If you can run a GPS antenna to the unit, you can get the API to performing timing for you. Enabling the smSetGPSTimebaseUpdate() allows the internal GPS to discipline the timebase of the unit. While the device is open, you can query the state of the lock with the smGetGPSState() function, you can wait for ‘disciplined’. Once disciplined the timestamps you received from the smGetIQ() function are referenced to the GPS. Technically you can get timestamps without the disciplining, but you will see timestamp drift between PPS’s.
If you want to do this manually with an external 10MHz and PPS, tell the instrument to use an ext 10MHz timebase with the smSetReference() function and connect the PPS to the trigger input port (and the 10MHz reference to the 10MHz ref input). Then when streaming I/Q, the PPS events will show up in the trigger output buffer. Its then easy to timestamp samples based on their location relative to the PPS trigger position in the stream. With this method though, you will have to acquire and parse the NMEA data from the GPS to know the absolute time of the PPS event. The advantage of doing this externally is being able to use higher accuracy GPS receivers than our internal one, and running multiple units off of one external GPS which will have better precision for certain tasks.
We have an example for the first method discussed above, but not for the second. We only have a generic ext trigger example which you can view to see how the triggers come in. Hopefully the explanation is enough to piece the rest together. If you have follow up questions as you work through this, please let me know.
Spectrums via the API sweep configuration are timestamped when using the internal GPS, but due to how we process the sweeps, it will be hard to get precise timing on an event in the sweep. It should really only be used as a coarse monotonically increasing timer. The I/Q sweep list functionality in the API allows you to sweep the I/Q acquisition with the same speed as our fast sweeps. Timestamping with the I/Q sweep lists only works with the internal GPS (rather than providing an ext 10M/PPS), but this approach would give you precise timestamped ‘steps’ of I/Q samples. The downside, is that you would have to FFT and construct the sweep if you wanted the classic sweep plot as well.
You can email me directly at aj@signalhound.com if needed.
Andrew
