mmediately after changing the CE pin from low to high level
When initializing registers with SPI
The PLL will be unlocked.
If you increase the time to start register initialization with SPI,
I lock it.
After CE pin goes high,
Immediately initializing registers with SPI
Is not it possible?
Make sure all voltages on the ADF4351 have settled to steady-state. If these haven't settled, then there is a risk that the wrong VCO band will be selected, which will cause unlock.