Hi Joe, It looks like you have made a good start. If you have not already, I suggest you review the design, code and particularly the write-up posted here https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/?all on eevblog by Lars Walenius. His hardware design is a good deal simpler than yours but is conceptually quite similar.
One significant difference is that his software incorporates a P-I (Proportional - Integral) controller and it appears that you are only using the P term. Unfortunately Lars has passed away, but I have developed several GPSDOs based on his concept and may be able to help if you have questions. My latest design uses a TDC7200 time to digital converter chip to measure the time between the PPS pulse and the oscillator. To answer your specific questions, At 10 Mhz,.a change of .00003 Hz is 3e-5 / 10e6 = 3e-12 or 3 ppt. At 5 MHz it would be twice that A change in the DAC voltage over several hours may or may not translate directly to a frequency change. It depends partly on how stable your oscillator is with temperature and other factors and how much it is drifting due to aging. The aging in turn depends on the quality of the oscillator, how long it has been running and how long it was powered down before that. You can get an indication of what to expect by studying OCXO spec sheets. The control loop is adjusting the DAC voltage to keep the oscillator at a constant frequency based on the PPS pulses from the GPS. You can get an indication of what is going on by looking at the DAC voltage, but you can't tell for sure, which leads to Bob's comments that the only way to tell for sure is to compare your GPSDO's frequency to a better reference. HTH -- --Jim Harman _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.