What is the use of divider after reference oscillator in pll

Can anyone help me out?

I know that the divider used in the feedback circuit of pll
is used to change the output frquency of VCO.


but in some block diagram of pll synthesizer there is given a divider after reference oscillator.
why we use this divider in pll?
is it necessary to use this divider in the circuit?
what will be the equation relating this divder ratio to output frequency?

If you need to step the frequency of your PLL in 1KHz steps, the easy way is to divide it down by N (division ration) to get one KHZ then compare its phase with a reference oscillator running at 1 KHZ. So you get a high stability 5 or 10 MHZ crystal and divide it down to 1 KHZ.
Frank