[time-nuts] First success with very simple, very low cost GPSDO
csteinmetz at yandex.com
Thu Apr 10 17:55:43 UTC 2014
>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.)
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