I am interested in an application of the SparkPNT to measure the performance of an externally supplied 10 MHz oscillator. I am curious if anybody has had success in bypassing the onboard oscillator for this purpose. I see in the schematics https://docs.sparkfun.com/SparkFun_GNSSDO/assets/board_files/schematic.pdf on the last page that there is a DPDT switch for this purpose. Is this as simple as opening the enclosure and changing the switch position, or are there more serious modifications required to do this?
Hi @ach ,
You may be heading into Time-Nuts territory? I’ll include a couple of links below. Those guys will be able to give you much better advice than I can.
It depends what you want to measure and achieve…
Yes, the GNSSDO can use an external 10MHz oscillator. All you need to do is move the internal switch and apply 10MHz to the SMA connector (50 Ohm, -14dBm min, +12dBm max). When it is in this mode, you can use the Bias figure from the display to trim your external oscillator. If you see the bias steadily increasing, you need to decrease the oscillator frequency, and vice versa. When the bias is static, you’re at 10.0…MHz. You would need to implement your own PID control loop to fully discipline your oscillator and drive the bias to zero. Perhaps by outputting the SBF PVTGeodetic message on mosaic-T COM2 (accessible via the TX on the green 10-way screw terminals) and parsing the RxClkBias manually.
Or maybe you just want to compare your 10MHz oscillator to the disciplined GNSSDO 10MHz? At 10MHz directly, or divided down to 1Hz? There you are heading towards measuring Allan Deviation, and definitely into Time-Nut territory.
The mosaic-T Breakout also supports an external clock input. But it doesn’t have an on-board disciplined oscillator, only the (non-disciplined) one internal to the mosaic-T itself. It doesn’t have a 10MHz output, or Ethernet connectivity, or the other bells and whistles of the GNSSDO. But, if you are looking to construct your own disciplined oscillator, the Breakout could be a good option for you.
I hope this helps,
Paul
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