My design initially used ADF4356 but when it came to production this chip was on long lead time so we switched to ADF4355-3 which in principle does all we need. However I am finding ADF4355-3 sometimes will not lock and to get around this my firmware, on detecting no lock after a timeout, increments the frequency and tries again. This scheme works but is not ideal. I have found if I use the Register 8 value for ADF4356 instead o the different value specified in the data sheet for ADF4355-3, the synthesiser locks ok, although sometimes takes a long time to do so.
In order to investigate further I figure it might be helpful to know what Register 8 does, whereas the datasheet simply dictates a preset value for Register 8.