PFD in PLL chip

In the NS' PLL chip such as LMX2332,The PFD is digital,But my reference and VCO back signals are analog.How dose it work ?

So,what's the differents betwen digital and analog signal if they all swing from vdd to ground.

You can input analog signal into digital PFD. But the swing requirement for digital PFD is larger than the one for analog PFD.

Yibin.