lotoy
The pll will vary the voltage out of the loop filter and into the vco, until the required freq is reached (indicated by the phase comp). In this condition the pll is locked.
If you intend to adjust a pll whose operating range is fmin to fmax.
First you should check that the VCO can go from fmin to fmax over the range of voltages of the loop filter (VCO input). To check it. Set the divider for the lowest freq and see if the pll locks. Then repeat for the highest freq. When the pll is locked you can measue the vco voltage with a high impedance voltmeter.
If you cannot lock the pll then you must adjust the value of the C or L on the VCO.
And repeat the process to ensure that the pll will lock on the limits and hence on all the operating band.
The voltages for fmin and fmax must be near to the limits of the loop filter but with enough tolerance to take into account variations in temperature, supply voltage, loading of the osc, etc.
To check the VCO (open loop) you can connect a potentiometer to the VCO control voltage and a freq meter to the output and plot the VCO response.
I assumed that you are talking about a programmable pll (freq synthesizer). If your pll does not have a programmable divider. just give me more details.