I recently did a design using the ADF4351 up to 1.8GHz. For that application i used a prescaler value of 4/5. I've just noticed in the datasheet that the prescaler should be set to 8/9 for RF frequencies above 3.6GHz. My new design operates to 2.2GHz, but i was wondering whether the 3.6GHz threshold is for final RF frequency or VCO frequency? My latest application would obviously require a VCO frequency of >3.6GHz, but not an RF frequency of >3.6GHz. My first application had neither a VCO frequency or RF frequency higher than 3.6GHz so i was fine to use a 4/5 prescaler all the time.
Are there any issues to be aware of when switching from 4/5 to 8/9 other than the increase in Nmin?