It is unclear to me whether the AD9552 reference monitor continuously monitors the REF input, or only does so on power-up. The part of the datasheet that leads me to think it may be the latter is PART INITIALIZATION AND AUTOMATIC POWER-ON RESET on page 17.
-10MHz crystal on XTAL pins. This clock is always present
-10MHz external reference on REF pin. This clock is a user input and may come or go during use.
Ideally, I would like the chip to automatically select the REF input when the external reference is present and switch back to the crystal when the reference goes away. Is this the default behavior?
-Does the VCO need to be re-calibrated when switching between references if no change is made to the input or output frequencies?
-Can the output be expected to be glitch-free when switching between references?