[time-nuts] Assistance needed to understand some V_OCXO stability concepts.

[Jim Harman]

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