TinyPFA is firmware from Erik Kaashoek for NanoVNA-H4 hardware for accurate measurement of oscillator and clock source stability.
This article documents calibration and stability comparison of a HP8656B signal generator’s high stability reference oscillator.
Above is a screenshot of the tinyPFA with only Trimble Thunderbolt GSPDO input, and after freq A is nulled (colours have been inverted for printing).
Above is the freq comparison, it is 380mHz low, -0.038ppm, and will move higher as the day warms up. It was calibrated yesterday using this type of display, and whilst it is easy to adjust to within 10mHz, it will not hold that in the short term, much less long term.
Above is a spot check after 24h running and at 20° ambient, it is very close but I doubt it would hold much better than 100mHz error over the short term.
A better measure of stability comes from an Allan Deviation plot.