it puts out the wfm info which includes absolute time stamp and relative time stamp (which would be great). However, both values come out the same, and are the amount of time since the recording system was booted. I would think that the absolute time stamp would use the system clock (which we have slaved to GPS).

Any help would be appreciated.

Thanks,

Sean

[TimmTheEnchanter]

Sean,

What hardware are you using?  You may need to post to the RF forums, since you are using the RFSA driver.
http://forums.ni.com/t5/RF-Measurement-Devices/bd-p/290

-Andrew

[Anthony_F]

Sean,

There may have been some misunderstanding with the name "absolute timestamp."

The following is the detailed context help of that output

Absolute  - The timestamp corresponds to the date and time of the acquisition of the first sample returned. 

Even though your system is controlled by a GPS clock, there is no built in method for the RF signal analyzer (or any other PXI device for the matter) to "know" what the current GPS time is.

If you are looking for GPS Time stamping functionality, you will need to use the timing stamping feature of the 6682 timing and sync card.

both values come out the same, 

Is this true for more than just the first record of data?

[go_sox]

I realize that the system clock doesn't automatically get set by the GPS, which is why I set the system clock to the GPS time via the kernal32.dll. But the timestamp doesn't give the system time, it is more like the time from when the system was powered up (always a really low value).

I only look at the first timestamp, so maybe I can't expect absolute and relative timestamps to be different?

Thanks,

Sean

[MikhailK]

Hi Sean,

Let me expand on what Anthony mentionned in his previous post.

 niRFSA Fetch IQ VI

Transfers acquired waveform data from device memory to computer memory; this process is also known as fetching. The data was acquired to onboard memory previously by the hardware after the acquisition was initiated.

• absolute timestamp—Returns the timestamp, in seconds, of the first fetched sample that is comparable between records and acquisitions.

• relative timestamp—Returns a timestamp that corresponds to the difference, in seconds, between the first sample returned and the Reference trigger location.

So if you don't have your reference trigger configured, absolute and relative timestamps will be the same. What is the value of "samples to read" input that you set in your niRFSA Fetch IQ VI? What is the model number of the PXI VSA that you use?