because free running frequency of the vco is typically in the 300 mhz to 20 ghz range, and trying to get the pll control loop to have more than 5 mhz of bandwdith will usually end up in an unstable condition due to delays and parasitics.
or you need to use a band step gain block and syncro block band stepping so the referance must be post proceed the gain loop band width /10 or /100 or /100
has to be harmus... level
and the gain block is for the loop filter also variable notch...
300mhz to 20ghz is a big leep and has to be banded
then there is stepping needed and you need to do this both at the loop and the referance
---------- Post added at 05:33 ---------- Previous post was at 05:28 ----------
you can use a simple 4059 or even 2 to achive this with just two external ic's /10
and two vco
the ratio = 1/n where 1 = input to loop and n = 1/(ipl/v)
loop vector does come in to it
most people use low freq to give accurate hi freq or in our case sshf fixed