the change in DC control voltage Vc during lock is given by the formula:
ΔVc=Δf/K0
where Δf is the difference between the input signal frequency and the free running frequency, and K0 is the voltage to frequency transfer coefficient of VCO.
Plugging in the given values, we get:
ΔVc=20−21/4=−0.25V
The change in DC control voltage Vc during lock is -0.25 V
