Timestamp Source (Input)

Menu Path:Input > Capture

Timestamps are added to the trace headers and recording files in VSA where the timestamp represents the absolute time of the T=0 point (often the trigger location) of the time domain data. The timestamp is in GPS Global Positioning System:L A worldwide radio-navigation system that was developed by the US. Department of Defense. In addition to military purposes it is widely used in marine and terrestrial navigation (for example car navigation systems). time and has a resolution (not accuracy) down to 1 attosecond. GPS time is a continuous time scale standard with time "0" being midnight January 6, 1980 UTC. The first point in the data can be known in time relative to this timestamp by using the XStart value which is a duration in seconds relative to the timestamp.

When recalling a recording and for playback the source of the timestamp will always be gotten from the file that was recalled.

When exporting recordings, VSA runs some filtering, resampling and corrections, and the XStart value will be adjusted to account for that during the file export operation.

Timestamp Source lets you select where you want the timestamp information to come from and provides feedback about where it actually comes from and why they may not be the same.

The following Timestamp Source choices are available for live measurements or when making a recording:

Desired: Select a desired timestamp source. If the timestamp source is supported with the configured hardware and setup, the selection will show in the Actual field. See Actual for more information.
- Auto (default): VSA will automatically choose where to get the timestamp from. The timestamp will come from the instrument if the instrument supports timestamps; otherwise it will come from the application.
- Application: VSA reads the clock of the computer it is running on and does its best to associate that with the data. This is what VSA has supported prior to the Timestamp Source feature.
- Analyzer Instrument: The timestamp will be gotten from the instrument that VSA is connected to so the instrument will determine how accurate it might be. Typically the hardware will have some running counter that counts the number of ADC Analog-to-Digital Converter samples since some point in time that the counter was synchronized with some clock and can report the point in time that the data starts to VSA when VSA reads the data from the instrument. The Analyzer Instrument choice does not sync the instrument's ADC counter with an external clock, so the timestamp's reference is the date time stamp from the instrument's computer clock. Doing a difference between two timestamps from the same instrument should result with fairly accurate duration values between acquisitions, but the absolute time may not be very accurate. However, it will still be much more accurate than previous approaches with added latencies between instrument and PC.
- User: The user can provide VSA with the timestamp to use for the data via some programmatic API 1) Access Preamble Indicator, or 2) Application Programming Interface call to VSA. The user must use VSA in single sweep mode and provide this information via a .NET API call to the VSA application after the measurement has been started. VSA will block the current measurement from completing until the user has provided this information. A status bar message will indicate if the measurement is waiting for the user to provide a timestamp.
Actual: Displays the actual timestamp source. When the desired and actual values differ, a icon is shown and a tooltip on it will indicate why the values differ. Often they can differ if the hardware does not support timestamps with some setups. It might depend upon the trigger type chosen, the span, or an input extension parameter (such as what IF Path is being used) as to whether timestamps are supported from hardware.