In my case, the cold-start frequency of my OCXO with EFC at midpoint was off sufficiently far that I needed a minimum number of remainder bits to know which way to initially steer it. Don't recall the exact number I needed, but it was more than eight at 10 MHz. Like this design, I had 16 bits to work with, which gave me a usable range.
Why even try to discipline an OCXO before it's warm? Just leave the control loop off for a predetermined time at startup. You can light up a bright red "unlocked" LED, and even inhibit the 10 MHz output until lock is achieved if you want.
Alternatively, you could figure out the EFC voltage needed to zero the cold oscillator and load the corresponding DAC code at startup. However, if the control loop is slow enough for good GPSDO performance at tau out to 100 seconds or more, it would probably be too slow to track the oscillator as it warms up -- so you would likely need switched fast and slow loop filters. (Switched "acquire" and "maintain" time constants are often very useful for a number of reasons, and a GPSDO can benefit from several different "maintain" time constants for best performance in noisy conditions and recovering from holdover.)
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
