if we consider the whole frequency generation loop, PN depends on many factors such as ref clk you use, your loop filter BW, your VCO and PLL.
If you check the datasheet of HMC764LP6CE, the part biff44 suggested, its PN at 100k offset is around 120dBc/Hz. You can improve it with a better reference like an OCXO or even better depending on your budget. But the main question is, do you really need much better phase noise than 100dBc/Hz @ 100K
You can check `dean`s book` for pll basics.