Dear James,

I have never implemented a tuning range algorithm for a crystal oscillator, but I would just like to mention that this might not be feasible
if phase noise is of importance. Typically, you will use a bank of binary scaled capacitors with MOS switches in series. The Q factor of these elements can never match the intrinsic Q of your crystal.

If phase noise is of no importance, I would use a binary scaled capacitor bank in combination with a binary search algorithm. The crystal oscillator clock drives a counter which counts for a fixed amount of time T. Every time T, you compare the count value with a wanted value.

Kind Regards

Peter