[time-nuts] Catching range of GPSDO

[Erik Kaashoek]

The GPSDO I'm building started with frequency locking but now I'm adding
phase locking so the time stamping counter can be on GPS time.
A first version works with a PI controller setting the vc-tcxo Vtune DAC
based on the phase difference of the 10 MHz with the PPS phase. Due to
tolerances the tcxo frequency range is big and is set by a 16 bit DAC where
1 bit is about 2e-11 frequency change.
Once the DAC value is close to the correct frequency the loop catches
nicely but if the setting is far off catching takes a long time.
A possible solution is to use the frequency error to set the DAC close to
the optimal frequency for catching.
Speed of catching is important as the design is intended to only be
switched on when needed.
Does anyone have pointers to info on how to do quick catching in such a
control loop?
Erik